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Weather
Grade
Earth Science Content Standard
Kindergarte
n
3. Earth is composed of land, air, and water. As a basis for understanding this concept:
b. Students know changes in weather occur from day to day and across seasons, affecting Earth and
its inhabitants.
1st
3. Weather can be observed, measured, and described. As a basis for understanding this concept:
a. Students know how to use simple tools (e.g., thermometer, wind vane) to measure weather
conditions and record changes from day to day and across the seasons.
b. Students know that the weather changes from day to day but that trends in temperature or of rain
(or
snow) tend to be predictable during a season.
c. Students know the sun warms the land, air, and water.
5th
4. Energy from the Sun heats Earth unevenly, causing air movements that result in changing weather
patterns. As a basis for understanding this concept:
a. Students know uneven heating of Earth causes air movements (convection currents).
b. Students know the influence that the ocean has on the weather and the role that the water cycle
plays in weather patterns.
c. Students know the causes and effects of different types of severe weather.
d. Students know how to use weather maps and data to predict local weather and know that weather
forecasts depend on many variables.
e. Students know that the Earth's atmosphere exerts a pressure that decreases with distance above
Earth's surface and that at any point it exerts this pressure equally in all directions.
6th
4. Many phenomena on Earth's surface are affected by the transfer of energy through radiation and
convection currents. As a basis for understanding this concept:
a. Students know the sun is the major source of energy for phenomena on Earth's surface; it powers
winds, ocean currents, and the water cycle.
b. Students know solar energy reaches Earth through radiation, mostly in the form of visible light.
c. Students know heat from Earth's interior reaches the surface primarily through convection.
d. Students know convection currents distribute heat in the atmosphere and oceans.
e. Students know differences in pressure, heat, air movement, and humidity result in changes of
weather.
Weather and climate
• Weather
– Weather is over a
short period of time
– Constantly changing
• Climate
– Climate is over a long
period of time
– Generalized composite
of weather
Weather and climate
• Elements of weather and climate
– Temperature
– Humidity
– Cloudiness
– Precipitation
– Air Pressure
– Winds speed and direction
The Atmosphere
Composition
Structure
Heating
Composition of the atmosphere
• Gases
–
–
–
–
Nitrogen (N) – 78%
Oxygen (O2) – 21%
Argon and other gases
Carbon dioxide (CO2) –
0.036%
Composition of the atmosphere
• Variable components of air
– Water vapor
– Aerosols
– Ozone
Structure of the atmosphere
• Pressure changes
– Pressure is the weight
of the air above
– Average sea level
pressure
– Pressure decreases
with altitude
Structure of the atmosphere
• Atmospheric layers based on temperature
– Troposphere
– Stratosphere
– Mesosphere
– Thermosphere
Earth-Sun relations
• Earth motions
– Rotates on its axis
– Revolves around the Sun
Earth-Sun relations
• Seasons
– Caused by Earth's changing orientation to the
Sun
• Axis is inclined 23½º
• Axis is always pointed in the same direction
Earth-Sun Relations
Atmospheric heating
• Heat is always transferred from warmer to
cooler objects
Atmospheric heating
• Incoming solar radiation
– Reflection – albedo
(percent reflected)
– Scattering
– Absorption
– Most visible radiation
reaches the surface
Atmospheric heating
• Outgoing Radiation
– Earth re-radiates radiation (terrestrial
radiation) at the longer wavelengths
– Terrestrial radiation is absorbed by
• Carbon dioxide and
• Water vapor in the atmosphere
• Lower atmosphere is heated from Earth's surface
– The greenhouse effect
Atmospheric Energy Balance
Temperature
Measurement
Controls
World Distribution
Temperature measurement
• Daily maximum and minimum
• Other measurements
– Daily mean temperature
– Daily range
– Monthly mean
– Annual mean
– Annual temperature range
Mean monthly temperatures
for Vancouver, British
Columbia and Winnipeg,
Manitoba
Mean monthly temperatures
for Eureka, California and
New York City
Temperature measurement
• Human perception of temperature
– Important factors are
• Air temperature
• Relative humidity
• Wind speed
• Sunshine
Controls of temperature
• Temperature variations
• Receipt of solar radiation is the most
important control
• Other important controls
– Differential heating of land and water
• Land heats more rapidly than water
• Land gets hotter than water
• Land cools faster than water
• Land gets cooler than water
Controls of temperature
• Other important controls
– Altitude
– Geographic position
– Cloud cover
– Albedo
Clouds reduce
the daily
temperature
range
World distribution of
temperature
• Global temperature patterns
– Temperature decreases poleward from the
tropics
– Isotherms exhibit a latitudinal shift with the
seasons
– Warmest and coldest temperatures occur over
land
World distribution of
temperature
• Global temperature patterns
– Isotherms show ocean currents
– Annual temperature range
• Small near equator
• Increases with an increase in latitude
• Greatest over continental locations
World mean sea-level
temperatures in January
World mean sea-level
temperatures in July
Formation of Weather
Atmospheric Circulation
Wind Patterns
Air Masses
Fronts
Cyclones & Anticyclones
Cloud Formation
Solar Heating and Atmospheric
Circulation
• Air at high elevations is: • Air at sea level is:
– Cooler
– Expands
– Water vapor tends to
condense
– Warmer
– More compressed
– Can hold more water
vapor
Air Circulation & Convection
Currents
Coriolis Force
Atmospheric Circulation &
Convection Cells
Wind
• Horizontal movement of air
– Out of areas of high pressure
– Into areas of low pressure
• Controls of wind
– Pressure gradient force
• Isobars – lines of equal air pressure
• Pressure gradient – pressure change over
distance
Wind Pattern Development
General atmospheric circulation
• Idealized global
circulation
– Equatorial low
pressure zone
• Rising air
• Abundant precipitation
General atmospheric circulation
• Idealized global
circulation
– Subtropical high
pressure zone
• Subsiding, stable, dry air
• Location of great deserts
• Air traveling equatorward
from the subtropical high
produces the trade winds
• Air traveling poleward
from the subtropical high
produces the westerly
winds
General atmospheric circulation
• Idealized global
circulation
– Subpolar low
pressure zone
• Warm and cool winds
interact
• Polar front – an area of
storms
General atmospheric circulation
• Idealized global
circulation
– Polar high pressure
zone
• Cold, subsiding air
• Air spreads
equatorward and
produces polar easterly
winds
• Polar easterlies collide
with the westerlies
along the polar front
Local winds
• Produced from temperature differences
• Small scale winds
• Types
– Land and sea breezes
– Mountain and valley breezes
– Chinook and Santa Ana winds
Illustration of a sea breeze
and a land breeze
The Santa Ana Winds
Air masses are classified on the
basis of their source region
Fronts
• Types of fronts
– Warm front
• Warm air replaces cooler air
• Shown on a map by a line with semicircles
• Small slope (1:200)
• Clouds become lower as the front nears
• Slow rate of advance
• Light-to-moderate precipitation
Fronts
• Types of fronts
– Cold front
• Cold air replaces warm air
• Shown on a map by a line with triangles
• Twice as steep (1:100) as warm fronts
• Advances faster than a warm front
• Associated weather is more violent than a warm
front
Cold Fronts and Warm Fronts
Rotating Air Bodies
• Bends in the polar jet create troughs and
ridges
• Forms cyclones and anticyclones
Rotating Air Bodies
• Low Pressure Zone
Formation
– Warm air rises
– Creates a low pressure
zone
– At the Earth’s surface,
air “feeds” the low
pressure zone, moves
counterclockwise
• High Pressure Zone
Formation
– Cool air sinks
– Creates a high
pressure zone
– At the Earth’s surface,
winds blow clockwise
Cyclones and Anticyclones
Changes of state of water
• Three states of matter
– Solid
– Liquid
– Gas
• To change state, heat must be
– Absorbed, or
– Released
Changes of state of water
Water Phase Changes
Humidity
• Amount of water vapor in the air
– Saturated air is air that is filled with water
vapor to capacity
– Capacity is temperature dependent – warm
air has a much greater capacity
– Water vapor adds pressure (called vapor
pressure) to the air
Adiabatic heating/cooling
• Adiabatic temperature changes occur
when:
• Air is compressed
– Motion of air
molecules increases
– Air will warm
– Descending air is
compressed due to
increasing air pressure
• Air expands
– Air parcel does work
on the surrounding air
– Air will cool
– Rising air will expand
due to decreasing air
pressure
Adiabatic cooling of rising air
Processes that lift air
Cloud Formation
• Condensation
– Ground: Grass, car windows, etc.
– Atmosphere: tiny bits of particulate matter
• Condensation nuclei
• Dust, smoke, etc
• Ocean salt crystals which serve as hygroscopic
("water seeking") nuclei
Cloud Formation
• Clouds
– Made of millions and millions of
• Minute water droplets, or
• Tiny crystals of ice
Cloud Formation
• Clouds
– Classification based on form (three
basic forms)
• Cirrus – high, white, thin
• Cumulus - globular cloud masses
often associated with fair weather
• Stratus – sheets or layers that cover
much of the sky
Cloud Formation
• Clouds
– Classification based on height
• High clouds - above 6000 meters
– Types include cirrus, cirrostratus, cirrocumulus
• Middle clouds – 2000 to 6000 meters
– Types include altostratus and altocumulus
• Low clouds – below 2000 meters
– Types include stratus, stratocumulus, and
nimbostratus (nimbus means "rainy")
Classification
of clouds
according to
height and
form
Classification of clouds according
to height and form (continued)
Precipitation
• Cloud droplets
– Less than 20
micrometers (0.02
millimeter) in
diameter
– Fall incredibly slow
Precipitation
• Formation of
precipitation
– Collision-coalescence
process
• Warm clouds
• Large hygroscopic
condensation nuclei
• Large droplets form
• Droplets collide with other
droplets during their
descent
• Common in the tropics
Precipitation
• Forms of precipitation
– Rain – droplets have at least a 0.5 mm
diameter
– Drizzle – droplets have less than a 0.5 mm
diameter
– Snow – ice crystals, or aggregates of ice
crystals
– Sleet – Small particles of ice
– Glaze, or freezing rain – impact with a solid
causes freezing
Precipitation
• Forms of precipitation
– Hail – Hard rounded pellets
• Concentric shells
• Most diameters range from 1 to 5 cm
• Formation
– Occurs in large cumulonimbus clouds with violent upand down drafts
– Layers of freezing rain are caught in up- and down drafts
in the cloud
– Pellets fall to the ground when they become too heavy
Precipitation
• Forms of precipitation
– Rime
• Forms on cold surfaces
• Freezing of
– Supercooled fog, or
– Cloud droplets
Precipitation
• Measuring
precipitation
– Rain
• Easiest form to
measure
• Measuring
instruments
– Standard rain gauge
Precipitation
• Measuring precipitation
– Snow has two measurements
• Depth
• Water equivalent
– General ratio is 10 snow units to 1 water unit
– Varies widely
– Radar is also used to measure the rate of rainfall
Global distribution of
precipitation
• Relatively complex pattern
• Related to global wind and pressure
patterns
– High pressure regions
• Subsiding air
• Divergent winds
• Dry conditions
• e.g., Sahara and Kalahari deserts
Global distribution of
precipitation
• Related to global wind and pressure
patterns
– Low pressure regions
• Ascending air
• Converging winds
• Ample precipitation
• e.g., Amazon and Congo basins
Average annual precipitation
in millimeters
Global distribution of
precipitation
• Related to distribution of land and water
– Large landmasses in the middle latitudes
often have less precipitation toward their
centers
– Mountain barriers also alter precipitation
patterns
• Windward slopes receive abundant rainfall from
orographic lifting
• Leeward slopes are usually deficient in moisture
Seasonal Pressure and Precipitation Patterns
Weather
Thunderstorms
Snow / Rain storms
Mid-latitude cyclones
Tropical cyclones
Types of Severe Weather
• Thunderstorms
• Snow / Rain storms
• Mid-latitude cyclones
– Blizzards
– Tornadoes
• Tropical cyclones
– Typhoons in the western Pacific
– Cyclones in the Indian Ocean
– Hurricanes in the U.S.
Stages in the development
of a thunderstorm
Thunderstorms
• How Lightning Works
Thunderstorms
Lightning Varieties
cloud-to-ground
Blue jets
Cloud discharge
Red sprites
Ball lightning
Elves
(NOVA: Science Now – Lightning http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html)
Lightning Varieties
Volcanic Lightning
Nuclear Lightning
Triggered Lightning
(NOVA: Science Now – Lightning http://www.pbs.org/wgbh/nova/sciencenow/3214/02.html)
Severe weather types
• Tornadoes
– How a Tornado Forms
• Moist air from Gulf of
Mexico
• Fast moving cold, dry
air mass from Canada
• Jet stream moving east
at 150 mph
• Sets up shearing
conditions
Severe weather types
• Tornadoes
– How a Tornado Forms
• Warm moist Gulf air
releases latent heat,
creates strong updraft
• Updraft sheared by
polar air, then twisted
in a different direction
by jet stream
Tornado Wind Patterns
Severe weather types
• Tornadoes
– Why do some thunderstorms spawn
tornadoes while others do not?
– Super Cell Thunderstorms
Severe weather types
• Tornadoes
– The Fujita-Pearson Scale
• The size of a
tornado is not
necessarily an
indication of its
intensity!
Tornadoes
• “Tornado Capitol of the World”
– CNN’s “10 deadliest U.S. tornadoes”
Source: cnn.com
Tornadoes
• Why don’t tornadoes strike large cities?
– Occur over large regions
– Cities are relatively small targets
– Oklahoma City Tornado (1999)
Mid-latitude Cyclones
• Idealized weather
– Middle-latitude cyclones move eastward
across the United States
• First signs of their approach are in the western sky
• Require two to four days to pass over a region
– Largest weather contrasts occur in the spring
Mid-latitude Cyclones
Mid-latitude Cyclones
• Blizzards
– Form when a long cyclone brings
• Cold 60 km/hr winds
• Freezing temperatures
• Lots of snow
– Can travel very slowly
– Storm itself usually doesn’t kill
• Shoveling snow, auto accidents, etc.
Mid-latitude Cyclones
• Ice Storms
– Formation:
• Falling snow and ice melt, change to rain, then
freeze again as they reach the ground
• Sleet
• Freezing rain
Hurricanes
• Only natural disaster that is given a
human name
• Actually large tropical cyclones
• Convert heat in the ocean into winds
• Exports excess heat from the tropics to
the midlatitudes
Hurricanes
• How a Hurricane Works
– Tropical disturbance
• Low pressure zone develops and draws in clusters
of thunderstorms and winds
Hurricanes
• How a Hurricane Works
– Tropical disturbance
– Tropical depression
• Surface winds strengthen, move about the center
of the storm
• Central core funnels warm moist air up towards
stratosphere
• Air cools, vapor condenses, latent heat released
• Fuels more updrafts, cycle repeats, storm grows
Hurricanes
• How a Hurricane Works
– Tropical disturbance
– Tropical depression
– Tropical Storm
• Storm has sustained surface wind speeds of +39
mph
Hurricanes
• How a Hurricane Works
– Tropical disturbance
– Tropical depression
– Tropical Storm
– Hurricane
• Surface winds consistently over 74 mph
Hurricanes
• How a Hurricane Works
– Tropical disturbance
– Tropical depression
– Tropical Storm
– Hurricane
– The Eye
• As wind speed increases, winds are spiraled upwards
prior to reaching the center
• A distinctive clear “eye” is formed
• Strongest winds are located on the walls of the eye
Hurricane Wind Patterns
Hurricane Origins
• Form in the tropics ~ 5° and 20 ° latitude
• Cannot form at the equator
(Coriolis effect = 0)
Hurricanes
• Hurricane Damages
– Storm Surges
• Large mound of water builds up beneath the eye
• Reaches land as a surge of water
Hurricanes
• Hurricane Damages
– Storm Surges
• Wind speed varies
depending upon which
side of the hurricane
you’re on
• Amount of damage on the
coastline will vary
accordingly
Hurricanes
• Hurricane Damages
– Heavy Rains
– Mudflows and Debris Avalanches
– Flooding
Hurricanes
• Hurricane paths
– Curves due to
Coriolis affect
– Storms must go
around high
• Strong and large –
storms to Atlantic
seaboard
• Small and to the
north – storms may
miss the U.S.
Hurricane Paths
Forecasting the Hurricane
Season
• Frequency of hurricanes in the North
Atlantic is affected by climate
– Wet Sahel region in Africa = more
thunderstorms
– Warm SST = more energy for tropical
depressions
– Low atmospheric pressure in Caribbean =
more cyclones
Forecasting the Hurricane
Season
• The La Nina / El Nino Connection
– La Nina present in Pacific = more winds to
move storms
– El Nino present in the Pacific = less winds,
disrupts storms
El Niño
Normal conditions in the tropical
Pacific Ocean
• Surface winds move from east to west
• From high pressure in S. America to low
pressure in Australia
• Drags water westward
• Warm water pools in the western Pacific
Every 3 – 8 years, system
reverses
• Called the Southern Oscillation
• Trade winds weaken or reverse
• Warm water migrates from Australia to S.
America
• Arrives in time for Christmas – Corriente
del Niño
El Niño and La Niña
What is El Niño?
• Basically, it's a giant puddle (or pod) of
heated water that sloshes across the
Pacific Ocean
• Similar to an iceberg
– Bulge on the surface
– Most of “pod” beneath the surface
– Due to difference in density
• National Geographic’s Model
ENSO - El Niño-Southern
Oscillation
• Typically lasts 1 year
• May last up to 3
• In multi-year events, first year not as
affected
• Affects both hemispheres
Recognizing an El Niño
• Sea Surface Temperatures (SST)
• Normal: 6-8° C warmer in the western
tropical Pacific than in the eastern tropical
Pacific
• Check SST to see if in “normal” range
La Niña
• Return to “normal” conditions from an El Niño
•
strong
Produces:
– Strong currents
– Powerful upwelling
– Chilly and stormy conditions along S. American coast
• Eastern Pacific cools rapidly, Western Pacific
•
warms rapidly
Renewed Trade Wind activity spreads the cooler
eastern Pacific waters westward