Visualizing Physical Geography
by Alan Strahler and Zeeya Merali
Chapter 4
Atmospheric Moisture and Precipitation
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Chapter Overview
Water and the Hydrosphere
Humidity
The Adiabatic Process
Clouds
Precipitation
Air Quality
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Objectives
Describe how water changes state and
interacts in the hydrosphere.
Define and explain humidity.
Explain the adiabatic process in
relationship to cloud formation.
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Objectives
Describe cloud classification and fog.
Describe precipitation in its various
forms.
Describe and explain air pollutants and
the damage they can do.
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Water and the Hydrosphere
Three States of Water:
•Solid (ice)
•Liquid (water)
•Gas (vapor)
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Water and the Hydrosphere
Hydrosphere: Total realm of water at
Earth’s surface.
•Oceans
•Ice
•Surface water
•Groundwater
•Atmospheric water
•Soil moisture
•Biota
Visualizing Physical Geography
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Water and the Hydrosphere
Distribution of water in the
hydrosphere
• Oceans: 97.5%
• Fresh water: 2.5%
Visualizing Physical Geography
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Water and the Hydrosphere
Hydrologic Cycle:
Water moves among the ocean, atmosphere and land
• Evaporation
• Precipitation
• Transpiration
from plants
• Runoff
• Sinks into soil
• Recharge of
groundwater
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Water and the Hydrosphere
Precipitation: Particles of
liquid water or ice that fall
from the atmosphere and
may reach the ground.
Visualizing Physical Geography
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Humidity
Humidity: the amount of water vapor in the air
The maximum
quantity of moisture
that can be held in
the air depends on
air temperature
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Humidity
Relative Humidity: compares the amount of
water vapor present in the air to the
maximum amount that the air can hold at
that temperature
Expressed as a percentage:
At 100% relative humidity, air is saturated.
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Humidity
Relative Humidity changes when:
1. Atmosphere gains or loses water vapor
• Evaporation
2. Temperature changes
• Lower temperature relative humidity rises
• Raise temperature relative humidity
decreases
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Humidity
Specific Humidity:
actual quantity of
water held by a parcel
of air
• Grams of water vapor
per kilogram of air
(g/kg)
• Highest in equatorial
zones
• Lowest near poles
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
Humidity
Dew-point temperature:
temperature at which air
with a given humidity will
reach saturation when
cooled without changing its
pressure
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Adiabatic Process
Adiabatic Principle: the physical principle
that a gas cools as it expands and warms
as it is compressed
Change in temperature:
• caused only by a change in pressure
• not caused by heat flowing in or out of the
gas
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Adiabatic Process
Atmospheric pressure decreases with altitude
so...
As a parcel of air rises
pressure on the parcel
decreases
air expands and cools
As a parcel of air descends
pressure on the parcel
increases
air is compressed and
warms
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Adiabatic Process
Dry adiabatic lapse rate: rate at which rising air is cooled by
expansion when no condensation is occurring:
10º C per 1000 m
(5.5 º F per 1000 feet)
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.
The Adiabatic Process
Wet adiabatic lapse rate: rate at which rising air is cooled by
expansion when condensation is occurring:
Ranges from
4-9º C per 1000 m
(2.2-4.9º F per 1000 feet)
Visualizing Physical Geography
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Clouds
Clouds consist of water
droplets, ice crystals, or both
Condensation nucleus: a
tiny bit of solid matter
(aerosol) in the atmosphere,
on which water vapor
condenses to form a tiny
water droplet
Visualizing Physical Geography
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Clouds
Cloud Families: High clouds, middle clouds, low
clouds, clouds of vertical development
Visualizing Physical Geography
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Clouds
Cumulus Clouds
Cumuliform clouds: globular masses of cloud,
associated with parcels of rising air
Visualizing Physical Geography
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Clouds
Stratus Clouds
Image ID: wea02050, NOAA's National Weather Service (NWS) Collection
Photographer: Ralph F. Kresge
http://www.photolib.noaa.gov/htmls/wea02050.htm
Stratiform clouds: blanket-like, cover large areas
Visualizing Physical Geography
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Clouds
Cirrus Clouds
Cirrus clouds: high, thin, wispy clouds composed
of ice crystals
Visualizing Physical Geography
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Clouds
Fog is a cloud layer at or close to Earth’s surface
• Radiation fog: formed
when temperature of the
air at ground level falls
below dew point
• Advection fog: forms when
warm moist air moves over
a cold surface
•Common over oceans
(“sea fog”)
Image ID: wea03250, NOAA's National Weather Service (NWS) Collection
Photographer: LCDR Mark Wetzler, NOAA Corps
http://www.photolib.noaa.gov/htmls/wea03250.htm
Visualizing Physical Geography
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Precipitation
Types of Precipitation
• Rain
• Snow
• Hail
• Ice storm
Rain formation in warm clouds
Visualizing Physical Geography
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Precipitation
To form precipitation, air must move upward
and chill by adiabatic processes.
Four ways for air to move upward:
1.Orographic precipitation
2.Convectional precipitation
3.Cyclonic precipitation
4.Convergence
Visualizing Physical Geography
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Precipitation
Orographic precipitation: precipitation induced when moist
air is forced over a mountain barrier
Visualizing Physical Geography
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Precipitation
Rain shadow: a belt of dry climate that extends down
and beyond the leeward slope of a mountain range
Visualizing Physical Geography
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Precipitation
Convectional precipitation: precipitation induced when
warm, moist air is heated at the ground surface, rises,
cools, and condenses to form water droplets, raindrops,
and rainfall
Lifting condensation level: level at which condensation begins
Visualizing Physical Geography
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Precipitation
Thunderstorm: intense local storm associated with a tall,
dense cumulonimbus cloud in which there are very strong
updrafts of air
Conditions:
1. Warm, moist air
2. An environmental
lapse rate in which
temperature
decreases more
rapidly with altitude
than it does for either
the dry or wet
adiabatic lapse rates
Visualizing Physical Geography
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Precipitation
Thunderstorm Cell
Hailstorm Frequency
Visualizing Physical Geography
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Air Quality
Air pollutant: an unwanted substance injected into the
atmosphere from the Earth’s surface by either
natural or human activities. Includes:
• Aerosols
• Gases
• Particulates
Generated
largely by
combustion
Smog (“smoke” + “fog”) contains nitrogen
oxides, volatile organic compounds, ozone
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Air Quality
Acid deposition: sulfur dioxide (SO2) and nitric oxide
(NO2) released into the air combines with water to form
sulfuric and nitric acids. May form:
• Acid rain
• Acid ice crystals
• Dry acid particles
Acidity of rainwater in U.S., 2005
Visualizing Physical Geography
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Air Quality
Effects of acid rain:
• Acid streams and lakes affect
aquatic life
• Damage to forests
• Damage to soils
• Damage to buildings
Visualizing Physical Geography
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Air Quality
Air pollution control:
• Improve controls on industrial
emissions
• Develop alternative, nonpolluting technologies
•Solar
•Wind
•Geothermal
• Reduce fossil fuel consumption
Visualizing Physical Geography
Copyright © 2008 John Wiley and Sons Publishers Inc.