Welcome to the Atmospheric Moisture and Precipitation Presentation. Here you will find visual resources that will help you understand this topic. These resources correspond to material presented in chapter 6 of Strahler and Strahler, PHYSICAL GEOGRAPHY,

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Welcome to the Atmospheric Moisture and
Precipitation Presentation. Here you will find
visual resources that will help you understand
this topic. These resources correspond to
material presented in chapter 6 of Strahler and
Strahler, PHYSICAL GEOGRAPHY, 3rd edition.
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The Three States of Water
Water can exist in three states - solid (ice),
liquid (water), and gas (water vapor).
Water exists in the air in the form of water
vapor, clouds, fog, and precipitation.
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The Three States of Water
A schematic diagram of the three states of water.
Arrows show the ways that any one state can
change into either of the other two states. Heat
energy is absorbed or released, depending on the
direction of change.
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Global Water Balance
Since our planet contains only a fixed amount of
water, a global balance must be maintained among
flows of water to and from the lands, oceans, and
atmosphere.
This balance also controls the proportion of
water found in the different reservoirs of the
hydrosphere.
What to look for
Notice that the oceans and glaciers contain most
of the water. Also notice that the remaining
freshwater, upon which we are so dependent,
comprises the least.
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Global Water Balance
The global water balance. Figures give average
water flows in and out of world land areas and
world oceans. Values are given in thousands of
cubic kilometers (cubic miles). Global
precipitation equals global evaporation.
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Humidity
The amount of water vapor present in the air is
called humidity. Specific humidity is a measure
of the actual quantity of water vapor held by the
air. Relative humidity compares the amount of
water vapor present to the maximum amount that
the air can hold at that temperature, expressed
as percentage.
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Humidity
The maximum specific humidity of a mass of air
increases sharply with rising temperature.
Relative humidity changes with temperature
because the capacity of warm air to hold water
vapor is greater than that of cold air. In this
example, the amount of water vapor stays the
same, and only the capacity changes.
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The Adiabatic Process
Physicists use the term adiabatic process to
refer to a heating or cooling process that occurs
solely as a result of pressure change. That is,
the change in temperature is not caused by heat
flowing into or away from a volume of air, but
only by a change in pressure on a volume of air.
See Movie on the Adiabatic Process in the
geodiscoveries section of your texts website.
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The Adiabatic Process
A schematic diagram of adiabatic cooling and
heating that accompanies the rising and sinking
of a mass of air. When air is forced to rise, it
expands and its temperature decreases. When air
is forced to descend, its temperature increases.
Adiabatic decrease of temperature in a rising
parcel of air leads to condensation of water
vapor into water droplets and the formation of a
cloud.
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Clouds
A cloud is made up of water droplets or ice
particles suspended in air.
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Clouds
Clouds are grouped into families on the basis of
height. Individual cloud types are named
according to their form.
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Types of Clouds
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Cirrus in Parallel Bands
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Altostratus Layer
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Stratocumulus Clouds
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Cirrocumulus Clouds
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AltoCumulus Clouds
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Cumulus Clouds
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Precipitation
Precipitation can form in two ways
1) Water droplets condense, collide, and coalesce
into larger and larger water droplets that fall
as rain. This is referred to as coalescence.
2) Ice crystals form and grow in a cloud tht
contains a mixture of both ice crystals and water
droplets. This occurs where the cloud temperature
is below zero degrees Celsius but despite that
some droplets are still liquid in form.
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Precipitation
Growth of raindrops is called coalescence.
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Types of Precipitation
The processes responsible for precipitation in
the atmosphere create different forms of
precipitation.
Rain falls from warm clouds where saturated air
has risen quickly and cooled, leading to
condensation. Snow, however, forms in cool clouds
where ice crystals collide with super cooled
water droplets. If they fall through air which is
below freezing, they remain as snow flakes. If
the air is above freezing, they begin to melt and
fall as sleet. Hail falls from tall cumulonimbus
clouds associated with thunderstorms. Large
hailstones grow as the updrafts in the clouds
successively add layer upon layer of condensing
water.
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Orographic Precipitation
In orographic precipitation, through-flowing
winds move moist air up and over a mountain
barrier. The term "orographic" means related to
the mountains.
See Movie on Orographic Precipitation in the
geodiscoveries section of your texts website.
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Convectional Precipitation
In convectional precipitation, unequal heating of
the surface causes an air parcel to become warmer
and less dense than the surrounding air. Because
it is less dense, it rises. As it moves upward,
it cools, and condensation with precipitation may
occur.
See Movie on Convectional Precipitation
(Thunderstorm) in the geodiscoveries section of
your texts website.
Anatomy of a thunderstorm cell. Successive
bubbles of moist condensing air push upward in
the cell. Their upward movement creates a
corresponding down-draft, expelling rain, hail,
and cool air from the storm as it moves forward.
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