PrecipitationPrecipitation

Why does it occur?Why does it occur?

What are the three major What are the three major types of precipitation?types of precipitation?

Precipitation: Precipitation: Why does it occur?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 PrecipitationTypes 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 PrecipitationTypes of Precipitation

• Relief Precipitation (also called Orographic Precipitation)

• Convectional Precipitation

• Cyclonic Precipitation (also called Frontal Precipitation)

Relief or Orographic Relief or Orographic PrecipitationPrecipitation

• 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 Relief or Orographic PrecipitationPrecipitation

Relief or Orographic Relief or Orographic PrecipitationPrecipitation

• 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 Relief or Orographic PrecipitationPrecipitation

• 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 PrecipitationConvectional 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 PrecipitationConvectional Precipitation

Convectional PrecipitationConvectional 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=RkgThul2El8&NR=1

• http://www.youtube.com/watch?v=n46P4j7Hrl8&NR=1

Cyclonic or Frontal Cyclonic or Frontal PrecipitationPrecipitation

• 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 Cyclonic or Frontal PrecipitationPrecipitation

Cyclonic or Frontal Cyclonic or Frontal PrecipitationPrecipitation

• 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=D88dYNFyBq8&feature=related

• http://www.youtube.com/watch?v=vNIPBfKbec8&feature=fvwp&NR=1