Principle
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. |
The
Importance of Clouds
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
- 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
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
- 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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