The factors controlling the amount and rate of evaporation are : (i) temperature; (ii) the moisture content or the degree of dryness or aridity of air; and (iii) movement of air.
(1) Increase in temperature increases the water absorbing capacity of the given column of air and vice versa.
(2) When and where aridity is high, air has a potential of absorbing and retaining a large amount of moisture. In air with higher humidity, this potential is reduced; hence evaporation is slow and small in amount. Evaporation is greater on the oceans in summer than in winters.
(3) The greater the movement of air, the greater is the evaporation. The movement replaces the moisture filled layer by a drier layer which has a larger capacity of absorbing moisture.
On the basis of formation, clouds are classified into two main classes :
(i) Cumuliform or Heap Clouds : They are of great vertical height.
(ii) Stratiform or Layer Clouds : They have layered structure.
On the basis of the height, clouds are classified into three classes. They are :
(i) Low Clouds : They are formed upto 2000 metres. Stratocumulus, nimbostratus, cumulus, cumulonimbus and stratus are well known low clouds.
(ii) Medium Clouds : They are formed upto 6000 metres. Alcocumulus and altostratus clouds are the well known medium clouds.
(iii) High Clouds (5-14 km) : Stratus clouds are layered clouds and with uniform layers. These clouds are formed mainly during winter and cause drizzle.
Convectional precipitation or rainfall :The convection currents are normally set up in the atmosphere as a result of local heating. The currents rise up at the centre and are drawn up at the sides. The very rapid rising of the warm moist air results in the formation of cumulonimbus clouds which may be several kilometres in depth and may give very heavy rain, but the rainfall does not last long. We speak of it as "thunder showers" rather than thunder rain.
Orographic precipitation:This is the most widespread form of rainfall. It occurs when mountains stand in the part of moisture laden winds and force them to rise. As the air rises, it expands and cools and the rain falls. The resultant rainfall may often be heavy especially where a high continuous mountain barrier lies close to a coast and the rain bearing winds meet it at right angles. Heavier rainfall occurs on the windward side; Cherrapunji in Meghalaya lies on the windward side of the Khasi hills and has an average of over 1,000 cm of rain. Similarly, the Konkan region on the windward side of the Western Ghats receives over 250 cm of rain. As the air crosses over to the other side (leeward side), it loses most of the moisture and becomes warm and dry in descending and their is less rain. The leeward side of the mountain is called as such a Rain Shadow area. The Deccan region is said to lie in the rainshadow of the Western Ghats.
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