Fundamental Of Physical Geography Chapter 11 Water In The Atmosphere

Following are the characteristics of the zonal distribution of relative humidity:

i. It is the highest at the equator and decreases towards poles.

ii. It is minimum in subtropical regions.

iii. It increases from 30°, north and south towards poles, because of the decrease in temperature.

iv. The belt of relative humidity shifts north and southwards with the annual movement of the sun.

Precipitation is a process by which condensed water vapours in the atmosphere fall to the earth in the form of rain, snow, hail, or sleet. Hence rainfall, snowfall, hail-stones and sleet are the various forms of the precipitation.

Necessary conditions for the precipitation : There are three necessary conditions for the precipitation. They are :

(i) There should be evaporation, so that the relative humidity is high in the air.

(ii) There should be adiabatic cooling of the moist air.

(iii) There should be dust particles in the air.

Three types of precipitation are:

i. Rainfall
ii. Snowfall
iii. Hailstone.

1. Cirrus: These are formed at high altitudes (8,000–12,000m).

2. Cumulus: They are formed at a height of 4,000–7,000m. They look like cotton.

3. Stratus: They are layered clouds covering large portions of the sky.

4. Nimbus: These are black or dark grey rain clouds. They are very near to the earth’s surface.

There are three types of humidity. They are explained below:

The process by which the water vapours in the atmosphere change into droplets or ice crystals is called condensation. It takes place when the air is saturated with vapour.

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.

The features are:
i. Cumulus clouds look like cotton wool.

ii. They are generally formed at a height of 4,000 - 7,000 m.

iii. They exist in patches and can be seen scattered here and there. They have a flat base.

The rainless area on the leeward side of a mountain is called the rain shadow. 

Areas:
(i) The plateau of the Deccan is in the rain shadow of the Western Ghats.

(ii) The plateau of Tibet is in the rain shadow of the Himalayas.

The salient features of the world distribution of precipitation are discussed below:

Following are the forms of condensation:
1. Dew 2. Frost 3. Fog 4. Clouds.
Dew: When the moisture is deposited in the form of water droplets on cooler surfaces of solid objects such as stones, grass blades and plant leaves, it is known as dew. The ideal conditions for its formation are clear sky and calm air, high relative humidity and cold and long nights. It is also necessary that the dew point is above freezing point.

Frost: When the dew point is below freezing point the water vapour will accumulate on the earth’s surface in the form of small white ice particles. The formation of the ice particles over a large area is called frost. The frost is formed when the temperature of the air falls rapidly so that the water vapour present in the air is directly turned into solid particles without turning into liquid state.

Clouds are formed mainly by the adiabatic cooling of air below its dew point. This cooling process is most effectively created by upward movement of light and warm moist air, which in turn reduces pressure, expands and reaches its dew point. With further cooling below dew point, condensation takes place in air forming clouds.

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.

The only difference between the mist and fog is that mist contains more moisture than the fog.

Sleet:
i. Sleet is frozen raindrops and refrozen melted snow-water.

ii. When a layer of air with the temperature above freezing point overlies a subfreezing layer near the ground, precipitation takes place in the form of sleet.

iii. Raindrops, which leave the warmer air, encounter the colder air below. As a result, they solidify and reach the ground as small pellets of ice not bigger than the raindrops from which they are formed.

On the basis of the total amount of annual precipitation, major precipitation regimes of the world are identified as follows.

i. The equatorial belt, the windward slopes of the mountains along the western coasts in the cool temperate zone and the coastal areas of the monsoon land receive heavy rainfall of over 200 cm per annum.

ii. Interior continental areas receive moderate rainfall varying from100 - 200 cm per annum.

iii. The coastal areas of the continents receive moderate amount of rainfall. The central parts of the tropical land and the eastern and interior parts of the temperate lands receive rainfall varying between 50 - 100 cm per annum.

iv. Areas lying in the rain shadow zone of the interior of the continents and high latitudes receive very low rainfall-less than 50 cm per annum.

i. Cloud : It is a mass of small water drops or the ice crystals formed by the condensation of water vapour in the free air at considerable heights above the earth’s surface. They are mostly formed in the upper regions of the atmosphere.

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.

Fig. 11.3 : Convectional Precipitation

Convectional rainfall is common within the tropics, especially in the equatorial belts where it occurs almost every day in the afternoons. Similarly, the summer rain in the Steppes and Prairies is largely of this type but the amount of rainfall is low as these areas are far away from the sea.

Cyclonic rainfall or precipitation : When hot and hurried airmass meets a cloud, the warm air tries to enter the cold air at the frontal region but the cold air pushes the warm air from below. This moves the warm air on to the cold airmass and warm air begins to ascend. The water vapours of the warm humid airmass and warm airmass are condensed in the ascent of the airmass and rainfall occurs.

This rainfall is due to the convergence of air masses.

Fig. 11.4 : Cyclonic Precipitation

There is a low pressure area at the centre of cyclones. The air from the surrounding high pressure area moves encircling towards the centre. This air does not find place to stay at the centre due to the cyclonic rotation and therefore has to rise in the atmosphere. Condensation by ascent causes rainfall.

condensation

1. It is the amount of actual water vapour present per unit volume of air.
2. The absolute humidity is measured in terms of grams per cubic metres of air.

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.

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.

 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.

2. The coastal areas of the world receive greater amount of rainfall than interior of the continents.

3. The rainfall is more over the oceans land mass.

4. The rain is heavier between 35° to 40° latitudes in both hemispheres.

5. The rain is heavier on eastern coasts and decreases towards the west.

6. The rain is greater on the windward side than the leeward side.