TYPES OF RAINFALL                                                                                            back to W. C. page

  1. CONVECTIONAL RAINFALL   Convectional rain is caused by convection where the surface layer of the atmosphere is heated causing the moisture laden air to rise. As it rises it cools to form clouds. The unequal heating of the earth’s surface causes convection. The resulting condition is that more widespread areas of colder air separate rising currents of warm air. The colder air slowly sinks to take the place of rising warm air.     

The condition of rising currents of warm air separated by more wide-spread areas of slowly sinking air is referred to as CONVECTION. This is typical of thunder storms during a hot summer. Convectional rain can also be found year round in regions near the equator.

2. CYCLONIC RAINFALL This rain is caused by depressions or Lows. Cyclonic rain originates where warm tropical air meets cold polar air. The warm air overrides the cold air.

When contrasting air masses make contact, an abrupt zone or boundary is formed. This boundary is called a FRONT and is accompanied by rather abrupt changes in temperature, pressure and humidity.      

When a mass of warm air moves into a region of cold air, the warm air overrides the cold air mass, forcing the cold air to retreat. This situation is called a WARM FRONT, which are characterized by several days of rain.       

A cold air mass moving into a warm air mass produces a frontal surface, which is more vertical than that of a warm front. This situation produces a COLD FRONT. Cold air masses advance rapidly and force the warm air mass upward where it becomes cooled. The movement of the air mass is rapid enough to produce cumulonimbus clouds. Rainfall is heavy but brief in duration.

  3. OROGRAPHIC RAINFALL Canada’s west coast. Rain which mountains standing in the path of moisture-laden air cause; the air is forced to rise and is therefore cooled. If enough water vapour is present it is deposited on the windward side of the mountains.