1.5.2 Precipitation

Precipitation

The term Precipitation denotes all forms of water that reach the earth from atmosphere. The usual forms are rainfall, snowfall, hail, frost & dew. Only first two contribute significant amounts of water. Precipitation is often classified according to the factors responsible for lifting.

Types of precipitation

Cyclonic precipitation – results from lifting of air masses converging into low pressure area of cyclone. A broader region between two adjacent air masses having different characteristics such as temperature and humidity is called a front. When a flow of warm and moist air mass from south, meets cold air mass from north, the colder one being heavier, under run the warm air in wedge shape forcing warm air to lift. This lifted warm air cools down at high altitude causing precipitation called frontal precipitation

In non frontal precipitation, the moist warm air mass is stationary and the moving cold air mass meets it. Due to lightness of warm air there is passive ascent of warm air over cold air owing to the active under cutting. When the lifted warm air cools down at high altitude precipitation occurs.

Convective precipitation – This caused by natural rising of warmer lighter air in colder denser surroundings. Difference in temperature may result from unequal heating at the surface, unequal cooling at the top of the layer or mechanical lifting when when air is forced to pass over a denser colder air masses. Convective precipitation is spotty and intensity varies from light showers to cloud bursts.

Orographic precipitation – is due to lifting of warm moisture overloaded air masses due to topographic barriers such as mountains. This type of precipitation is rich in moisture content because of their long travel over oceans.

Forms of precipitation

• Rain – When size of droplet is more than 0.5mm to 6.25mm.Rain is considered light if intensity is 2.5mm/hr moderate from 2.5 to 7.5mm/hr and heavy over 7.5mm/hr.
• · Snow- This is precipitation in the form of ice crystals normally hexagonal in shape resulting from sublimation. 10cm of snowfall is equivalent to 1.0 cm of rainfall.
• · Drizzle – When the size of water droplet is under 0.5mm and intensity < 0.01mm/hr
• · Glaze – When the drizzle or rain freezes as it comes in contact with cold objects it is known as glaze.
• · Sleet – It is frozen rain drops falls through air of subfreezing temperature, the drops freeze to form grains of ice called sleet.
• · Hail – is lumps or bulbs of ice over 5mm diameter formed by alternate freezing or melting as carried up and down in highly turbulent air currents.
• Dew- During nights when surface of the objects on earth cools by radiation, the moisture present in atmosphere condenses on surface of these objects forming water droplets called dew.