Download File

WATER’S ROLE IN
THE ATMOSPHERE
HUMIDITY
 Amount
of water vapor in the air
Water vapor adds pressure
(called vapor pressure) to 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
Measurements of humidity
Specific humidity
Quantity of water vapor in a given
mass of air
Often measured in grams per
kilogram
Relative humidity
Ratio of the air’s actual water vapor
content to its potential water vapor
capacity, at a given temperature
Expressed as a percent


Saturated air
 Content equals capacity
 Has a 100 percent relative
humidity
Relative humidity can be changed in
two ways
 Add or subtract moisture to the air
•Adding moisture raises the
relative humidity
•Removing moisture lowers the
relative humidity
 Changing the air temperature
•Lowering the temperature raises
the relative humidity
•Raising the temperature lowers
the relative humidity


Dew point
 Temperature at which the air
is saturated and the relative
humidity is 100 percent
 Cooling the air below the dew
point causes condensation
•E.g., cloud formation
•Water vapor requires a
surface condense on
Two types of hygrometers are used
to measure humidity

 Psychrometer
•Compare the temperatures of
Wet-bulb thermometer, and
Dry-bulb thermometer
•If the air is saturated (100 percent
relative humidity) then both
thermometers read the same
temperature
•The greater the difference between the
thermometer readings, the lower the
relative humidity
 Hair
hygrometer – reads the
humidity directly
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 rates

Dry adiabatic rate
Unsaturated air
Rising air expands and cools at 1°C
per 100 meters
Descending air is compressed and
warms at 1°C per 100 meters

Wet adiabatic rate
Commences at condensation level
Air has reached the dew point
Condensation is occurring and latent
heat will be liberated
Heat released by the condensing
water reduces the rate of cooling
Rate varies from 0.5°C to 0.9°C per
100 meters
STABILITY OF AIR
 Two
types of air stability
STABLE AIR

Resists vertical displacement

(sinking air, or air sitting at the surface)
 Cooler
than surrounding air
 Denser than surrounding air
 Wants to sink
 No adiabatic cooling
 Stability occurs when the
environmental lapse rate is less than
the wet adiabatic rate
 Often results in widespread clouds with
little vertical thickness
 Precipitation, if any, is light to
moderate
UNSTABLE AIR
Acts like a hot air
balloon
 Rising air
 Warmer than
surrounding air
 Less dense than
surrounding air
 Continues to
rise until it
reaches an
altitude with the
same
temperature
 Adiabatic cooling

Air is unstable when
the environmental
lapse rate is greater
than the dry adiabatic
rate
 Clouds are often
towering
 Often result in heavy
precipitation
 Conditional instability
occurs when the
atmosphere is stable
for an unsaturated
parcel of air but
unstable for a
saturated parcel of air


Determines to a large degree
Clouds that develop
Intensity of the precipitation

PROCESSES THAT LIFT AIR
Orographic lifting
Elevated terrains act as barriers
Result can be a rain shadow desert
 Frontal wedging
Cool air acts as a barrier to warm air
Fronts are part of the storm systems
called middle-latitude cyclones
 Convergence where the air is flowing
together and rising

LIFTING AIR
CONDENSATION AND CLOUD FORMATION
 Condensation
Water vapor in the air changes to
a liquid and forms dew, fog or
clouds
Water vapor requires a surface to
condense on

Possible condensation surfaces on
the ground can be grass, a car
window, etc.
Possible condensation surfaces in
the atmosphere are tiny bits of
particulate matter
 Called condensation nuclei
 Dust, smoke, etc
 Ocean salt crystals which serve as
hygroscopic (“water seeking) nuclei

Clouds

Made of millions and millions of
Minute water droplets, or tiny
crystals of ice

Classification based on
Form (three basic forms)
 Cirrus – high, white, thin
 Cumulus
•Globular cloud masses
•Often associated with fair
weather
 Stratus
•Sheets or layers
•Cover much of the sky

Height
 High clouds
•Above 6000
meters
•Types
 Cirrus
 Cirrostrat
us
 Cirrocumu
lus
 Middle clouds
•2000 to 6000
meters
•rainy)
•Types (alto as
part of the name_
 Altocumulus
 Altostratus
 Low clouds
•Below 2000
meters
•Types
 Stratus
 Stratocumulus
 Nimbostratus
(nimbus means
 Clouds
of vertical development
•From low to high altitudes
•Called cumulonimbus
•Often produce
Rain showers
Thunderstorms
HIGH CLOUDS
MIDDLE CLOUDS
LOW CLOUDS
FOG
 Considered
an atmospheric hazard
 Cloud with its base at or near the
ground
 Most fogs form because of
Radiation cooling, or
Movement of air over a cold
surface
FOG
 Types
of fog
Fogs caused by cooling
Advection fog – warm, moist air
moves over a cool surface
Radiation fog
 Earth’s surface cools rapidly
 Forms during cool, clear, calm
nights
ADVECTION FOG
RADIATION FOG
Upslope fog
 Humid air moves up a slope
 Adiabatic cooling occurs
 Evaporation fogs
 Steam fog
 Cool air moves over warm water and moisture
is added to the air
 Water has a steaming appearance
 Frontal fog, or precipitation fog
 Forms during frontal wedging when warm air
is lifted over colder air
 Rain evaporates to form fog

UP-SLOPE FOG
EVAPORATION FOGS

Steam Fog

Frontal Fog
PRECIPITATION
 Cloud
droplets
Less than 10micrometers in
diameter
Fall incredibly slow
Formation of precipitation

Bergeron process
Temperature in the clouds is
below freezing
Ice crystals collect water vapor
Large snowflakes form and
 Fall to the ground as snow, or
 Melt on their descent and form
rain

Collision-coalescence process
Warm clouds
Large hygroscopic condensation
nuclei
Large droplets form
Droplets collide with other
droplets during their descent
Forms of precipitation
Rain and drizzle
Rain – droplets have at least a
0.5 mm diameter
Drizzle – droplets have less
than 0.5 mm diameter
Snow – ice crystals, or
aggregates of ice crystals


Sleet and glaze
Sleet
 Wintertime phenomena
 Small particles of ice
 Occurs when
•Warmer air overlies colder air
•Rain freezes as it falls
Glaze, or freeing rain – impact with
a solid surface causes freezing

Hail
 Hard rounded pellets
•Concentric shells
•Most diameters range from 1-5 cm
 Formation
•Occurs in large cumulonimbus
clouds with violent up-anddowndrafts
•Layers of freezing rain are caught
up in up-and-downdrafts in the
cloud
•Pellets fall to the ground when they
become too heavy

Rime
 Forms on cold surfaces
•Freezing of super-cooled fog,
or
•Cloud droplets
Rime
Measuring precipitation
Rain
Easiest form to measure
Measuring instruments
 Standard rain gauge
•Uses funnel to collect and conduct
rain
•Cylindrical measuring tube
measures rainfall in centimeters or
inches
 Recording gauge


Snow has two measurements
Depth
Water equivalent
 General ratio is 10 snow units
to 1 water unit
 Varies widely
AIR PRESSURE AND WIND
Atmospheric Pressure
 Force exerted by the weight of the
air above
 Weight of the air at sea level
14.7 pounds per square inch
1 kilogram per square centimeter
 Decreases with increasing altitude

 Units
of measurement
Millibar (mb) – standard sea
level pressure is 1013.2mb
Inches of mercury – standard sea
level pressure is 29.92 inches of
mercury

Instruments for measuring
Barometer
Mercury barometer
 Invented by Torricelli in 1643
 Uses a glass tube filled with mercury
Aneroid barometer
 “Without liquid”
 Uses an expanding chamber
 Barograph (continuously records air
pressure)
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 changes
over distance
Coriolis Effect
Apparent deflection in the wind
direction due to Earth’s rotation
Deflection is
 To the right in the northern
hemisphere
 To the left in the southern
hemisphere

Friction with earth’s surface
Only important near the
surface
Acts to slow the air’s
movements
 Upper
air winds
Generally blow parallel to isobars
– called geostrophic winds
Jet stream
“River” of air
High altitude
High velocity (120-140 km/h)
CYCLONES AND ANTICYCLONES
 Cyclones
A center of low pressure
Pressure decreases toward the
center

Winds associated with
In the Northern Hemisphere
 Inward (convergence)
 Counterclockwise
In the Southern Hemisphere
 Inward (convergence)
 Clockwise
Associated with rising air
Often bring clouds and precipitation

 Anticyclone
A center of high pressure
Pressure increases toward the
center

Winds associated with
In the Northern Hemisphere
 Outward (divergence)
 Clockwise
In the Southern Hemisphere
 Outward (divergence)
 Counterclockwise
Associated with subsiding air
Usually bring “fair” weather

GENERAL ATMOSPHERIC CIRCULATION
 Underlying
cause is unequal
surface heating
 On the rotating earth, there are
three pairs of atmospheric cells
that redistribute the heat
 Idealized
global circulation
Equatorial low pressure zone
Rising air
Abundant precipitation
Subtropical high pressure zone
Subsiding, stable, dry air
Near 30° latitude
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


Subpolar low pressure zone
Warm and cool winds interact
Polar front – an area of storms

Polar high pressure zone
Cold, subsiding air
Air spreads equator-ward and
produces polar easterly winds
Polar easterlies collide with the
westerlies along the polar front
Influence of continents

Seasonal temperature differences
Influence is most obvious in the
Northern Hemisphere


Monsoon
Seasonal change in wind direction
Occur over continents
 During warm months
•Air flows onto land
•Warm, moist air from the ocean
 Winter months
•Air flows off the land
•Dry, continental air
CIRCULATION IN THE MID-LATITUDES
Complex
 Occurs in the zone of the westerlies
 Air flow is interrupted by cyclones
Cells move west to east in the
Northern Hemisphere
Create anti-cyclonic and cyclonic flow
Paths of the cyclones and anticyclones
are associated with the upper-level
airflow

I CAN…
I can explain the dynamics of the El NinoSouthern Oscillation and its effect on continental
climates.
 I can explain differences between maritime and
continental climates with regard to oceanic
currents.
 I can describe the various conditions of formation
associated with severe weather.
 I can describe the seasonal variations in severe
weather.

EL NINO
A
counter current (ocean
current that flows the
opposite way) that flows
southward along the
coasts of Ecuador and
Peru
THINK ABOUT THESE QUESTIONS…
Can
you think of another ocean
current that travels along the
coast of a continent?
How does it affect the areas
weather?
Can you explain what the
weather would be like without
an ocean current’s effect?
Warm
Usually appears during the
Christmas season
Blocks upwelling of colder,
nutrient filled water, and
anchovies starve from lack of food
Strongest El Nino on record
occurred in 1997 and 1998 and
caused
Heavy rains in Ecuador and Peru
Ferocious storms in California

NOW, THINK ABOUT THESE
QUESTIONS…
How
could the loss of the
anchovies affect us?
What do you think a warm ocean
current would do to the
atmosphere of a location that
normally had cold ocean water
near it?
How do you know this?
 Related
to large-scale atmospheric
circulation
Pressure changes between the
eastern and western Pacific
called the Southern Oscillation
Changes in trade winds creates a
major change in the equatorial
current system, with warm water
flowing eastward
WHICH MAP
SHOWS
CONDITIONS
THAT YOU
WOULD LIKE
TO
EXPERIENCE?
 Do
you feel we
are
experiencing
one of these
phenomena
this year?
 Explain.
Effects
are highly variable
depending in part on the
temperatures(how warm or
cold they are) and size of the
warm water pools
LOCAL WINDS
 Produced
from temperature
differences
 Small scale winds
 Types
Sea and land breezes
Valley and mountain breezes
Chinook and Santa Ana winds
WIND MEASUREMENT
 Two
basic measurements
Direction
Speed

Direction
Winds are labeled from where they
originate (e.g., North wind – blows
from the north toward the south)
Instrument for measuring wind
direction is the wind vane
Direction indicated by either
Compass points
Scale of 0° to 360°
Prevailing wind comes more often
from one direction

Speed – often measured with a cup
anemometer
 Changes
in wind direction
Associated with locations of
Cyclones
Anticyclones
Often bring changes in
Temperature
Moisture conditions
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

Low pressure regions
Ascending air
Converging winds
Ample precipitation
e.g., Amazon and Congo basins
RELATED TO DISTRIBUTION OF LAND AND WATER
Large landmasses in the middle
latitudes often have less
precipitation than toward their
centers
Mountain barriers also alter
precipitation patterns
Windward slopes receive
abundant rainfall from
Orographic lifting
Leeward slopes are usually
deficient in moisture
