Precipitation: Falling of water from the atmosphere

The term precipitation refers to the falling of water from the atmosphere in any form. Most of the moisture on the land is received through precipitation, however, its distribution everywhere is not homogeneous.

The process by which the products of condensation, water droplets, ice crystals, sleet, etc., fall to the ground is known as precipitation.

Process of Precipitation

As we know, the only process bringing about cloudy condensation and resultant precipitation from extension air masses is the adiabatic cooling, but at the same time condensation is not followed by precipitation all the time. Adiabatic cooling is the process of reducing heat through a change in air pressure caused by volume expansion. Only when the cold droplets, ice pellets or ice crystals grow to such a large size as to overcome the normal buoyancy, precipitation does occur.

Conditions necessary for precipitation

Precipitated moisture falling on the ground depends on the following conditions:

1. The air must be saturated
2. The temperature, at which condensation takes place, must be present
3. The air must contain small particles of matter such as dust around which the droplets form
4. The air must be cooled below its dew point.
The common forms of precipitation are rainfall, snowfall, hailstorm, drizzle, sleet, etc.

Distribution of Precipitation

The mean annual rainfall on Earth is about 100cm but it is not evenly distributed. Most of the annual rainfall is received during a few months of the year, while the rest of the months either remain dry or receive little rainfall. In spite of the great variations in the distribution pattern, some generalizations can be made about the average annual distribution of precipitation in the world.

1. The total amount of rainfall is the highest in the equatorial region and decreases gradually towards the poles with a great deal of irregularity.
2. The coastal areas receive greater amount of rainfall than the interior of the continents which are generally far from the main source of moisture.
3. The mountain barriers tend to concentrate precipitation on the windward slope. Rainfall decreases as one move towards the leeward slope.
4. Between 35 degree and 45-degree latitudes in both the hemisphere, there occurs heavy rain on the eastern coast which decreases gradually towards the western coast.
5. Between 45 degree and 65-degree latitudes in both the hemispheres, the rainfall from the westerlies is first received on the western coast and decreases gradually towards the eastern coast.
6. Rainfall is moreover the oceans than over the continents. This is due to the greater amount of evaporation from the ocean surface.

Questions:

1. What is the only process that brings about cloudy condensation and precipitation?
(i) Adiabatic expansion
(ii) Adiabatic heating
(iii) Evaporation
(iv) Adiabatic cooling

2. Which region receives the highest rainfall?
(i) Polar region
(ii) Temperate region
(iii) Arctic region
(iv) Equatorial region

3. Which among the following is a necessary condition for precipitation?
(i) The air should be dry
(ii) The weather should be windy
(iii) The air should not have dust particles
(iv) The air must be saturated

4. Through which process do the products of condensation fall on the ground?
(i) Condensation
(ii) Evaporation
(iii) Adiabatic cooling
(iv) Precipitation

5. What is the mean annual rainfall of the Earth?
(i) 200 cm
(ii) 150 cm
(iii) 120 cm
(iv) 100 cm

Read More: Condensation: Fog and Mist, Dew and Frost - Class 9 - Geography