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Cloud Formation
Ingredients for Cloud Formation:
•Warm, moist air (unstable air) – why?
•Lifting mechanism
•Convectional lifting – heating of the air at the surface. Less dense,
warm air rises and cools.
•Orographic lifting – landform (mountains) lifts air, air cools.
•Convergence / Frontal Lifting – Air lifted and cooled over
frontal boundaries.
•Radiative Cooling – clear nights just before dawn when the surface
air is coolest.
•“Dirty Air” – Dust, Pollen particles suspended in the air.
Changing from a liquid to a gas
Rising (warm/moist) Air Cools
How does it cool?
What happens when you cool water vapor?
Changing the gas back to a liquid.
As the air rises and
cools it approaches
the Dew Point
temperature.
The Dew Point Temperature
is the temperature at which
the air is saturated and
condensation can occur.
This happens when the
Air Temperature = Dew Point Temperature.
When the
Air Temperature =
Dew Point Temp...
…condensation can occur
and droplets will form
Calculating Dew Point and Relative Humidity
Using a sling psychrometer
Dry Bulb thermometer –
measures the air temperature
Wet Bulb thermometer –
Measures the rate of evaporation
Let’s say a parcel of
air begins rising with
a T at the surface of
30oC. As it rises
what will happen to
its T?
The Dew Point T for
this parcel of air is
20oC.
At what altitude will
the air be saturated
enough for
condensation to
begin and a cloud to
form?
1.2Km – the altitude
at which the cloud
begins to form is called
“Cloud Base”. Here the
Air T = DPT.
Formation of Clouds and Precipitation
In summary:
Droplets merge, grow in size, and fall to the ground as precipitation
Droplets form and gather into masses called Clouds…
and condensation occurs on condensation nuclei…
The air is then saturated…
Reaches it’s Dew Point Temperature.
As the warm/moist air rises it cools adiabatically.
This make the air warm, moist and less dense so it can be
displaced by cooler air.
The sun evaporates water.
Convectional Lifting
Orographic Lifting
The COLD FRONT
Warm Moist Air
Cold air mass will displace a warmer air mass upwards.
Warm air rises ahead of the front.
Causes heavy rain just ahead of the front,
thunderstorms, hail, tornadoes, and
Frontal Lifting
rapidly decreasing temperatures.
The WARM FRONT
Warm Air Mass
Warm air will rise up the back end of a slower moving cooler air mass.
Warm air rises, cools, condenses,
Causes extended periods of rain,
Slowly increasing temperatures.
Frontal Lifting
Frontal Lifting
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Fog from Radiative Cooling
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