Three Types of Precipitation

ReliefPrecipitation

• 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.