CLIMATE
and
WEATHER
Part II
PRECIPITATION
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QuickTime™ and a
TIFF (Uncompressed) decompressor
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Precipitation
Three types of precipitation:
• RELIEF PRECIPITATION
• CONVECTIONAL PRECIPITATION
• CYCLONIC PRECIPITATION
Why does precipitation occur?
• Air cools as it rises
• As air cools, water vapour
condenses more than it evaporates
Why does air rise?
1. It rises to cross an area of high elevation.
This causes RELIEF PRECIPITATION
2. It rises because it has absorbed heat from
the earth’s surface. This causes
CONVECTIONAL PRECIPITATION
3. It rises because there is a cooler, denser air
mass flowing beneath it that forces it up.
This causes CYCLONIC PRECIPITATION.
RELIEF PRECIPITATION
• Mountain barriers create relief precipitation
• As the moist air rises up the slope of a mountain,
it expands and cools
• As the air cools, the rate of evaporation
decreases while the rate of condensation
increases
• The result is water droplets in the air
• The heavier and more condense the droplets
become, rain falls
• In colder temperatures, water vapour condenses
to snow
Quic kT ime™ and a
T IFF (Uncompress ed) decompress or
are needed to s ee this pi cture.
RELIEF PRECIPITATION
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TIFF (Uncompressed) decompressor
are needed to see this picture.
CONVECTIONAL
PRECIPITATION
• Very common in summer
• Common in location such as the Prairie
provinces, Ontario and Québec
• The land is subject to intense heating
• As the ground heats up, the air above it rises
• As it rises it expands and cools
• The water vapour within it condenses,
forming puffy white clouds
• As more water vapour condenses into these
clouds, the clouds develop vertically and
water droplets get larger and larger
CONVECTIONAL
PRECIPITATION
• At high altitudes, the water droplets may
even freeze, forming ice pellets
(hailstones)
• The weight of the water droplets or
hailstones cause them to fall to the
ground, often violent downpours
CONVECTIONAL
PRECIPITATION
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are needed to see this picture.
CYCLONIC
PRECIPITATION
• A cyclonic storm (also known as a front) is a
large, low-pressure system that forms when a
warm air mass and a cold air mass collide
• This often occurs under the polar-front jet
stream which separates cold dry arctic air
from warm, moist tropical air
• The rotation of the earth causes the air to
circulate in a counterclockwise direction
CYCLONIC
PRECIPITATION
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Homework
• Textbook - page 139 #1-10 a), 11