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Cloud Development
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Principle
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Clouds are masses of condensed water
vapor which are visible signs of atmospheric processes at
work. Clouds help regulate the earth's energy balance by
reflecting and scattering solar radiation and by absorbing
the earth's infrared radiation. In addition, clouds help
redistribute surplus heat from the equator toward the poles
and return water (in the form of precipitation) to the
oceans and land masses across the globe.
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The Importance of Clouds
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Clouds are essential to the earth-atmosphere system.
Clouds complete the following functions:
- Clouds help regulate Earth's energy balance by
reflecting and scattering solar radiation and by
absorbing Earth's infrared energy.
- Clouds are required for precipitation to occur and,
hence are an essential part of the hydrologic cycle.
- Clouds indicate what type of atmospheric processes
are occurring (e.g., cumulus clouds indicate surface
heating and atmospheric turbulence).
- Clouds help redistribute extra heat from the equator
toward the poles.
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Making a Cloud
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- Clouds are masses of condensed
water vapor.
- Clouds are formed when water
vapor is condensed into liquid water (cloud
droplets).
- There are three basic
requirements for clouds to occur:
- Water vapor must be present
in sufficient amounts so that saturation can be
reached by some means.
- Cloud condensation nuclei
(CCN) must be present to provide a surface on which
water will condense. (Examples of CCN include dust in
the air from the earth's surface, salt particles from
the sea, combustion products, and volcanic or
meteorite dust.)
- Cooling mechanism is
required to cool the air temperature to the dewpoint
temperature.
The requirements for sufficient
water vapor and CCN are rarely the limiting factors for
cloud development. Typically, the limiting factor is a
cooling mechanism. Thus, the air temperature needs to be
lowered to the dewpoint temperature for a cloud to form.
If a cooling mechanism is not present, clouds will not
form.
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Common Cooling Mechanisms
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The following are common mechanisms in the atmosphere
that cause cooling and, hence, may be associated with cloud
formation:
- Radiative cooling from the ground
- Evaporative cooling
- Cooling by lifting (as air rises in the troposphere,
it cools)
- Mechanical lifting
- Lifting near a front
- Lifting in low pressure systems
- Local circulations
- Thermal instability
- Cooling by horizontal motion (warm, moist air moves
over cool surface or cool air moves over warm, moist
surface)
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Atmospheric Stability
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- Warm air is less dense than cold air at the same
pressure.
- Moist air is less dense than dry air at the same
pressure and temperature.
- If the air is stable, an air parcel which is moved
upward or downward will return to its original position.
If the air is unstable, an parcel which is moved upward
or downward will continue to move in the direction of
initial motion and will accelerate in that direction
until stopped by an opposing force.
An air "parcel" refers to an
imaginary blob of air to which may be assigned certain
properties such as a temperature, pressure, water vapor
content, etc. Generally thought of as the size of a beach
ball.
- As an air parcel rises, it will expand (because the
air pressure decreases with increasing height) and cool.
The environmental temperature may decrease at the same
rate as the expanding air parcel, but in general the
environmental temperature decreases (or even increases)
at a rate different from that of the rising air parcel.
The "environment" refers to the
average properties of the air nearby the air
parcel.
- Stability can tell us whether or not clouds will
form, where they will form, when they will form and what
type of clouds they will be.
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End
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