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Three Types of Precipitation
Relief
Precipitation
• Mountain Barriers create Relief
Precipitation. As moist air rises up
the windward (The side of a
mountain that faces the prevailing
wind) slope of a mountain, it
expands and cools.
• This cooling produces the rate of
condensation to increase. When the droplets
of water that form get too heavy they fall as
precipitation. In temperatures below freezing,
the water vapor sublimates into snow.
*sublimates – Changes directly from a gas to a
solid.
• As cool air descends on the leeward
(The side of a mountain that faces
away from the prevailing wind) side
of the mountain, it contracts and
becomes warmer. Since more
evaporation than condensation takes
place as the air descends, cloud
formation and precipitation
decrease.
• The result is a drier climate, or rain
shadow on the leeward side of the
mountain.
Convectional Precipitation
• Convectional precipitation results from the heating of
the earth's surface. The warm ground heats the air
over it. As the air warms, the air molecules begin to
move further apart.
• With increased distance between molecules, the
molecules are less densely packed. Thus, the air
becomes “lighter” and rises rapidly into the
atmosphere.
• As the air rises, it cools. Water vapour in the air
condenses into clouds and precipitation. This type of
precipitation is common in the Prairies and Ontario
Frontal Precipitation
• When masses of air with different
density (moisture and temperature
characteristics) meet, warmer air
overrides colder air. The warmer air
is forced to rise and if conditions are
right becomes saturated, causing
precipitation. In turn, precipitation
can enhance the temperature and
moisture contrast along a frontal
boundary.
• Fronts cause sudden changes in general
temperature, and in the humidity and
pressure in the air. Warm fronts occur
where the warm air scours out a
previously lodged cold air mass. The
warm air 'overrides' the cooler air and
moves upward. Warm fronts are
followed by extended periods of light
rain and drizzle, because, after the
warm air rises above the cooler air
(which sinks to the ground), it gradually
cools due to the air's expansion while
being lifted, which forms clouds and
leads to precipitation.
• Cold fronts occur when a mass of
cooler air dislodges a mass of warm
air. This type of transition is sharper,
since cold air is more dense than
warm air. The rain duration is less,
and generally more intense, than
that which occurs ahead of warm
fronts. A wide variety of weather can
be found along an occluded front,
with thunderstorms possible, but
usually their passage is associated
with a drying of the air mass.