Download METEOROLOGY, PAGE I. The Atmosphere

METEOROLOGY, PAGE 1
I. The Atmosphere
- envelope of gases and tiny, suspended particles encircling the Earth
A. Meteorology
- study of the atmosphere and weather processes
- studies of atmosphere involves many sciences
B. Weather
- the state of the atmosphere at a given time and place
C. Climate
- sum of all statistical weather information that helps describe a region
Climatology - the study of climate
D. The Scientific Method
- means of discovering basic scientific principles
1. Pose a question or problem
2. Collect observations (DATA; "facts")
3. Analyze data
4. Propose hypotheses (tentative explanations)
5. Predict what would happen if hypothesis were true
6. Test predictions and discard incorrect hypotheses (= concept of Multiple Working Hypotheses)
Null Hypothesis- no systematic relationship exists among the observations
7. Theory
- hypothesis that passes testing and has a good chance of being true
8. Scientific Law
- fundamental principles that are invariably found to be true
9. Model
- a conceptual, graphic, mathematical or physical image that is consistent with the data
a. Conceptual Model
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- describes general relationships among components of a system
b. Graphical Model
- assembles and displays data in an organized and easily-interpreted format
c. Physical Model
- miniaturized version of a system
d. Numerical Model
- consists of mathematical equations that illustrate behavior of a particular physical system;
computerized numerical models are very important for weather forcasting
E. Formation of the Atmosphere during PreCambrian times (4.6 to 0.6 billion years ago)
The Earth's Atmosphere was formed by:
1. Degassing of Earth's interior
- an initial atmosphere was probably formed during differentiation, then "swept away" when the
early Solar System was cleared of debris by a strong "Solar Wind" (charged particles moving
away from the Sun)
- volcanic activity then produced a second atmosphere of water vapor, hydrogen, hydrogen
chloride, nitrogen, carbon dioxide, carbon monoxide (and secondary chemical reactions in the
atmosphere produced methane and ammonia)
2. From Comets/"Space Ice"
- comet-like material supplies ammonia, methane, water vapor, etc. to partially create the
atmosphere
3. Photosynthesis
- early photosynthetic organisms, such as blue-green algae (cyanobacteria), create oxygen
- but there was evidently little "free" oxygen present in the Precambrian (production of oxygen
by plants was probably not important prior to 2 billion years ago)
F. Subdividing the Atmosphere
1. By Homogeneity
a. Homosphere
- lower 80 km of atmosphere where gases mix; gases mostly nitrogen (78%) and oxygen (21%); also
argon (0.93%), carbon dioxide (0.035%) and aerosols (minute liquid and solid particles)
b. Heterosphere
- above 80 km where gases stratified; mostly nitrogen and oxygen
- molecules break down due to radiation (photodissociation) to produce oxygen in its atomic form (O)
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2. By Temperature
a. Troposphere
- lowest layer; up to approximately 18 km (equator) or 8 km (poles)
- where most weather occurs; temperature usually decreases with altitude; upper boundary = Tropopause
b. Stratosphere
- from tropopause to approximately 50 km
- temperature does not change to about 20 km (= isothermal); temperature increases above this to
Stratopause
c. Mesosphere
- up to approximately 80 km; temperature decreases upward; upper boundary = Mesopause
d. Thermosphere
- above 80 km
- temperature isothermal to approximately 90 km, then increases (temperature highly variable)
3. Ionosphere
- located primarily in thermosphere
- with high concentration of ions (charged atomic particles), especially in lower thermosphere
- Affects Include:
a. "bounces" AM radio waves
b. produces Aurora Borealis (northern lights) and Aurora Australis (southern lights)
- produced where solar wind (charged subatomic particles) deflects Earth's magnetic field into a
magnetosphere, excites atoms and produces radiation (forms mostly blue-green lights)
- especially important when sun produces abundant solar flares
G. Probing the Atmosphere
1. Radiosonde
- instrument package with radio transmitter carried aloft by a hydrogen (or helium) balloon
- transmit soundings (continuous altitude measurements of temperature, pressure and humidity) to
ground stations
a. Rawinsonde
- a radiosonde that is tracked (to determine wind direction)
b. Dropwindsonde
- like rawinsonde but dropped from plane by parachute
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2. Weather Satellites
- very important meteorological instruments
II. Radiation
A. Solar Radiation
1. Electromagnetic radiation emitted by Sun's surface
- measured by pyranometers
a. usually described by wavelength (distance between wave crests) and wave frequency (number of
crests/troughs that pass per time unit)
b. primarily ultraviolet (UV; 9%), visible light (45%) and infrared (IR; 46%)
2. Global Radiation Balance
- total energy absorbed by Earth equals total energy emitted by Earth-atmosphere back to space
B. Global Insolation
- incoming solar radiation
1. Amount of Global Insolation depends on:
a. output energy of the sun
b. distance between Earth and Sun
c. angle at which the sun's rays strike the Earth's surface (i.e. Solar Altitude)
d. duration of daylight
e. atmosphere composition
2. Therefore the amount of insolation depends primarily on the latitude and seasons
C. Earth's Motions in Space
1. Axis of Rotation
a. Earth rotates on its axis (counterclockwise; 15° per hour = time zones) and revolves about the sun
(counterclockwise in approximately 365 1/4 days)
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Perihelion - orbital position closest to the sun; approximately January 3
Aphelion - orbital position farthest from the sun; approximately July 4
b. axis inclined 23 1/2° from the perpendicular
c. north axis "always" points to North Star
D. The Seasons
- due to angle Earth tilts toward and away from the sun
- If sun is directly overhead, solar rays are most concentrated and less atmosphere traversed
- air temperature usually lags 1-2 months behind periods of minimum and maximum solar radiation
1. Equinox
a. Earth's axis at a 90° angle with a line drawn to the sun
b. Noon sun directly overhead at Equator and at horizon at the Poles
c. Day and night are equal length
d. Vernal (Spring) Equinox
- on or about March 21
Texas Spring - most turbulent season
e. Autumnal (Fall) Equinox
- on or about September 22
Texas Autumn - by September one or more cool fronts penetrated most of the state; often drier air and
wide temperature range
2. Solstice
a. Points farthest from Equator at which Noon Sun is directly overhead (subsolar point) = 23 1/2° North
(Tropic of Cancer) and 23 1/2° South latitude (Tropic of Capricorn)
b. Points from the Equator at which Noon Sun is at the horizon= 66 1/2° North and South latitude;
define the Arctic and Antarctic Circles
c. Summer Solstice
- on or about June 22; Northern hemisphere tipped toward sun; noontime rays of sun have greatest
poleward displacement (23.5°)
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Texas Summer - not much variation (hot; East Texas humid; West Texas dry)
d. Winter Solstice
- on or about December 22; Southern hemisphere tipped toward sun
- In Northern Hemisphere intensity of light from the sun is at a minimum
Texas Winter = day and night temperatures steadily drop to minimum in late December or January; with
drastic temperature variation
E. Effects of the Atmosphere and Earth's Surface Upon Insolation
1. Only 1/2 - 2/3 of incoming radiation reaches Earth's surface (highly variable)
Due to:
a. length of passage of Sun's rays through atmosphere; depends on latitude
b. Effects of a Clear Sky
- Scattering of light by gas molecules (Rayleigh scattering) higher for short wavelengths of light (blue)
than for long wavelengths (red); causes blue sky
- Scattering of light by dust and haze causes long wavelengths of light scattered as much as short
wavelengths (white/milky sky)
- No scattering at very high altitudes (black sky)
c. Effects of a cloudy sky
- Locally can reduce insolation from approximately 1/3 to nearly all
F. Global Energy Distribution
1. Daily variation in net radiation and temperature affected by:
a. length of day (season)
b. albedo (total radiation reflected by a surface)
c. moisture content of air (deserts versus oceans; amount of vegetation cover, transpiration)
d. cloud cover
e. snow cover
2. Absorption by CO2 and water vapor in the lower atmosphere
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- allow short-wave radiation to pass through but opaque to longwave radiation leaving earth (heats up
atmosphere = Greenhouse Effect)
3. Annual variation of gain and loss of energy
a. Net radiation varies with latitude
a1. Energy surplus between 40°North and 40°South Latitude
a2. Energy deficit poleward of 40°North and 40°South Latitude
a3. Net energy imbalance causes continuous horizontal exchange of energy (causes atmospheric and
oceanic circulation and weather patterns)
b. Effect on annual temperature variation
- fairly uniform in equatorial regions
- moderately large range in subtropics
- very large range at higher latitudes
- little variation at poles
c. Contrasts between areas of land and water
- water heats more slowly than land and evaporation cools water surface
Thermal Inertia - resistance to temperature change
d. Contrasts greater at higher altitudes
4. Urbanization and agriculture also affects global energy
III. Heat and Temperature
A. Kinetic Energy
- energy of motion
B. Heat
- total kinetic energy of atoms or molecules composing a substance
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1. Heat Measurements
a. Calorie (cal)
- heat required to raise temperature of 1 gram of water 1°C; 1 cal = 4.1868 J (joules)
b. British Thermal Units (Btu)
- heat required to raise temperature of 1 pound of water 1°F; = 252 cal or 1055 J
2. Heat Transport
a. Conduction
- kinetic energy transferred due to atom/molecule collision
- substances (solid, liquid, gas) in contact
b. Convection
- kinetic energy transferred due to fluid (liquids/gases) motion
Sensible Heating = heat transported by conduction + convection
c. Radiation
- electromagnetic waves traveling at speed of light
- can travel through a vacuum
- primary way Earth-atmosphere system gains heat from sun and loses heat into space
3. Specific Heat
- heat required to change temperature of 1 gram of a substance by 1°C
- water with high specific heat
a. Thermal Stability
- resistance to temperature change; water has greater thermal stability than land
b. Index of Continentality
- degree of maritime influence on average air temperature
C. Temperature
- average kinetic energy of atoms or molecules composing a substance OR the degree of molecular
activity of a substance
1. Temperature Scales
a. Fahrenheit (°F)
- boiling point of water at sea level at 212°F; freezing point 32°F
b. Celsius (formerly Centigrade)
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- boiling point at 100°C; freezing point 0°C
c. Kelvin
- number of degrees above absolute zero (= temperature at which molecular motion stops;
= -273.15°C)
c. Temperature Conversion Formulas
°F = (1.8 X °C) + 32°
°C = (°F - 32°) X 0.56
°K = °C + 273.15
2. Types of Thermometers
a. Liquid-in-glass
- the most common type of thermometer, which have a sealed glass tube attached to a glass bulb filled
with liquid (mercury or red-colored alcohol)
b. Maximum thermometer
- has a small constriction just above the bulb; it is designed to measure the maximum air temperature
c. Minimum thermometer
- a thermometer designed to measure the minimum air temperature during a desired time period
d. Bimetallic (expansion) thermometer
- a temperature-measuring device usually consisting of two dissimilar metals that expand and contract
differentially as the temperature changes
e. Electrical Thermometers
- thermometers that use elements that convert energy from one form to another (transducers)
- common electrical thermometers include electrical resistance thermometers (the resistance of a
platinum or nickel wire increases as temperature increases), thermocouple thermometers (the
temperature difference at the junction of two dissimilar metals sets up a weak electric current) and
thermistors (made from ceramics; the resistance increases as temperature decreases)
g. Thermograph
- an instrument that continuously measures and records air temperature
h. Infrared Sensors (Radiometers)
- an instrument designed to measure the intensity of infrared radiation emitted by an object
- used in satellites
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3. Measuring Temperature
- thermometers should be protected from direct or reflected light (measure air temperature)
- be consistent with measurements
Isotherm - a line on a map connecting points of equal air temperature
4. Heating and Cooling Degree-Days
a. Heating Degree-Day Units
- a form of the degree-day used as an index for fuel consumption
- usually days when mean the outdoor air temperature is less than 65°F
b. Cooling Degree-Day Units
- a form of degree-day used in estimating the amount of energy necessary to reduce the effective
temperature of warm air
- a cooling-degree day is a day on which the average temperature is one degree above a desired base
temperature (typically when mean outdoor air temperature is greater than 65°F)
- air conditioning needed if greater than 700
5. Windchill
a. Frostbite
- freezing body tissue
- ears, nose, fingers and toes especially susceptible
b. Windchill Equivalent Temperature (WET; Windchill Index)
- gauges sensible heat loss from exposed skin due to low air temperature plus wind
IV. Heat Imbalances and Weather
A. Atmosphere versus Surface Heating/Warming
1. Atmosphere with cooling rate (due to infrared emission) greater than warming (due to absorption of
solar radiation)
2. Earth's Surface with warming rate greater than cooling
- therefore with heat imbalance between atmosphere and Earth's surface
B. Flow of energy from Earth's surface to Atmosphere is by:
1. Sensible Heating (23%)
- by conduction/convection
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2. Latent Heating (77%)
Latent heat = stored-up energy in water vapor
a. Energy is stored during evaporation, released during condensation (Latent Heat of Vaporization)
b. Energy is stored during melting, released during freezing (Latent Heat of Fusion/Melting)
c. Energy is stored when ice transforms directly to vapor, released when vapor transforms directly to
ice (Latent Heat of Sublimation)
C. Bowen Ratio
- ratio of sensible heating to latent heating
- varies from one locality to another (approximately 1:10 for oceans; 2:1 for deserts)
D. Poleward Heat Transport
- heat transported from tropics to higher latitudes
- approximately 50% heat transfer by air mass exchange; 20% by ocean currents; 30% by release of
latent heat by storms
- energy of weather (storms, etc.) greater with more heat imbalance
E. Air Temperature Variation
1. Local Air Temperature is Affected by:
a. Time of day
Diurnal Temperature Variation (Daily Range of Temperature) = difference between the maximum and
minimum temperatures for any given day; highest temperature is typically in the early- to midafternoon;
lowest temperatures is typically near sunrise
b. Day of year
c. Cloud cover
d. Nature of surface cover
- air over dry surfaces warms faster (is why extreme temperatures accompany droughts)
- above (# a-d) are radiation controls
e. Air Mass Controls
Air Mass = huge volume of air (thousands of kilometers) with relatively uniform temperature and water
vapor concentration
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Air Mass Advection - movement of air mass from one locality to another (one air mass replaces another)
e1. Cold Air Advection
- wind blows across regional isotherms (lines of equal temperature on weather map) from colder to
warmer area
e2. Warm Air Advection
- wind blows across regional isotherms from warmer to colder area
e3. Temperature Variation by Advection Depends Upon:
- Initial temperature characteristics of new air mass
- modification of air mass by underlying surface
V. Air Pressure
A. Pressure
1. Pressure
- force per unit area of surface confining a fluid
2. Air Density
- mass per unit volume
B. Atmospheric pressure
1. At sea level about 14.7 lb/in2
Millibar (mb) = conventional air pressure unit
- average sea-level pressure = 1013.25 mb
- worldwide range usually 970 - 1050 mb
2. Rate of decreasing pressure much greater at low altitudes than at higher altitudes
- at 16 km = 10% sea level pressure
3. Standard Atmosphere
- mean vertical profiles of temperature, pressure and density within the atmosphere
- Temperature = 59°F; pressure= 1013.25 mb
4. Pressure at a given location will change during a single day; also seasonal changes
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5. Influences
a. Temperature
- air heating increases distance between molecules, therefore less air density and less air pressure
b. Humidity
- the greater the water vapor the less the density (molecular weight of water less than dry air)
c. Therefore, cold and dry air with greater pressure than warm, humid air
- Therefore, air mass advection with change in air pressure
d. Divergence
- wind pattern with net outflow from a central point; with descending air from above
e. Convergence
- wind pattern with net inflow toward a central point; with ascending air from surface
C. Weather Maps and Pressure
1. Isobars
- lines on a map connecting equal pressures
Air Pressure Tendency = change of air pressure with time
2. High Pressure ("H" or "HIGH")
- pressure higher than surrounding areas
- usually fair weather system
3. Low Pressure ("L" or "LOW")
- low pressure
- falling pressure; usually stormy weather system
D. Measuring Atmospheric Pressure
1. Barometers
- measure atmospheric pressure
a. Mercurial barometer
- at sea level column of Hg about 29.92 inches or 760 mm high
- falling air pressure with drop in level
- mercurial barometers are calibrated to “sea level pressure”
b. Aneroid barometer
- a nonliquid barometer; it is less precise than a mercury barometer but more mobile and easier to use
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c. Barograph
- aneroid barometer that records data
d. Altimeter
- an instrument that indicates the altitude of an object above a fixed level
- pressure altimeters use an anemoid barometer with a scale graduated in altitude/elevation rather than
pressure
E. Gas Law
- the pressure exerted by air is directly proportional to the product of its temperature and density
(Air Pressure = Constant X Density X Temperature)
VI. Humidity and Stability
A. The Hydrologic Cycle
- the system of moving water at the earth's surface
Water Budget - balance of input and output of water to and from global water reservoirs
1. Water returns to the atmosphere by:
a. Evaporation
- Rate depends on
a1. temperature of the water
a2. amount of water vapor already in the air
a3. wind speed
--------------------------------------------- Distribution
a1. greater over oceans than over continents
a2. changes with latitude
a3. directly related to air temperature and relative humidity
b. Transpiration
- process by which water is released into the atmosphere by plants
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Evapotranspiration = water returned to atmosphere by evaporation and transpiration
2. Water returns to land by means of precipitation - rain, drizzle, snow, ice pellets, hail
3. Water is temporarily stored:
a. in bodies of water
b. in glaciers
c. as groundwater
4. Runoff
- water that flows over the land surface
B. River System
- consists of a main channel and all of the tributaries that flow into it
Drainage Basin - total area that contributes water to a single river system
C. Humidity
- amount of water vapor in the air
1. Importance
- source of precipitation
- principal absorber of solar and radiant energy
- greatly affects temperature distribution
- important energy source
2. Atmospheric humidity
- capacity of air to hold water vapor
Saturated air = contains maximum amount of water vapor for a given temperature (saturation point)
3. Measuring humidity
a. Absolute humidity
- mass of water vapor/unit volume of air (g/m3)
b. Specific humidity
- mass of water vapor/unit mass of air
c. Relative Humidity
- ratio of water vapor in air to maximum allowable amount for a given temperature
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- varied by (1) adding moisture by evaporation or transpiration (2) changing the temperature (warmer air
with greater moisture than cooler air)
4. Measurement of Atmospheric Humidity
a. Psychrometer
- an instrument used to measure the water vapor content of the air
- it consists of two thermometers (wet bulb and dry bulb)
- after whirling the instrument, the dew point and relative humidity can be obtained by comparing the
difference of wet and dry bulb thermometer readings; the relative humidity and dew point can be
determined by using special graphs or tables (psychromatic tables)
- accurate but cumbersome
b. Electrical Hygrometer
- electrical resistance is a function of conductor dampness; electrical hygrometers use a plate coated with
carbon
- hygrometers are portable (they are used in radiosondes), easy to use, and are fairly accurate
c. Infrared Hygrometer
- measures atmospheric humidity by the amount of infrared energy absorbed by water vapor in a sample
of air
d. Dew Cell
- an instrument used to determine the dew point temperature
- it measures the amount of water vapor in the air by measuring the air's actual vapor pressure
e. Hydrograph
- recording hygrometer
D. Condensation
1. Dew Point
- temperature where air becomes saturated with water vapor
2. Condensation occurs if temperature falls below dew point
- amount depends on how much the air is cooled below the dew point
- warm air with greater potential for precipitation than cooler air (warmer air holds more water vapor)
- suitable surface must be present before condensation can occur (hygroscopic nuclei)
E. Adiabatic Processes
- cooling or warming air parcel (unit mass of air) by moving vertically
- with expansion (cooling) or compression (warming)
- no heat additions or losses occur
- major cause of temperature change in large air masses that move vertically
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1. Rising parcel of unsaturated air (less than 100% relative humidity)
- air with Dry Adiabatic Lapse Rate [temperature decreases at 10°C/1 km (5.5°F/1000 ft)
- dew point decreases about 2°C/1 km (1°F/1000 ft) = Dew Point Lapse Rate
- condensation occurs when 2 lapse rates meet; depends on the initial relative humidity of air parcel
2. Rising parcel of saturated air
- altitude where saturation first occurs (= base of clouds)
- condensation (with latent heat release) works against adiabatic process
- reduced lapse rate above cloud base = Wet (moist, saturation) Adiabatic Lapse Rate
- varies with temperature and moisture content [as low as 3°C/1000 m (2°F/1000 ft) for tropical air;
slightly less than dry adiabatic rate for cold dry air]
- In theory, if air parcel continues to rise, all water vapor is condensed; if this parcel descends
temperature increases at dry adiabatic lapse rate
F. Vertical Stability
1. Stable air
a. Parcel of air resists upward vertical movement
- cooler than surrounding air (sinks to original position)
- cool surface air will not rise
b. Absolute Stability
- environmental lapse rate less than wet adiabatic rate
2. Unstable Air
- warm air rises until it reaches altitude with same temperature
a. Absolute Instability
- ascending air always warmer than environment and continues to rise (environmental lapse rate greater
than dry and wet adiabatic lapse rates)
- most common in warm, humid areas
b. Conditional Instability
- moist air with lapse rate between dry and wet adiabatic rate (between 5°C and 10°C/ km)
- Conditional Unstable air = begins ascent as stable air but above condensation level is unstable (with
latent heat release); probably most common instability type
3. Neutral Air Layer
- rising or descending air parcel with same temperature as surroundings
- does not prevent or enhance vertical motion
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G. Factors Modifying Air Stability
- most due to movement of air, also daily temperature changes
- instability if environmental lapse rate changes
1. Instability enhanced by:
a. Intense solar heating warms air from below
b. Heating of air mass from below as it traverses a warm surface
c. Forceful Lifting of air
c1. Orographic Lifting
- sloping terrain (Ex.= mountains) act as barriers to flow of air and forces air to ascend
- air on leeward side of mountains loses moisture (creates Rain Shadow Deserts; Ex.= Great Basin
Desert of U.S.)
c2. Frontal Wedging
- cool air acts as a barrier over which warmer, lighter air rises
- responsible for most precipitation in many areas
c3. Upward movement of air due to Convergence (air flowing together); often associated with forceful
lifting
c4. Radiation cooling from tops of clouds that are trapping heat below may cause instability in the
cloud; produce many nocturnal thunderstorms
2. Stability is enhanced by:
a. Radiation cooling of earth's surface after sunset
b. Cooling of air mass from below as it traverses a cold surface
c. Subsidence - downward air flow
- upper portion of subsiding air heated by compression; air near surface usually not involved in
subsidence and with temperature unchanged
- is stable air (air aloft warmer versus surface air)
- warming by subsidence evaporates clouds
3. Temperature Inversion
- an increase in air temperature with height
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a. Low-Level Inversion
- most stable condition; air near surface cooler than air aloft and with little vertical mixing
- this may be caused by nighttime radiational cooling of the surface, by an influx of cold air brought in
by the wind (cold advection), or by air moving over a cold surface
b. Upper Level (Subsidence) Inversion
- Air subsides and is warmed by compression; cool air is overlain by compressed warmer air
- these sometimes occur at the surface, but more often are observed aloft
- may create severe pollution problems
VII. Dew, Frost, Fog and Clouds
A. Low Level Saturation Processes
- takes place at Earth surface where relative humidity is 100% due to cooling
- causes dew, frost or fog
1. Dew and Frost
- due primarily to nocturnal radiational cooling (surface emits infrared radiation)
a. Dew
- water droplets formed by condensation at the surface
b. Frost
- water vapor deposited as ice crystals where air temperature is less than 0°C
Frost Point = temperature (below 0°C) at which saturation occurs
Hoarfrost - humid, cold air deposits water vapor on vegetation as fern-like crystals
Frost Prevention - either conserve heat (reduce the heat lost at night by covering plants with insulating
material) or add heat to warm the lowermost layer of air by employing water sprinklers, air-mixing
techniques using wind machines, or using orchard heaters
2. Fog
- cloud layer in contact with Earth's surface
- restricts visiblity to 1 km or less (otherwise is termed mist)
a. Radiation fog
- fog produced over land when radiational cooling reduces the air temperature at (or below) its dew point
(calm, humid air overlies a chilled land surface)
- is also termed "ground fog" or "valley fog"
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- usually occurs over marshy areas or wet soils
b. Advection fog
- warm, humid air moves over a chilled surface (land or water)
- most common when warm, humid air is forced over a cold ocean current
c. Evaporation (Mixing) Fog
- when moist air mixes with cold air, the air becomes saturated and fog forms
Types Include:
Steam Fog - cold air moves over warm water and evaporates it; looks like rising streamers; common
over lakes on autumn mornings
Frontal Fog (Precipitation Fog) - warm raindrops evaporate as they fall through a cool air mass, creating
fog
d. Upslope fog
- fog formed where moist, stable air flows upward over a topographic barrier
B. Clouds
1. Cloud Dynamics
a. Warm air rises (must be warmer than surrounding air)
- four common causes are:
Convection
Orographic uplift
Convergence of air
Lifting along fronts
b. Rising air expands (decreasing pressure) and cools
c. If water vapor present condensation begins when dew point is reached
Convective Condensation Level (CCL) - altitude at which condensation begins to occur through
convection; coincides with the altitude of the cloud base
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- hygroscopic nuclei must be present for condensation to take place (hygroscopic nuclei have a chemical
affinity for water molecules)
d. condensed particles form a cloud
e. Water droplets and ice particles remain suspended due to air resistance and upward moving air in the
cloud
2. Types of clouds
a. Classified by height and form (most with great horizontal extent, Stratus; or great vertical extent,
Cumulus)
b. Basic Cloud types
---------------------------------------High clouds- base greater than approximately 6 km altitude; formed from ice crystals; thin
Cirrus (Ci) - fibrous, feathery, "mares' tails" formed from ice crystals
Cirrostratus (Cs) - transparent, whitish cloud veil that produces halo around Sun or Moon
Cirrocumulus (Cc)- small "globules"; when arranged in a regular pattern form fish scale-like pattern
called "mackerel sky"
---------------------------------------Middle clouds - base approximately 2 to 6 km
Altocumulus (Ac) - "globules" often in rows or waves
Altostratus (As) - formless layer of grayish clouds covering a large portion of the sky; the Sun is visible
through the clouds as a bright spot
---------------------------------------Low clouds- base below 2 km
Stratus (St) - thick, looks like fog but above ground
Stratocumulus (Sc) - thick but wave-like "globules"
Nimbostratus (Ns) - thick, dark, fairly steady precipitation
---------------------------------------Clouds with Vertical Development
Cumulus (Cu) - dense, dome-shaped tops; isolated or together
Cumulonimbus (Cb) - extreme vertical development; "thunderheads"; isolated or along a line; anvil top
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----------------------------------------c. Cloud Varieties and Variations
c1. Cirrus Uncinus
- cirrus clouds shaped like a comma resting on its side; often precursors of bad weather
c2. Fractus
- adjective used when stratus or cumulus clouds appear to be broken into small pieces
c3. Cumulus Congestus
- cumulus clouds crowded into heaps
c4. Mammatus
- clouds with rounded protuberances on their bottom surface; often associated with stormy weather and
cumulonimbus clouds
c5. Lenticular Clouds
- Lens-shaped clouds
Lenticular Altocumulus - typically form in the turbulent flow that develops on the lee side of mountains
c6. Nacreous (Mother-of-Pearl) Clouds
- usually observed at dusk during winter at high latitudes; form in stratosphere from supercooled water
c7. Noctilucent Clouds
- rare, silvery-blue clouds observed at high latitudes during summer around midnight; water-ice crystals
formed on meteoric dust particles(?) within the mesosphere
c8. Condensation Trails (Contrails)
- typically produced where hot, humid exhaust gases from jets mix with cold, dry air; contrails may
impact weather and climate
c8. Cloud Streets
- strong vertical shear in horizontal wind speed or direction cause clouds to line up into rows; occur in
cirrocumulus, stratocumulus, and cumulus clouds
VIII. Precipitation, Weather Modification and Atmospheric Optics
A. Precipitation
- water in solid or liquid form that falls to the Earth
1. Speed of cloud droplet or ice crystal depends on gravity versus air resistance
METEOROLOGY, PAGE 23
Terminal velocity = speed (constant) at which particle descends
For particle to remain suspended updraft is greater than terminal velocity
B. Raindrop Formation
- two processes responsible for growth:
1. Warm Clouds
- Collision-Coalescence Process
a. water droplets of unequal size have different terminal velocities and collide
b. collisions and "wake capture" cause droplets to coalesce
c. probably most common process in the tropics
2. Cold Clouds
- Bergeron Process
a. Both ice crystals and supercooled water droplets present in clouds with temperature = -10 to -20°C
b. Water droplets evaporate (higher vapor pressure) and water vapor deposited on ice crystals (which
grow)
c. ice crystals fall into lower atmosphere and melt (if not, snow)
- some evaporate when encounter drier air, some may reach surface
d. probably the major precipitation process at mid-latitudes
C. Forms of Precipitation
1. Drizzle
- 0.2-0.5 mm water droplets that drift slowly to surface
- from stratus clouds
- associated with fog
2. Rain
- most common type of precipitation
- 1-8 mm diameter (but rarely exceed 2 mm diameter)
- often start as snowflakes or hail in cumulonimbus or nimbostratus clouds
3. Freezing Rain/Freezing Drizzle
- water condenses in warmer atmosphere above and freezes at the ground (therefore need temperature
METEOROLOGY, PAGE 24
inversion)
- may produce hazardous glaze
4. Sleet (Ice Pellets)
- transparent/translucent ice less than 5 mm in diameter
- raindrops form in atmosphere and fall into lower air with freezing temperature (usually due to upper air
inversion)
5. Snow
- water vapor deposits directly as solid six-sided crystal
- most common just north of center of low pressure; warm moist air overrides colder air located north of
the low
Blizzard - a violent and extremely cold wind laden with dry snow picked up from the ground
6. Snow pellets (graupel)
- 2-5 mm white, spherical ice grains
- grow as supercooled droplets freeze on ice crystals
7. Hail
- large ice pellets (5-190 mm in diameter); with concentric milky rings (rapid freezing) alternating with
clear rings (slow freezing)
- due to cycling pellets in large updrafts and downdrafts in mature cumulonimbus clouds
- commonly occurs with thunderstorms 15 km or more in height
8. Virga
- shaft of falling precipitation that evaporates before reaching the ground
Fall Streaks - falling ice crystals that evaporate before reaching the ground
D. Measuring Precipitation
- Measure in units of depth per unit of time
1. Rain
a. Standard Rain Gauge
- with cone-shaped funnel at top to resolve rainfall to increments of 0.01 inches
b. Tipping Bucket Rain Gauge
- with two small, free-swinging containers that collect 0.01 inches of rainfall each and alternately spill
their contents and recorded
c. Weighing Bucket Rain Gauge
- with continuously recording scale that calibrates weight of rainwater as water depth
METEOROLOGY, PAGE 25
2. Snow
- measure depth each 24-hour period or depth on ground at observation time
- sometimes measure meltwater equivalent (average 1/10 snow depth)
E. Weather Modification
- change in weather induced by human activity
1. Cloud Seeding
- stimulate precipitation processes by injecting nucleating agents (silver iodide, dry ice) into clouds
2. Fog Dispersal
- use jet engines or helicopters to disperse fog or use seeding agents to form rain
F. Atmospheric Optics
1. Crepuscular Rays
- also termed "Sunbeams" (and called "Jacob's Ladder" in England)
- alternating light and dark bands of light that appear to fan out from the sun's position, usually at
twilight, where dust, tiny water droplets or haze scatter sunlight
- they also form when the sun shines through a break in a layer of clouds, when dust, tiny water droplets
or haze beneath the cloud scatters light
2. Halo
- ring of light around sun or moon due to refraction of sunlight by tiny ice crystals suspended in cirriform
clouds
- the most common type of halo is a 22°Halo (there is also a 46° Halo)
3. Sun Dogs (Mock Suns / Parhelia)
- a colored luminous spot produced by refraction of light through ice crystals that appears on either side
of the sun
4. Sun Pillar
- a vertical streak of light extending above (or below) the sun; it is produced by the reflection of sunlight
off ice crystals
5. Rainbow
- arc of concentric colored bands due to refraction and internal reflection of sunlight by raindrops
- in order to observe a rainbow, you must be looking at a distant rain shower with the sun at your back
- a Primary Rainbow exhibits brilliant colors, with each individual raindrop reflecting/refracting a
particular color
- Secondary Rainbows are larger and usually much fainter than primary rainbows; they appear above the
METEOROLOGY, PAGE 26
primary rainbow and are created where the sunlight enters the raindrops at an angle that allows the light
to make two internal reflections in each drop
6. Optical Corona
- colored rings around the moon or sun
- due to diffraction of light (bending light wave) around spherical cloud droplets
7. Glory
- colored rings about the shadow of an observers head that appear on a cloud below the observer
- due to same optics as rainbow + diffraction
8. Heiligenschein
- a faint white ring surrounding the shadow of an observer's head on a dew-covered lawn
- is formed when sunlight is focused and reflected back from the nearly-spherical dew drops
9. Mirages
- an optical effect of the atmosphere caused by refraction in which the image of an object appears
displaced from its true position
a. Inferior Mirage
- a mirage in which the image appears below the true location of the object
- hotter air near the ground causes light to be bent upward (includes the classic "desert mirage")
b. Superior Mirage
- a mirage in which the image appears above the true location of the object
- cooler air near the ground causes light to be bent downward; typically created in polar regions or over
cool ocean surfaces [may cause objects such as ships to appear to be suspended above the horizon
(termed looming)]
c. Fata Morgana
- a complex mirage that is characterized by objects being distorted in such a way as to appear as castlelike figures
- it appears where the air temperature increases with height (slowly at first, then more rapidly, then more
slowly); it is often seen where warm air rests above a cold surface (such as large bodies of water or in
polar regions)
IX. The Wind
A. Forces that produce atmospheric motion
1. Forces Independent of Air Velocity:
a. Gravity
METEOROLOGY, PAGE 27
- directed toward center of Earth
- varies with latitude and altitude
- magnitude proportional to mass of air parcel (Force = mass times acceleration)
b. Pressure
b1. air moves from high pressure to low pressure areas
- pressure difference due to unequal surface heating
b2. Pressure differences cause wind to blow
- Pressure represented by isobars (lines of equal pressure)
- usually broad curves or circular cells
- isobars spacing indicates amount of pressure change over a given distance = pressure gradient (closely
spaced isobars = steep pressure gradient and high winds)
2. "Forces" dependent on velocity of the air
a. Coriolis Effect
a1. free-moving objects (Ex.= wind) are deflected from straight-line path due to Earth's rotation
- deflection is to right in Northern Hemisphere and left in Southern Hemisphere
- force strongest at poles and nonexistent at the equator
a2. Deflection amount depends on wind speed (stronger winds with greater deflection)
a3. If no other forces are significant, wind will eventually blow parallel to the isobars (Geostrophic
Wind); occur primarily at high altitude
a4. Buys- Ballot's Law
- In Northern Hemisphere, if you stand with your back to the wind the low pressure will be to your left
(in theory!)
b. Friction
b1. Effective up to approximately 1 km above surface
b2. Amount depends on roughness of terrain
b3. Acts in direction opposite that of wind
b4. Friction Layer Wind
- below 1 km friction on wind currents change direction of the wind (air crosses isobars at angle; moves
from high to low pressure)
c. Centrifugal force
METEOROLOGY, PAGE 28
c1. Tendency of moving object to continue moving in a straight line
c2. Resultant force from the pressure gradient force, Coriolis force, and centrifugal force depends on
whether the wind direction is clockwise or counterclockwise
Cyclone - in Northern Hemisphere counterclockwise winds moving around a center of low pressure
Anticyclone - in Northern Hemisphere clockwise winds moving around a center of high pressure
Trough - isobars curve to form an elongate region of low pressure with cyclonic flow
Ridge - isobars curve to form an elongate region of high pressure with anticyclonic flow
c3. wind direction for cyclones/anticyclones opposite in Southern Hemisphere
c4. Cyclone wind is weaker than anticyclone due to effects of centrifugal force (in theory!)
B. Convective circulation (thermal circulation)
- Due to unequal heating of atmosphere
1. Two winds develop (one high level, one low level) blowing in opposite directions
2. Convective System
- with rising air over one area (warm, less dense air) and sinking air over other areas (cooler, more dense
air)
C. Wind Measurement
1. Wind Direction
- determined by wind vanes and wind socks
- direction from which wind is blowing
2. Wind Speed
- measured by cup anemometer (cups spin) or hotwire anemometer (wind cools hot wire)
- classified by Beaufort Scale (0-12) or measured in knots, miles per hour, or kilometers per hour
One Knot = 1 nautical mile (1.85 km) per hour; 1 knot = 1.15 miles per hour; 1 knot = 0.51 meters per
second; 1 mile per hour = 0.44 meter per second
3. Wind Pressure
- force per unit area produced by wind on an object
METEOROLOGY, PAGE 29
- wind pressure is directly proportional to the square of wind speed (F = V2)
- wind pressure also increases with altitude (due to higher velocity winds)
- useful for designing buildings, etc.
D. Scales of Weather Systems
1. Planetary-Scale Circulation (Global-Scale Circulation)
- large-scale wind systems of planet
2. Synoptic-Scale Weather
- the typical weather map scale that shows features such as high-and low-pressure areas and fronts over a
distance spanning a continent
3. Mesoscale Weather
- range in size from a few kilometers to about 100 kilometers
- it includes local winds, thunderstorms and tornadoes
4. Microscale Systems
- the smallest scale of atmospheric motions
- examples would be small, turbulent eddies that disperse smoke, sway branches and swirl dust and
papers into the air
X. Planetary-Scale Circulation
A. General circulation
Semipermanent Pressure Systems - global-scale persistent cyclones and anticyclones; change seasonally;
include subtropical anticyclones, equatorial trough and subpolar lows
1. Equatorial Trough/Doldrums/ Intertropical Convergence Zone (ITCZ)
- equatorial
- light, variable winds; Coriolis force weakest
- maximum insolation, air rises, heavy precipitation
2. Hadley Cell
- rising equatorial air moves poleward, deflected eastward due to Coriolis force
- air cools, descends
- air dry with little precipitation; results in large continental deserts at 25-30° latitude: high pressure
- light winds over ocean = "Horse Latitudes" (Subtropical Anticyclones/Subtropical High Pressure Belt)
- air moves poleward or toward the equator
3. Trade winds
- surface air in Hadley cell moves toward equator
METEOROLOGY, PAGE 30
- Coriolis deflection results in Northeast Trades (Northern Hemisphere) and Southeast Trades (Southern
Hemisphere)
- persistent winds
4. Westerlies
- 35° to 60° North and South latitude
- descending air in Hadley cell moves poleward
- deflected by Coriolis force to northeast (Northern Hemisphere) and southeast (Southern Hemisphere)
but direction often variable due to polar air
- disrupted by land in Northern Hemisphere but Southern Hemisphere with strong winds
5. Subpolar Lows/Cyclones
- at approximately 60° north and south latitude
- including Aleutian Low (North Pacific) and Icelandic Low (North Atlantic)
- low pressure belt surrounds Antarctica
6. Polar Easterlies
- well defined in Southern Hemisphere over Antarctica
- in Northern Hemisphere wind directions variable and regionally-developed
Polar Front = where surface westerlies meet and override polar easteries; often with Synoptic-scale
storms
7. Polar Anticyclones
- centered over poles
- year-round in Southern Hemisphere, develop over continents in winter in Northern Hemisphere
- upper air flow in mid- to polar-latitudes is westerly
- Global Circulation Patterns shift toward poles in spring and equator in autumn
B. Northern Hemisphere Low Level Pressure Centers
1. Caused by large continental masses separated by oceans
2. Seasonal Patterns
a. Winter
- high pressure centers (Ex.= Siberian and North American Highs) over cold land areas and low-pressure
(Ex.= Aleutian and Icelandic Lows) over warmer oceans; with cloudy, stormy weather
b. Summer
- "opposite" of winter pattern
- high-pressure shifts north over oceans and become stronger (Azores/Bermuda and Hawaiian Highs)
3. Oceanic high-pressure affects adjacent coastal areas
METEOROLOGY, PAGE 31
C. Singularities
- weather event occurring with regularity
1. January Thaw
- mild New England weather approximately January 20-23
- warm air from shifting Bermuda-Azores anticyclone
2. Arizona "Monsoons"
- North Pacific Subtropical Anticyclone shifts northward in July
3. Indian Summer
- in eastern United States in October and November with stagnating anticyclones (mild days, cool nights
with frost)
D. Upper Air Westerlies
- mid- to upper-troposphere (lower than 500 mb-level) with 2-5 waves (Rossby Waves) of ridges (with
anticyclonic curvature) and troughs (cyclonic curvature)
- determine weather in U.S. and Canada
- stronger in winter (higher pressure gradient)
1. Long-Wave Patterns of Upper Air Westerlies
a. Meridional Component
- oriented north-south
- with north-south air mass exchange and poleward heat transport
- Meridional Flow Pattern with deep troughs and ridges
b. Zonal Component
- oriented east-west
- zonal flow pattern west to east; little mixing of air
- wave patterns shift abruptly (often less than 1 day)
2. Blocking Systems
- develop with extreme meridional flow
- form whirlpools (cutoff lows and highs) that block weather systems moving west to east
- sometimes develop weather extremes (drought, heat, cold)
3. Jet Stream
- zone(s) of very high speed wind, typically forming at the tropopause (between 10 and 15 kilometers
altitude, but may also occur at higher and lower altitudes)
- due to unequal heating of poles versus equator
The Major Types are:
METEOROLOGY, PAGE 32
a. Polar Front Jet Stream (Polar Jet)
- most important feature of upper atmosphere circulation
Distribution - at approximately 35°N during winter (Jet Maximum) with wind speed up to 350 km/hr
(average 130 km/hr), average 160 km wide, 2-3 km thick and several 100 kms long; it may split, creating
northern and southern branches
- summer at approximately 50° north latitude and average velocity approximately 65 km/hr
- jet maximum with both horizontal divergence and convergence aloft (divergence aids cyclone
development; convergence with anticyclones)
b. Subtropical jet stream
- due to excess heating in tropical regions and north and south movement of air in the upper atmosphere
- north wind becomes westerly wind at 25-30° latitude
- velocities usually lower than polar jet stream
E. El Niño/Southern Oscillation (ENSO)
- higher sea-surface temperature and less upwelling (cold, nutrient-rich bottom water moves upward) and
reduced fish catch off Peru/Ecuador coast
- usually lasts a few months
- develops where air pressure gradient of Indonesia/Australia versus tropical eastern Pacific weakens and
trade winds die
La Niña = opposite conditions and strong trade winds; may cause mid-latitude weather extremes
XI. Synoptic-Scale Weather
A. Air Masses
- body of air with uniform temperature and moisture over large area
1. Characteristics
a. may cover large part of continent
b. with distinctive temperature, environmental lapse rate and specific humidity
c. Acquire properties of region over which it occurs or slowly passes
d. Move from one region to another due to barometric pressure changes caused by upper level winds (in
part)
2. Classification of air masses
1. Classified according to:
METEOROLOGY, PAGE 33
a. Latitudinal position (determine thermal properties)
Air Mass
Arctic
Antarctic
Polar
Tropical
Equatorial
Source Region
Ocean and adjacent land (ice)
Ocean and continent (ice)
Oceans and continents (50°-60° latitude)
Oceans and continents (20°-35° latitude)
Oceans
b. Underlying surface (determines moisture content)
Air Mass
Maritime
Continental
Source region
Oceans
Continents
c. Combinations result in approximately 6 major air masses
3. North American Air Masses (learn Diagram)
a. Invasion of Continental Arctic (cA) air produces severe cold wave
- originates over Siberia, Arctic Basin, Greenland and northern Canada
b. Continental Polar (cP) air originates in northern Canada
c. Maritime Polar (mP) air originates over north Pacific, Bering Strait and north Atlantic
d. Maritime Tropical (mT) air originates mainly from the Gulf of Mexico; also from Atlantic east of
Florida and Pacific southwest of Baja, California
e. Continental Tropical (cT) air originates over northern Mexico and southwestern U.S. (ArizonaTexas)
f. there is No direct influence of Maritime Equatorial (mE) air in North America
4. Modification of Air Masses
Modified By:
a. exchanging heat/moisture with surface over which they travel
b. radiative cooling (infrared emission)
c. large-scale vertical motion
METEOROLOGY, PAGE 34
B. Weather Fronts
- boundaries separating air masses of different types
1. General Features
a. air masses do not easily mix
b. Air on either side of front and along front is in motion
c. Boundary always slopes upward over colder air
d. fronts associated with trough, wind shift and convergence
e. Frontogenesis - front grows stronger (density contrasts greater)
Frontolysis - front grows weaker
2. Cold front
a. cold air mass invades region occupied by less cold air mass
b. cold air forms wedge, pushes warm air up
- "Blunt" leading edge lifts warmer air, often results in thunderstorms (with cumulonimbus clouds)
c. usually narrow and fast-moving (about twice as fast as warm front)
- Squall Line = band of intense thunderstorms at or ahead of front
d. Front passage causes drop in temperature and humidity and shift in wind direction
e. Symbol = triangle-shaped points
3. Warm Front
a. warm air mass moves into region of colder air
b. warm air slides up over cold air
- Frontal surface broad, gently sloping
- If warm air stable with cirrus, cirrostratus, altostratus, nimbostratus, stratus succession (steady,
prolonged precipitation)
- if unstable thunderstorms develop in and above stratus layer
- Freezing rain may develop if cold lower air is below freezing
METEOROLOGY, PAGE 35
c. Tend to be wide and slow-moving
d. Passage of front causes gradually rising temperature and wind shifts
e. Symbol
- semicircles extending into cooler air
4. Occluded Front
- type of precipitation depends on stability of the warm air
Symbol = triangles and semicircles on same side of line
a. Cold-Type Occlusion
- cold air wedges under warm and cool air masses; warm air mass is Occluded (lifted off ground)
- most common type of occlusion
b. Warm-Type Occlusion
- cool air rides over colder air and occludes warm air between
5. Stationary Front
- front between air masses that have stalled
a. Stalling may occur due to:
- Presence of mountains
- Weather front parallels jet stream
b. Front oriented east-west with strong southerly and northerly flow towards front
c. Precipitation often with warm moist air Overrunning cold air
- widespread cloudiness, drizzle, light rain, light snow
d. Symbol
- triangular points on one side and semicircles on other
C. Extratropical/Midlatitude Cyclones
- low pressure systems
- principle weather-maker at mid-latitudes
1. Cyclogenesis
- formation of mid-latitude cyclone
- use Norwegian Cyclone Model
a. form along the Polar Front
METEOROLOGY, PAGE 36
b. strong horizontal divergence aloft creates surface air pressure drop (Deepening)
- cyclonic circulation begins (Incipient Cyclone)
- west of Low with cold front; east with warm front
c. if central pressure drops more with wave pattern formed (Wave Cyclone)
- often with comma cloud
d. cold front overtakes warm front (Occlusion Stage)
e. occluded front may begin counterclockwise rotation (bent-back occlusion)
f. another cyclone may form at Point of Occlusion/Triple Point
g. Type of precipitation depends on position in wave cyclone
2. Cyclolysis/Filling
- cyclone weakens
- due to loss of warm, moist air; latent heat loss; increased friction; loss of upper-level support
3. Global patterns (Northern Hemisphere example)
a. Large wave cyclones may travel 1/3-1/2 way around the earth
b. Larger sizes with greater intensity; smaller ones lead to spells of cloudy or rainy weather
c. Storm tracks
- usually move to east in Northern Hemisphere
- may originate in eastern North America, cross north Atlantic to northern Europe OR originate in far
eastern Asia, cross north Pacific to Alaska and northwest North American coast
D. Cold-core Lows
- cyclones that occupy relatively cold columns of air
- migrating midlatitude low-pressure systems with greater circulation at higher altitude
- Ex. = occluded cyclones
E. Warm-Core/Thermal Lows
- stationary cyclone due to intense solar heating (especially over deserts)
- circulation weakens with altitude
F. Anticyclones
- with subsiding air and surface divergence
- often with uniform air masses and clear skies
METEOROLOGY, PAGE 37
1. Types
a. Cold-Core Anticyclones
- domes of Continental Polar Air (cP; Polar Highs) or Continental Arctic/Arctic (cA/A; Arctic Highs)
- form north of Polar Front by radiation cooling over snow-covered areas
- may break off and move south as lobes of bitterly cold air
b. Warm-Core Anticyclones
- Subsiding warm, dry air
- Exs. = Bermuda High, summer highs over interior U.S.
2. Anticyclone Weather
a. Fair Weather
- subsiding, warming air, lower relative humidity
- light, calm winds
- often intense radiation cooling at night (fog, dew, frost)
XII. Local and Regional Circulation Systems
A. Monsoon winds
1. best developed over southeast Asia and Africa
2. Seasonal Changes
a. Summer
- low pressure over Afghanistan and eastward with warm, humid air; moves northward into Asia
- air rises over land, causes precipitation
- occurrence, duration, and intensity varies from year to year
Monsoon Active Phase = cloudy weather and frequent rain
Monsoon Dormant Phase = sunny and hot; approximately 15-20 day cycles between active and dormant
b. Winter
- air from Siberian High flows southward across Asia
- dry air picks up moisture across ocean; with rain on western mountain slopes of Pacific Islands
B. Land and Sea (or Lake) Breezes
1. Daytime
- land heats more rapidly than water
METEOROLOGY, PAGE 38
- air over land expands and rises, causes low pressure
- opposite occurs over water and thermal cell develops, produces a pressure gradient
- wind blows from water to land; produces a Sea (or Lake) Breeze and higher relative humidity
2. Nighttime
- opposite conditions from daytime with wind blowing from land toward water, produces a Land Breeze
C. Lake-Effect Snows
- local snowfall downwind from open lake
1. Occurrence
- often in autumn/early winter with mild lake surface temperature
- arctic air mass evaporates water, clouds develops and snows
- best developed over hilly terrain, large lake-land temperature difference and large fetch (amount of lake
surface over which wind blows)
2. Snowbelts
- downwind areas affected by lake-effect snows
3. Snowbursts
- extreme lake-effect snowfall
D. Urban Heat Island Effect
- urban areas with higher temperature due to more heat sources, better heat conduction by asphalt, etc.
and low evapotranspiration
- best developed where synoptic-scale winds weak
1. Dust Dome
- warm air over city gathers aerosols and rises
- surrounded by cooler denser air
2. Dust Plume
- dust blown downwind
E. Katabatic Winds
1. Cold, dense air accumulates on high plateau and spills over low divides or mountain passes and
flows into adjacent valleys or lowlands
2. sometimes high speed winds in narrow valleys
3. similar winds are termed Bora (in former Yugoslavia), Mistral (in Rhone Valley, France) and the
Columbia Gorge Wind (Pacific Northwest U. S.)
METEOROLOGY, PAGE 39
F. Warm Winds due to Compressional Heating
1. Chinook Winds (east side of Rocky Mountains) and Foehn (in the Alps)
- Air forced to move down leeward side of mountains and loses moisture
- temperature raised at dry adiabatic rate
2. Santa Ana Wind
- easterly warm air from high pressure over desert region of southern California is funneled down
through canyons in mountains; it's warmth is derived from compressional heating
- high velocity increases fire danger (Ex. = Oakland/Berkeley Fire of 1991)
G. Desert Winds
- heating produces atmospheric instability
1. Dust Devil (Whirlwinds)
- swirling mass of dust due to intense solar heating of dry surface areas
2. Haboob
- dust storm due to downdraft of desert thunderstorm
H. Mountain and Valley Wind ("breezes")
1. Daytime
- valley slopes facing sun heat and cause air to move upslope, produces a Valley Breeze
2. Nightime
- valley slopes cool by radiation; cool air moves down slope (greater density) as Mountain Breeze
I. Texas Northers ("Blue Northers")
- a strong, cold wind from between the northeast and northwest that is associated with a cold outbreak of
polar air that brings a sudden drop in temperature
J. Northeaster (Nor'easter)
- a name given to a strong, steady wind from the northeast that is accompanied by rain and inclement
weather
- it often develops when a storm moves northeastward along the eastern seaboard of North America
- it may develop hurricane-like characteristics (and even develop a hurricane-like "eye"!)
XIII. Thunderstorms
- mesoscale weather system due to strong convection currents
- reach to great altitudes within the troposphere
METEOROLOGY, PAGE 40
A. Life Cycle
- form in intense thermal/convection cells
- air rises in small parcels, condenses, forms cumulus cloud at CCL (Convective Condensation Level)
1. Cumulus Stage
- latent heat released, air parcel rises and cools (wet rate), combines with surrounding drier air (=
Entrainment) and evaporates
- more condensation, latent heat released, cloud builds higher, etc. etc., etc. (Cumulus Congestus =
upward-building convective cauliflower-like cloud)
- leads to tremendous updrafts and instability (prevent raindrops and ice crystals from falling)
2. Mature Stage
- entrainment cools air, and a downdraft is created (the formation of downdrafts marks the beginning of a
mature thunderstorm)
- the downdraft and updraft within a mature thunderstorm constitute a Cell
- the mature stage exhibits maximum vertical development of the cloud (often 40,000, or even 60,000,
feet); in some storms the updrafts may intrude above the cloud top into the stable atmosphere (termed
Overshooting)
- raindrops and ice crystals become large enough to fall
- Ice crystals/snow melts and forms rain (often intense)
- downdraft creates high pressure at ground surface and causes cold, dry air to spread out (Gust Front);
often associated with Shelf Clouds (also termed Arcus Cloud; these are low, elongate, wedge-shaped
clouds with a flat base that form when stable air rises up and over cooler air at the surface) and Roll
Clouds (dense, roll-shaped, elongated clouds that appear to slowly spin about a horizontal axis behind
the leading edge of a thunderstorm's gust front)
- gust fronts may trigger formation of other cells (termed Multicell Storms)
- late mature stage with anvil-shaped cumulonimbus cloud tops up to 60,000 feet (heavy rains, lightning,
hail)
3. Dissipating stage
- downdraft begins to overcome the updraft
- with less precipitation, less latent heat, and updraft disappears and cloud dissipates
- single-cell life cycle usually 1/2 to 1 hour
B. Thunderstorm Genesis
- most form due to destabilized (especially uplifted) maritime tropical (mT) air
1. Uplift by:
a. frontal activity
b. orographic effects
c. air convergence at surface
METEOROLOGY, PAGE 41
d. intense solar heating of ground
2. Geographical Distribution
a. most in continental interiors of tropics
b. in U.S. #1 in Florida (convergence of mT air) and #2 in Rocky Mountain Front Range (orographic
uplift)
c. rare in winter in mid- to high- latitudes
C. Thunderstorm Types
1. Ordinary (Air Mass) Thunderstorms
- thunderstorms produced by local convection within a conditionally unstable air mass
- develop almost randomly in mT air
- usually short-lived and rarely produce strong winds or large hail (typically not severe weather)
- common as summer thunderstorms, usually during warmest part of day
- often Multicell Storms (these are thunderstorms that form in a line, each of which may be in a different
stage of development)
2. Severe Thunderstorms
- are capable of producing local damaging winds, lightning, large hail, flash floods and tornadoes
- the most severe storms have the highest tops (up to 60,000 feet above the surface) and have strong
winds aloft that create wind shear that "tilts" the updraft [the updrafts move up and over the downdrafts,
which allows the updraft to remain strong for an extended period of time; precipitation may fall beside
rather than through the updraft]
- severe thunderstorms form in a region of strong vertical wind shear (there is rapidly increasing wind
speed with height and changing wind direction (from southerly at low levels to westerly at high levels);
this may cause the updraft inside the storm to spin cyclonically (forming a Mesocyclone; see discussion
of tornadoes below)
- as some of the falling precipitation evaporates, it cools the air and enhances the downdraft; the cool air
reaching the surface may wedge warm, moist air up into the storm system (feeding it)
a. Supercell Thunderstorms
- enormous, rotating severe thunderstorm whose updrafts and downdrafts are nearly in balance, allowing
them to maintain themselves for several hours
- they can produce updrafts that exceed 90 knots, grapefruit-size hail, and large, long-lasting tornadoes
- some supercells produce heavy precipitation and large hail, which appears to fall near the center (or
eastern) part of the storm [these are called HP Supercells (with high precipitation) and may result in
extreme downbursts, flash flooding and large hail]
b. the Dry Line and Dry Line Thunderstorms
METEOROLOGY, PAGE 42
- the dry line is a boundary that separates warm, dry air from warm, moist air; it usually represents a
zone of instability along which thunderstorms form
- dryline thunderstorms are special types of supercell thunderstorms that most often occur in the western
half of Texas and Oklahoma, especially during the Spring and Early Summer; the dryline is situated
between three air masses (it lies between warm, moist mT air to the east and warm, dry cT air to the
west, with a cold dry air (cP air) situated to the northwest; see the illustration provided); the higher
topography on the High Plains causes the dry air to overide the more humid Gulf air (this sets up
unstable atmospheric conditions and potentially damaging supercell thunderstorms)
- many of the supercell thunderstorms associated with the dry line are low precipitation (LP Supercell)
types (therefore LP supercells are often referred to as Dryline Storms)
3. Mesoscale Convective Complex (MCC)
- a large, organized convective weather system comprised of a number of individual thunderstorms
- often form a nearly circular cluster of many interacting thunderstorms covering many thousands of
square kilometers (the size of a MCC can be 1000 times larger than an individual airmass thunderstorm)
- usually warm-season, nocturnal and typically form within the eastern U.S.
- often form where upper level winds are weak and beneath a high-pressure ridge; if a cool front stalls
beneath the ridge and there is enough surface heating and moisture, thunderstorms may be generated on
the cool side of the front; nocturnal cooling at night helps to enhance the unstable situations
D. Thunderstorm Hazards
1. Lightning
- visible electric discharges produced by thunderstorms
a. Formation
- in general, lower part of thundercloud with negative charge, upper part with positive charge (with huge
electrical potential); probably due to interaction between graupel/ice pellets and ice crystals
- induces positive charge on surface and on objects beneath thunderstorm
- cloud-to-ground lightning begins within the cloud when the localized electric potential gradient
exceeds 3 million volts per meter along a path of perhaps 50 meters long
- Discharge (Lightning) is initiated as a Stepped Leader Stroke (there is a discharge of electrons that rush
toward the cloud base and then toward the ground in a series of steps, each about 50 to 100 meters long);
the stepped leader is typically invisible to the human eye
- as the stepped leader approaches the ground, the potential gradient (voltage per meter) increases, and a
current of positive charge starts upward from the ground to meet it; after they meet, electrons flow to the
ground and a much larger Return/Streamer Stroke surges upward to the cloud
- the leader-and-stroke process is typically repeated several times along the ionized channel at intervals
of about four-hundredths of a second (the subsequent leaders are termed Dart Leaders); typically there
are 3 or 4 leaders per lightning flash
- the lightning continues until the electrical potential is reduced
b. Types of Lightning
METEOROLOGY, PAGE 43
Forked Lightning - cloud-to-ground lightning that exhibits downward-directed crooked branches; this is
the most common type of lightning
Ribbon Lightning - lightning that appears to spread horizontally into a ribbon of parallel luminous
streaks when strong winds are blowing parallel to the observer's line of sight
Ball Lightning - a rare form of lightning that consists of a reddish, luminous ball of electricity or charged
air
Sheet Lightning - occurs when the lightning flash is not seen, but the flash causes the cloud (or clouds)
to appear as a diffuse luminous white sheet
Heat Lightning - distant lightning that illuminates the sky, but is too far away for its thunder to be heard
St. Elmo's Fire - a bright electric discharge that is projected from objects (usually pointed) when they are
in a strong electric field, such as during a thunderstorm
c. Protection against Lightning
- kills about as many people as tornadoes and floods
- Warning - hair often stands on end before lightning hits you; if hair stands on end crouch as low as
possible
- Dangerous Areas - under tall trees, on hilltops, open water, golf courses and golf carts, wire fences,
tractors; also telephones, TV, water pipes, metal appliances, sinks, bathtubs
- Lightning Safety - avoid areas mentioned above (automobiles and metal buildings usually safe; also
lighting rods work!)
Lightning-Direction Finder - locates cloud-to-ground lightning by detecting radio waves produced by
lightning (termed Sferics)
d. Thunder
- lightning discharge heats air up to 54,000°F
- air expands rapidly, producing shock waves which produce tremendous sound
- the sound of thunder travels at about 330 meters per second (1100 feet per second), so it takes the
sound of thunder about 5 seconds to travel one mile
2. Downbursts
- a severe localized downdraft that can be experienced beneath a severe thunderstorm
a. Macrobursts
- a strong downdraft (downburst) greater than 4 kilometers (2.5 miles) wide that can occur beneath
thunderstorms; winds up to 130 mph; duration up to 1/2 hour
b. Microburst
- a strong localized downdraft (downburst) less than 4 kilometers (2.5 miles) wide that occur beneath
METEOROLOGY, PAGE 44
thunderstorms; winds up to 168 mph; duration less than 10 minutes
- microbursts trigger Wind Shear (an abrupt change in wind speed and direction); wind shear is
dangerous to aircraft because there is a sudden loss of lift and a subsequent decrease in the performance
of the aircraft
- at a number of airports, a ground-based wind-shear detection system has been installed (the LLWSAS,
or Low-Level Wind Shear Alert System); unfortunately, LLWSAS can only detect a microburst when it
hits the ground-based instruments
- the installation of Doppler Radar has also provided a better system for detecting microbursts and other
severe weather phenomena
3. Straight-Line Winds
- damaging winds associated with a cluster of severe thunderstorms; wind speed may exceed 90 knots
- if the wind damage extends for at least 400 kilometers (250 miles) along the storm's path, the winds are
termed a Derecho (these are usually associated with thunderstorms that form in the early evening and
last throughout the night)
3. Flash Floods
- due to stationary or slow-moving thunderstorms
- often when prolonged rains of greater than 0.3 inch per hour
4. Hail
- precipitation in form of rounded or jagged ice chunks with internal concentric layering
- associated with thunderstorms with strong updrafts and large moisture content
- ice pellet transported through cumulonimbus cloud; cycles in updrafts and downdrafts and grows larger
- often with alternating transparent layers (glaze; freezes slowly) and opaque white layers (rime; freezes
quickly with air bubbles inside)
- once they are large enough the hail may fall out the bottom of the cloud with the downdraft, a strong
updraft may throw them out the side of the cloud, or they may fall from the base of the anvil
- often forms a Hailstreak (hail accumulates in long, narrow path along ground; average 1 by 6 miles
across)
XIV. Tornadoes and Hurricanes
A. Tornado
- very intense, highly localized cyclonic vortex associated with cumulonimbus clouds
Funnel Cloud - a tornado whose circulation has not reached the ground
1. Characteristics
a. Small diameter [most are between 100 to 600 meters wide (300-2000 feet), although the largest ones
have diameters exceeding 1600 meters (one mile)]
METEOROLOGY, PAGE 45
b. Averages 20 to 40 knots forward speed
c. Very high winds (but most wind speeds are less than 125 knots, and few exceed 220 knots)
- pressure changes up to 200 mb
d. Duration usually short (but can be several hours)
e. Generally move east or northeast (Northern Hemisphere)
f. Most common over larger continents during warmer weather
g. Can be Very Destructive
- until recently, tornado destruction was measured by the Fujita Tornado Intensity Scale [F-Scale;
classified as weak (F0,F1), strong (F2,F3) or violent (F4,F5)]; in U.S. approximately 2/3 are Weak;
approximately 2/3 total fatalities in Violent
- on February 1st, 2007 the Enhanced Fujita Scale (EF Scale) was introduced in the U.S., as
meteorologists believed that the wind speeds in the original F Scale were over-estimated
- the EF scale is as follows: EF0 (65-85 mph winds); EF1 (86-110 mph); EF2 (111-135 mph); EF3 (136165); EF4 (166-200 mph); EF5 (over 200 mph); the first EF5 tornado was the Greensburg, Kansas
Tornado of 4 May, 2007
2. Tornado Association
a. In U.S.:
- 2/3 occur in warmest hours of day
- 3/4 occur from March to July
- the greatest number of tornadoes in the U. S. occur in Tornado Alley (on the Great Plains, from Texas
to Nebraska)
- others tornadoes are associated with hurricanes (about one-fourth of the hurricanes in the U. S. produce
tornadoes)
b. Spring Maximum
- with steep lapse rate (warm near ground, cold above); this creates a very unstable situation in the
atmosphere (this is especially characteristic of Tornado Alley tornadoes)
- approximately 80% of North American tornadoes associated with Mid-Latitude Cyclones (especially
fronts between mP or cP air and mT air)
c. Stages of Tornado Formation
Dust-Whirl Stage - dust swirling upward from the surface marks the tornado's circulation on the ground;
a short funnel extends downward from the thunderstorm's base; damage is typically light
Organizing Stage - tornado increases in intensity, and the funnel extends downward
METEOROLOGY, PAGE 46
Mature Stage - damage is typically most severe, with the funnel reaching its greatest width and is most
vertical
Shrinking Stage - the funnel's width decreases, the funnel's tilt increases, and the damage swath at the
surface narrows (although the tornado may still be dangerous)
Decay Stage - the tornado typically becomes rope-like, becomes greatly contorted, and dissipates
- note that tornadoes do not have to reach all of these stages (for example, the mature stage may be
skipped entirely)
d. Tornadoes and Thunderstorms
- tornadoes are typically associated with Supercell Thunderstorms (very energetic thunderstorms with
updraft speeds exceeding 150 miles per hour)
- for Tornadoes on the Great Plains (see power point slide), a typical situation is when (at the surface) an
open-wave middle latitude cyclone with cold, dry air is moving in behind a cold front, and warm, humid
air is pushing northward from the Gulf of Mexico behind a warm front; above the warm surface air a
wedge of warm, moist area is streaming northward; directly above the moist layer is a wedge of colder,
drier air moving in from the southwest; higher up (500 mb level) a trough of low pressure exists to the
west of the surface low and higher (at the 300 mb level) the polar front jet stream swings over the region
(this provides an area of divergence that initiates surface convergence and rising air; the stage is set for
the development of severe storms
- in order to form a tornado, the updraft must rotate; severe thunderstorms form a region of strong
vertical wind shear [with rapidly increasing wind speed with height (vertical wind speed shear) and with
changing wind direction with height (from southerly at lower levels to westerly at high levels; termed
Vertical Wind Direction Shear)]; this rising, spinning column of air 5 to 10 kilometers across is termed a
Mesocyclone
- the mesocyclone narrows and spirals downward as a funnel (this narrowing increases the wind speed,
creating a Tornado Cyclone)
- air rushes into the low-pressure vortex from all directions; it expands, cools, and if sufficiently moist it
condenses into a visible cloud (the Funnel Cloud)
- the air beneath the funnel is drawn into the core; it cools rapidly and condenses, and the funnel cloud
descends toward the surface
- some tornadoes develop along gust fronts (Gustnadoes; these are relatively weak and short-lived)
e. Observing Tornadoes
- the first sign that a thunderstorm may give birth to a tornado are rotating clouds at the base of the storm
(if the area of rotation in the cloud lowers, it becomes a Wall Cloud)
- usually within the wall cloud, a smaller funnel extends toward the surface (sometimes it can't be seen
because of the intensity of falling rain or dust); sometimes the air is so dry that the swirling wind
remains invisible until it reaches the ground and picks up dust - these "Invisible Tornadoes" are
sometimes mistaken for dustdevils (which could be a fatal mistake)
- tornadoes may have a distinctive roar ("like a thousand freight trains") that can be heard for several
METEOROLOGY, PAGE 47
kilometers (but other tornadoes are silent!)
- tornadoes often with mammatus cloud formation (they look like pouches hanging from the underside of
the cloud)
- on conventional radar, the circulation of tornadoes is indicated by a hook-shaped pattern; on Doppler
Radar the speed at which precipitation is moving horizontally toward or away from the radar can be
calculated
3. Tornado Watches and Warnings
a. Tornado Watch
- a forecast issued to alert the public that tornadoes may develop within a specified area
b. Tornado Warning
- a warning is issued when a tornado has actually been observed, either visually or on a radar screen
- it is also issued when the formation of tornadoes is imminent
3. Damage due to:
a. High winds
b. Strong Updraft
c. Subsidiary Vortices
- Suction Vortices are small, rapidly rotating whirls perhaps 10 meters in diameter that are found within
large tornadoes
4. Tornado Safety
a. Seek secure shelter (avoid wide-roof buildings)
b. Go to small room in interior
c. Avoid windows
d. If outdoors lie in ditch
e. Avoid mobile homes
- account for about 45% of fatalities
5. Waterspouts
- tornado-like disturbance that occurs over ocean or large lake
Tornadic Waterspout - a tornado that originated from land and then traveled over water
METEOROLOGY, PAGE 48
"Fair Weather" Waterspouts - form over water (especially over the warm, shallow coastal waters off the
Florida Keys) during the summer; they are smaller, weaker and shorter-lived than a tornado
B. Tropical Cyclones
- are also termed Hurricanes (Atlantic and eastern Pacific), Typhoons (western Pacific Ocean), or
Cyclones (Indian Ocean and Australia)
1. Origin
a. Originate only at latitudes of approximately 4° to 20°
b. Originate only over water
- Sea-surface temperature at least 80°F through 200 meters (600 foot) water depth
- die quickly if move over cold water or land
c. Usually originate in easterly waves (disturbances in trade winds)
- Center of low pressure forms in the wave trough = cyclonic vortex
- Low pressure deepens rapidly
2. Hurricane Developmental Sequence
a. Tropical Disturbance - no strong winds or closed isobars
b. Tropical Depression - with one or more closed isobars; winds 37-63 km/hr (23-39 mph)
c. Tropical Storm - distinct coriolis rotation around central low pressure with several closed isobars
around it; winds 63 to 119 km/hr (39-74 mph)
d. Hurricane - pronounced rotation around eye (air sinks in center of eye); spiralling air around it with
winds greater than 119 km/hr; often moves west, then north, then northeast (due to coriolis effect and jet
stream influence); radar shows spiral bands of heavy rainfall
3. Hurricane Destruction - most in September
a. High winds (up to 250 km/hr or more)
b. Flooding due to heavy rains (average 13-25 cm)
c. Storm surge
- low pressure and winds with "wave set up"; is especially bad in high tides
d. Measured by Saffir-Simpson Hurricane Intensity Scale [Categories 1 through 5 with those above a 3
classified as Major (includes about 20% of hurricanes)]
- the Saffir-Simpson Scale measures central pressure, wind speed, storm surge potential and potential for
METEOROLOGY, PAGE 49
property damage
e. Naming Hurricanes
- since 1978 (eastern Pacific) and 1979 (North Atlantic) hurricane names have been alternately assigned
male and female names
- a storm only gets a name when it reaches tropical storm strength
- if a storm has caused great damage, its name is retired for at least ten years
4. Hurricane Safety
a. Obey warnings
Hurricane Watch = indicates that a hurricane poses a threat to an area (often within several days) and
residents of the watch area should be prepared
Hurricane warning = hurricane conditions are expected in 24 hours or less
- Evacuate if requested
b. Enter hurricane season prepared with boards, radio with batteries, flashlight, nonperishable foods
and water
c. avoid low-lying areas, mobile homes
d. listen for tornado warnings
XV. Weather Analysis and Forecasting
A. The National Weather Service
- Congress established U.S. Weather Bureau in 1890 (now the National Weather Service; NWS)
- an agency of NOAA (National Oceanic and Atmospheric Administration)
1. NWS Surface Weather Observations
- approximately 1000 weather stations (and others)
a. Synoptic Weather Network
- provide data for weather map preparation and forecasting
- reports every 3 hours sky condition (cloud type, sky cover), visibility, air temperature, dew-point,
barometric pressure, wind speed and direction, weather occurrences
- begin at midnight GMT (Greenwich Mean Time) = Universal Coordinated Time (UCT)
- data transmitted electronically to other NWS stations and numerous other facilities
b. Basic Weather Network
- mostly for aviation and supplemental data
METEOROLOGY, PAGE 50
- hourly observations
- report above observations + cloud heights + altimeter settings
2. NWS Upper-Air Observations
- 126 radiosonde stations
- vertical Air temperature, pressure and relative humidity monitored twice daily (0000 GMT and 1200
GMT)
3. National Substation Program
- includes approximately 11,590 weather stations
a. Federal Aviation Administration (FAA)
- collect weather observations like those of NWS
b. Automated Surface Observing System (ASOS)
- temperature, pressure, wind, precipitation each hour in unmanned stations and transmitted to NWS
- there are about 1700 ASOS stations
c. Airline observers
- submit hourly reports
4. Soil-temperature gauging stations
- gauges soil temperature and evapotranspiration (loss of soil moisture)
- issues agricultural weather advisories to farmers
5. Weather Radar
- NWS with approximately 120 radar stations
a. Conventional radar
- waves scattered by precipitation
- return signal (Radar Echo) displayed on cathode ray tube (scans 360°)
- time interval between emission and reception gives distance
- larger drops and hailstones with greatest reflectivity
- PPI (Plan-Position Indicator) Radar - sweeps horizontally up to 250 mile radius
- RHI (Range-Height Indicator) Radar - scans up to determine thunderstorm types
- conventional radar units use PPI and RHI modes
b. Doppler Radar
- can determine motion of precipitation (and air circulation pattern!) within a weather system based on
frequency shift between outgoing and returning radar
- multiple units yield 3-D color image of clouds, thunderstorms, mesocyclones, tornadoes, strong wind
shears and gust fronts
- NEXRAD Doppler System was placed in service in 1993-1994
METEOROLOGY, PAGE 51
6. NOAA Weather Radio
- first used to warn of natural disasters and emergencies
- now provides current and anticipated weather conditions on a 24 hour basis
B. Texas Meteorology
1. Early observations
- first begun on military outposts on Texas frontier between 1840 and 1860
- some individual weather observers kept records also
2. The Cooperative Weather Observer
- rainfall measured every day at more than 600 locations in Texas
- cooperative observers (private citizens) do approximately 90% of observations
- daily data on precipitation and temperature recorded on forms supplied by NWS and submitted to the
National Climatic Center (NCC)
- this data is available on the web (for example, Google "Texas Climate Data")
C. World Meteorological Organization (WMO)
- headquartered in Geneva, Switzerland; coordinates 178 nations and territories for global weathermonitoring (= World Weather Watch; WWW)
- monitors data from about 10,000 land stations and more than 7,000 ships, satellites, etc.; transmits data
to the three World Meteorological Centers in Washington, D.C., Moscow, Russia, and Melbourne,
Australia
D. Weather Satellites
1. Types of Satellites
a. Polar-Orbiting Satellites
- low orbit; moves pole-to-pole; moves over same spot twice daily
b. Geosynchronous/Geostationary Satellites
- scans same region all the time
2. Satellite Imagery
a. Visible
- black and white photos/movies
b. Infrared (IR)
- measures heat (land, sea surfaces, cloud tops)
- cloud top temperature measures thunderstorm intensity
METEOROLOGY, PAGE 52
E. Weather Maps
1. Station Model
- uses symbols to indicate weather conditions at a locality
- be able to analyze the station model provided
2. Surface Weather Maps
a. Synoptic Weather Charts
- weather maps that provide a summary of weather data
- Surface Synoptic Weather Maps are drawn every 3 hours for North America and 6 hours for Northern
Hemisphere
b. Prognosis Chart (Prog Chart)
- predicts what weather will be like in the (near) future
3. Upper-Air Weather Maps
- radiosonde data plotted on constant-pressure surfaces (500-mb, etc.)
- draw "contour maps" twice daily
- useful for finding upper-level steering winds (jet stream at 250- and 300-mb), troughs and ridges
- solid lines = feet above sea level
- dashed lines = temperature in °C
- Warm Air Advection (winds blow from warm to cold regions) - causes 500-mb surface to rise;
strengthens ridges and weakens troughs
- Cold Air Advection = vice versa
F. Weather Prediction
1. Numerical Weather Forecasting
- computer programmed with numerical model of atmosphere
- predicts weather for next 12, 24, 36 and 48 hours
- AWIPS (Advanced Weather Interactive Processing System) has data communications, storage,
processing and display capabilities to help individual forecasters to assimilate and analyze data
2. Special Forecast Centers
a. National Hurricane Center (NHC)
- Coral Gables, Florida
- also Central Pacific Hurricane Center, Honolulu, Hawaii
- goal to provide 12 hours daylight warning to coastal residents
- tracks hurricanes, predicts storm surge
METEOROLOGY, PAGE 53
b. Storm Prediction Center (SPC) and National Severe Storms Laboratory (NSSL)
- in Norman, Oklahoma
- monitors severe thunderstorms, tornadoes and blizzards and conducts severe weather research
3. Forecast Skill
- better now due to:
a. better understanding of atmospheric processes
b. better computers
c. better observation (satellites, doppler radar, etc.)
d. more worldwide observation networks
4. Long-Range Forecasting
- Long Range Prediction Branch of Climate Analysis Center (Camp Springs, Maryland) provides 30- and
90-day "outlooks" that identify expected positive and negative anomalies (departure from average)
- also looks at Teleconnections (long-range changes in widely separated regions; Ex. = ENSO)
5. Single-Station Forecasting
- short-term weather forecasts based on observations at 1 station
XVI. Air Pollution
Pollution - atypical contribution of substances to the environment
A. Types of Pollutants
1. Oxides of Carbon
a. Carbon Dioxide (CO2)
- released through respiration, burning fossil fuels, brush fires, volcanoes
- "greenhouse" gas
b. Carbon Monoxide (CO)
- released by burning fossil fuels and slash and burn agriculture
- causes drowsiness, slows reflexes, may kill you (especially in tunnels and parking garages)
2. Hydrocarbons
- volatile organic compounds
METEOROLOGY, PAGE 54
- often smog-forming and/or carcinogenic (cancer-causing)
a. Methane (CH4)
- most from anaerobic decay and natural gas
- "greenhouse" gas
b. Vegetation Emissions
- may help form smog
c. Motor Vehicle Emissions
- with many types of hydrocarbons
d. Tobacco Smoke
3. Oxides of Nitrogen
- produced by soil bacteria, burning fossil fuels, motor emissions
- nitrogen dioxide (NO2) causes heart, lung, liver and kidney damage; bronchitis and pneumonia; may
form nitric acid and smog
4. Sulfur Compounds
- produced by volcanoes, anaerobic decay, burning fossil fuels (especially coal and oil), smelting
sulfides, auto emissions
- may form sulfuric acid; causes respiratory problems
5. Photochemical Smog
- autos make oxides of nitrogens and hydrocarbons, combine with sunlight
- produce ozone, formaldehyde, ketones and PAN (peroxyacetyl nitrates)
- causes respiratory damage
- ozone also damages crops and trees, degrades rubber and fabrics
6. Aerosols
- suspended particulates
a. Dust
- most by soil erosion
b. Soot
- carbon particles
- incomplete combusion of fossil fuels and refuse
c. Fungal Spores and Pollen
- often cause allergic reactions
d. Many Others produced by mining, milling and manufacturing
METEOROLOGY, PAGE 55
- examples = asbestos fibers, pesticides, fertilizer dust
B. Air Pollution Episodes
- atmospheric conditions inhibit dilution of air pollutants, causing health-hazards
1. Wind Speed
- if wind speed doubles, it cuts air pollutants by one-half
- air pollution episodes greater at center of anticyclone, or where greater surface roughness (buildings,
etc.)
- smoke plume may be trapped in Wake [turbulence on leeward side of obstruction (building, etc.)]
2. Atmospheric Stability
- enhances air pollution episodes
a. Mixing Depth
- vertical distance between Earth's surface and altitude at which convection currents reach
- if mixing depths shallow (less than 1km), more air pollutants per air volume
b. Temperature Inversions
- air temperature increases with altitude (stable air with pollution episodes)
b1. Subsidence Temperature Inversion
- due to stalled anticyclone and subsiding air (forms "lid" = Fumigation)
b2. Radiational Temperature Inversion
- due to nocturnal radiational cooling of ground surface
3. Air Pollution Potential
- increases with above conditions, also with high topographic relief
- in western U.S. air quality usually worst in winter, in eastern U.S. in Fall
- during pollution episodes advisories issued based on PSI (Pollution Standards Index = for 6 pollutants
including carbon monoxide, sulfur dioxide, nitrogen dioxide, particulate matter, ozone and lead)
4. Natural Cleansing Processes
a. Dry Deposition
- includes Impaction (particulates stick to buildings, etc.) and gravitational settling
b. Precipitation Scavenging
- by hygroscopic nuclei
- most important (up to 90%)
C. Air Pollution Impact on Weather
METEOROLOGY, PAGE 56
1. Urban Weather
- with greater fog, clouds and precipitation (due to more hygroscopic nuclei and heat island effect)
2. Acid Deposition
- oxides of sulfur and nitrogen "washed" from air or dry (acid) deposition as sulfuric and nitric acids
- may produce Acid Rain (water pH less than 5.6); destroyes wildlife and plants in lakes, streams and
forests)
3. Ozone Shield
- absorbs UV light (especially important is UV-B = biologically effective radiation; causes skin cancer)
a. Chlorofluorocarbons (CFC's)
- used in refrigeration and foams (insulation, fast food packages); formerly in aerosol sprays
- CFC's break down to chlorine in the upper atmosphere, which destroys ozone
b. Antarctic Ozone Hole
- ozone depleted during Southern Hemisphere "Spring" (September-October)
- by CFC's(?); also due to Circumpolar Vortex keeping warmer air out and greater catalytic reactions
XVII. World Climates
A. Climate
- average weather over time plus seasonal distribution of weather plus extremes in weather behavior
- described by normals, means and extremes of temperature, precipitation, wind, etc.
- compiled by National Climate Data Center (NCDC) in Asheville, North Carolina
1. Climatic Norm/Normal
- average plus extremes for a locality, usually over 30 years
2. Climatic Anomalies
- departures from long-term climatic averages
- upper air westerlies determine weather extremes (very cold temperature; drought, etc.)
Agroclimatic Compensation - poor growing weather in one area offset by better growth in another
B. Air Mass Climatology
- describes climate by frequency of occurrence of various air masses
- air masses originate in anticyclones and modify as they move away from them
1. Streamlines
- mean paths of air mass winds moving horizontally
2. Confluence Zone
METEOROLOGY, PAGE 57
- average frontal positions
- separate different types of climate
3. Climate Controls
a. Latitude
b. Elevation
c. Topography
d. Proximity to large bodies of water
- a-d are fixed and predictable
e. Atmospheric Circulation - combined influence of all weather systems; relatively unpredictable
C. Global Patterns of Climate
1. Temperature
- mean annual isotherms approximately parallel latitude
a. Heat Equator
- latitude with highest mean annual temperature
- approximately 10°N latitude due to less albedo, more land, more warm ocean water in Northern
Hemisphere
b. Global Temperature
- January and July = coldest and warmest months
- larger shift of isotherms over land than water
2. Precipitation
- global mean annual precipitation = rain + melted snow
- tends to be zonal (east-west) pattern
- rain more reliable in maritime climates, less in continental
Equator = trade winds converge and with abundant rain
Tropical Monsoon Circulation = shifts of ITCZ and subtropical highs
Subtropics (20-35°) = with subtropical anticyclones and deserts
- at 35-40° with influence by prevailing westerlies and subtropical anticyclones; west side of continents
with moist winters and dry summers; east side with little variation in rain throughout year
METEOROLOGY, PAGE 58
- Poleward of 40° with less precipitation; more rain in summer
D. Köppen Climate Classification
Genetic Climate Classification = grouped on meteorological basis of climate
Empirical Climate Classification = grouped on environmental effects of climate
- Wladimir Köppen combined these classifications:
1. Tropical Humid (A)
- high temperature, little temperature variation
- greater than 100 cm rain per year; divided into tropical wet (Ar or Af; with rainforests), monsoonal
(Am), and tropical wet-and-dry (Aw; tropical savannas) by length of rainy season
2. Dry (B)
- evaporation greater than precipitation; due to subtropical anticyclone or rain shadows
- largest climate group
- rain limited, variable and unreliable
a. Steppe/Semiarid (BS)
- usually transitional zone between arid and humid climates
- includes warm, dry tropical (BSh; due to subtropical anticyclone; short rainy season) or cold, dry
midlatitude (BSk; due to rain shadow; meager rainfall, mostly summer) climates
b. Arid/Desert (BW)
- constantly dry
- includes BWh (tropical-subtropical; due to subtropical anticyclone) and BWk (temperate-boreal; rain
shadow) deserts
- anomalous deserts (Ex. = Coastal Deserts) = BWn or Bn
3. Humid Middle-Latitude Climates with Mild Winters (often termed “Subtropical”) (C)
- poleward of Tropics of Cancer and Capricorn
- the term “subtropical” may be misleading, as some regions do not have near-tropical characteristics
a. Marine West Coast (Cfb)
-on west side of continents from about 40-65°
-dominated by the onshore flow of oceanic air
-is especially well-developed on the west coast of Europe
b. Subtropical Dry Summer/Mediterranean (Cs)
- on west side of continents at 30-45°
- 40-80 cm rain (mostly winter); temperature variable
METEOROLOGY, PAGE 59
- during the summer is dominated by stable conditions associated with oceanic subtropical highs
c. Subtropical Humid (Cfa)
- on east side of continents at 25-40°
- abundant rain (76-165 cm) throughout year; hot summers and mild winters
-a good example is in the southeastern United States
4. Humid Middle-Latitude Climates with Severe Winters (sometimes termed “Temperate”) (D)
- midlatitude climates; usually occur in the interiors of their continents or on their east coasts north of
40° North Latitude
- with average temperatures above 50° F in their warmest month and below freezing in their coldest
a. Humid Continental / Humid Temperate (Df)
- Northern Hemisphere at 40-50° N
- inland on leeward side of continents
- variable weather (polar front, etc.)
- precipitation throughout year
- divided into Dfa (cool winters, warm-hot summers) and Dfb (cold winters, mild summers)
5. Subarctic / Boreal / Taiga (Dfc)
- Northern Hemisphere at 50/55-65°N
- extreme continentality; less than 50 cm/year rain
- short, cool summers and long, cold winters (where cP and A air masses originate)
- is characterized by the presence of conifer forests (termed Boreal Forests, or Taiga)
6. Polar (E)
- poleward of Arctic and Antarctic Circles
- extreme cold and snow (less than 25 cm/year melted)
- includes tundra (ET) and ice cap (EF) climates
7. Highland (H)
- mountainous areas
- every 300 m elevation change equals a 500 km poleward shift of climate
XVIII. Climate Record and Climate Variability
A. Earth History and Climate Change
- Earth has had many climate changes through time
1. Pleistocene
- 1.65 million years before present to approximately 10,000 years before present
- "ice ages" develop with widespread continental and alpine glaciers
- "Glacials" separated by warmer "Interglacials"
METEOROLOGY, PAGE 60
a. Evidence
a1. Sedimentary Structures
- transported boulders, ridges plowed up by glaciers, glacial striations
a2. Paleontology
- use distribution of animals and plants (especially pollen) to determine "shifts" in climate
a3. Oxygen Isotopes
ratios indicate approximately 18 glacial expansions (seas with greater O18)
16O/18O
a4. Deep Sea and Lake Cores
- give complete glacial/interglacial history
2. Climate Optimum
- approximately 7,000-5,000 years before present with global temperature approximately 2°C warmer
than now
3. Little Ice Age
- approximately 1400-1850 A.D. with global temperature approximately 1-2°C less than now
B. Causes of Glaciation
1. Plate Tectonics
- Earth's crust is divided into lithospheric plates; ride on the moving asthenosphere (ductile material
separating lithosphere from the lower mantle)
- during ice ages continents near or over poles
2. Elevation of Mountains
- alpine glaciers build
3. Volcanic Activity
- probably most influence by sulfur oxide gases rather than ash
- convert to sulfuric acid - remains suspended, absorbs radiation, cooler temperature (by approximately
1°C?)
- greater effects locally (higher temperature?)
4. Changes in Solar Irradiance (total radiative output)
- may vary with 11- and 22- year sunspot cycle (dark blotches on face of sun)
- sunspot mimima result in cooler temperature(?)
- double-sunspot (22 year) cycles correspond to droughts on High Plains(?)
- area of sunspot Umbra (dark central area) to Penumbra (outer lighter area) parallels temperature
variation (?)
METEOROLOGY, PAGE 61
5. Increase in Albedo
- sun's energy reflected by glaciers and clouds
6. Changes in Ocean Currents
- change temperature and rainfall/snowfall patterns
7. Milankovitch Cycles
- proposed by Milutin Milankovitch (1879-1958) in 1920's -30's
- Earth's Climate Changes due to:
Angle of the Ecliptic (Axial Tilt) - primary factor producing seasons; now 23.5°; varies from
approximately 22.1° to 24.5° every 41,000 years
Precession of the Equinoxes = Earth "wobbles" on it's axis (22,000 year cycle)
Eccentricity of the Earth's Orbit = 92,500 year cycles; approximately corresponds to 18 Pleistocene
cooling cycles
C. Climate Change due to Human Activities
1. Alter radiational properties (ex. = albedo) of Earth's surface
2. Vent Waste Heat into the Atmosphere
- # 1 and # 2 local effects only(?)
3. Change Gaseous and Aerosol Components of Atmosphere
a. Greenhouse Gases
- Carbon Dioxide increase primarily due to fossil fuel combustion and deforestation
- also increase in methane, nitrous oxides and CFC's
- greenhouse effect may cause more rain and snow near poles and equator, less in midlatitudes
b. Turbidity
- increase in dust
- may cause colder temperature (more albedo) or warmer temperature (backradiation to surface and
larger particles absorb heat) (?)
D. Earth's Surface and Climate Variability
1. Oceans
- principle absorber of solar radiation
- greatly affects atmospheric characteristics
METEOROLOGY, PAGE 62
2. Snow Cover
- leads to colder temperature due to refrigeration and greater albedo
E. Factor Interaction
- many factors affect climate
- feedback loops may amplify (positive feedback) or weaken (negative feedback) climate fluctuations
- climate cause and effect is hard to establish