Transcript Precipitation

Precipitation
Why does it occur?
What are the three major
types of precipitation?
Precipitation:
Why does it occur?
In order to fully understand why
precipitation occurs, it is important to
understand two key ideas:
1) Precipitation occurs when air containing
water vapour cools to the point where
condensation takes place
2) Air cools when it is forced to rise
Types of Precipitation
There are three types of precipitation:
each occurs when air is forced to rise
for a different reason.
All three types of precipitation occur
in some part of Canada at some time
of the year.
Types of Precipitation
• Relief Precipitation (also called
Orographic Precipitation)
• Convectional Precipitation
• Cyclonic Precipitation (also called
Frontal Precipitation)
Relief or Orographic
Precipitation
• Occurs when air is forced to rise over a
barrier or an area of high elevation
• As the air mass rises up the windward
side of the mountain, the temperature
drops, and the moisture content rises.
• Rain falls, and then changes to freezing
rain and/or snow at higher elevations as
the temperature drops further.
Relief or Orographic
Precipitation
Relief or Orographic
Precipitation
• Once the air mass reaches the top of the
mountain, precipitation usually stops,
because the air mass has a low moisture
content.
• As the air mass falls, its temperature
increases.
• This warm, dry wind on the mountain’s
leeward side is known as a chinook.
Relief or Orographic
Precipitation
• This type of precipitation is common in
the Western Cordillera, and in the
Southern Ontario snowbelt around Lake
Huron and Georgian Bay.
• The chinook wind is a weather
phenomenon associated with the Western
Cordillera and the Interior Plains.
• Relief rainfall - YouTube
Convectional Precipitation
• Occurs when air is forced to rise as it
absorbs heat radiating from the Earth’s
surface
• On a warm or hot spring or summer day,
the ground is heated by solar radiation
• The air above the ground also becomes
heated, and it begins to rise
• This rapidly rising air cools quickly,
leading to precipitation
Convectional Precipitation
Convectional Precipitation
• If the air rises quickly, and cools rapidly, this
type of precipitation can have dangerous
consequences
• Thunderstorms, lightning, violent winds,
hail, and heavy rainfall are all associated
with convectional precipitation
• In extreme circumstances, tornadoes may also
result
• This type of precipitation is common in the
Prairies, Ontario, and Quebec in the late
spring and the summer months
• http://www.youtube.com/watch?v=RkgThul
2El8&NR=1
• http://www.youtube.com/watch?v=n46P4j7
Hrl8&NR=1
Cyclonic or Frontal
Precipitation
• Occurs when air is forced to rise upwards by a
cooler, denser air mass flowing beneath it
• Systems of air masses move across North America
from west to east, and as they move, they will
often collide with one another along “fronts”
• At these fronts, the cooler, denser air mass forces
the warmer air mass upwards
• This upward movement of air starts the cooling
process, which leads to condensation and
precipitation
Cyclonic or Frontal
Precipitation
Cyclonic or Frontal
Precipitation
• As a result of the westerly (or eastward)
movement of air masses in North America,
cyclonic storms may affect Calgary on a given
day, move on to Winnipeg the following day, and
Ottawa the next
• Thus, people who live in Toronto can look to the
west of North America to “predict” what their
weather will be like in the near future
• Cyclonic precipitation is common in Ontario,
Quebec, and the Maritimes
• From time to time, cyclonic storms may form
over the Pacific Ocean and bring precipitation to
the coast of British Columbia
• http://www.youtube.com/watch?v=D88dYN
FyBq8&feature=related
• http://www.youtube.com/watch?v=vNIPBfK
bec8&feature=fvwp&NR=1