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Condensation
This is the process by which water vapour in the atmosphere is converted into a liquid or,
if the temperature is below 0 degrees Celsius, a solid. This process occurs when air is
cooled until it is saturated. This cooling which causes condensation can occur in the
following ways:
1.
Cooling by radiation: The ground loses heat rapidly through terrestrial
radiation and the air in contact with it is then cooled by conduction. If the air
is moist, some vapour will condense to form radiation fog or dew. If the
temperature is below freezing point hoar frost is produced. Dew, hoar frost
and radiation fog are all produced under calm, clear, anticyclonic conditions
when there is rapid terrestrial radiation at night. Radiation is a form of
horizontal movement of air and as a result, limited condensation is achieved.
2.
Advection cooling: This process occurs when warm, moist air moves over a
cooler land or sea surface. This occurrence produces advection fogs, which
can assist in providing sufficient moisture for a limited vegetation cover as in
parts of the Atacama Desert, South America. Advection is a form of
horizontal movement of air and as a result, limited condensation is achieved.
3.
Orographic and Frontal uplift: This process involves vertical movements of
air. Orographic uplift is the forced ascent of air when it collides with a
mountain. As air strikes the windward side, it is uplifted and cooled.
Windward slopes of mountains tend to be the rainy sides while the leeward
side is dry. Dry climates like steppes and desert are often found in the "rain
shadow" of tall mountain systems that are oriented perpendicular to the flow
of air. Frontal uplift occurs when greatly contrasting air masses meet along a
weather front. For instance, when warm air collides with cool air along a
warm front, the warm air is forced to rise up and over the cool air. As the air
gently rises over the cool air, horizontally developed stratus-type clouds form.
If cold air collides with warm air along a cold front, the more dense cold air
can force the warmer air ahead to rise rapidly creating vertically developed
cumulus-type clouds.
4.
Convective or Adiabatic cooling: This is when air is warmed during the
daytime and rises in pockets as thermals. As the air expands, it uses energy
and so loses heat and the temperature drops. The air is cooled by the reduction
of pressure with height rather than by a loss of heat to the surrounding air, this
is referred to as adiabatic cooling. Both Orographic and adiabatic cooling
involve vertical movements of air and are regarded as being more effective
mechanisms of condensation.
Condensation does not occur readily in clean air. If air is pure, it can be cooled below its
dew point (point where cooled air becomes saturated, just before condensation) to
become supersaturated. Clean unsaturated air can be cooled to -40 degrees Celsius before
condensation or in this case, sublimation. Pure air, however, is seldom encountered in the
atmosphere.
Products of condensation
Condensation results in the formation of small droplets of water. Condensation occurs
either near the surface or aloft. Condensation at the surface results in dew formation
when the near surface air temperature drops to the dew point temperature. This often
occurs at night under cloudless skies when the air is humid. Under cloudless skies,
emitted terrestrial radiation penetrates to space, cooling the surface which then cools the
near surface air.
Fog
Fog is a cloud that forms near the ground. Fog, like any other product of condensation,
requires the air temperature to decrease to the dew point temperature where upon the air
is at saturation. Cooling of the near surface air is accomplished either through contact
cooling (a diabatic process) or adiabatic cooling.
There are different types of fogs: advection fog, radiation fog, steam fog, frontal fog and
upslope fog.
An upslope fog forms when moist air is forced up a slope. These certainly occur as the air
encounters hilly terrain and is forced to rise, or if moist air travels up a very long slope.
Such might be the situation when air moves out of the Gulf of Mexico travelling west up
the Great Plains toward the east slope of the Rocky Mountains. As the air rises it expands
and adiabatically cools. Once the air temperature reaches the dew point temperature the
air becomes saturated, and condensation occurs to form the fog.
Cloud
Clouds form by the condensation of water into extremely small droplets of liquid or ice.
Clouds are classified according to the height at which they form and their structure. High
clouds form above 7,000 m (23,000 ft) and are primarily composed of ice crystals.
Meteorologists call these cirrus-form clouds. Mid-level clouds form between 2,000 and
7,000 m. The prefix alto is applied to these clouds. Low clouds form between the surface
and 2,000 m.