Study Notes on Precipitation

Precipitation

Precipitation is water released from clouds in the form of rain, freezing rain, sleet, snow, or hail on the Earth's surface . When air is lifted in the atmosphere, it expands and cools and leads to the formation of clouds. The clouds floating overhead contain water vapour and cloud droplets, which are small drops  of condensed water. For precipitation to happen, first tiny water droplets must condense on even tinier dust, salt, or smoke particles, which act as a nucleus. Some vapour freezes into tiny ice crystals which attract cooled water drops. The drops freeze to the ice crystals, forming larger crystals known as snowflakes. When the snowflakes become heavy, they fall as it exceeds the cloud updraft speed. When the snowflakes meet warmer air on the way down, they melt into raindrops.

Rainfall and Its Types

Rain is the most common form of precipitation. When minute droplets of water are condensed from water vapour in the atmosphere on to nuclei, they may float in the atmosphere as clouds. If the droplets coalesce, they will form larger drops which will be enough to overcome by gravity and will fall as RAIN to the surface of the earth.

There are three main types of rainfall

a) CONVENTIONAL RAINFALL:

It occurs when moist air, having been warmed by Conduction from a heated surface, expands, rises and is adiabatically cooled to the Dew Point. Cumulus clouds develop and may fall accompanied by Thunder.Convectional rainfall occurs commonly during the afternoon near the equator, due to high temperature and high humidity

b) OROGRAPHIC RAINFALL: 

This type of rainfall occurs when air is forced to ascend the side of a mountain range. When land barriers such as mountain ranges, hilly regions or even escarpments of plateaus lie in the path of prevailing winds, large portion of the atmospheric air is forced to rise above these barriers. This resultant precipitation is termed as orographic. Because the air has deposited on the windward side of the mountain, there will normally less rainfall on the leeward side which is known as Rain Shadow Area

c) FRONTAL RAINFALL: 

Cyclonic or Frontal precipitation results when the warm, moist air mass (warm front) meets a cool and dry air mass (cold front). The molecules in the cold air are more tightly packed together (i.e., more dense), and thus, the cold air is heavier than the warm air. The warmer air mass is forced up over the cool air. As it rises, the warm air cools, the water vapour in the air condenses, and forms clouds and results in precipitation. This type of system is called Frontal Precipitation because the moisture tends to occur along the front of the air mass