Notes-Class 9-Geography-Chapter-5-Precipitation-Maharashtra Board

Topics to be Learn : Introduction Precipitation Snow Hail Rain Convectional Rainfall Orographic Rainfall Cyclonic Rainfall Distribution of Rainfall in the World Fog, Dew and Frost Effects of Precipitation Acid Rain

Introduction :

• 70.8% surface of the earth is full of water. The distribution of water is very uneven. At some places, the water storage is limited while it is ample at other places.
• Changes in the forms of condensation observed as the atmospheric conditions changed.
• Snow, rain, hails, fog, dew and frost are some of the important forms of precipitation.
• In areas located at higher altitudes, snowfall occurs while it rains in other places.

Precipitation :

• Water falling either in solid state or in liquid state from the clouds to the ground is called precipitation.
• Snowfall, rainfall, hailstorm, fog, dew and frost are the various forms of precipitation.

Snow :

Sublimation : The process of gaseous water i.e. water vapour directly transforming into solid snow is called as sublimation.

When the temperature falls below freezing point, water vapour directly turns into snowflakes through the process of sublimation.

Such precipitation of solid water in form of snow particles is known as snowfall.

Hail :

Formation of hails :

• When there is lot of heat on the earth’s surface, the upward air flow blows at a greater speed.
• Because of this upward flow, the temperature of the air reduces and the condensation of the water vapour takes place.
• Dark clouds are formed.
• Because of the upward movement of air, these water droplets go to a higher altitude. Here, solidification of these droplets occur and hailstones are formed.
• Hails are hard and big.

Concentric Layers : When the hailstones are formed, they move up and down continuously because of upward movement of air currents. As they go to higher altitude, a new layer of ice gets deposited around it. It happens repeatedly and such layers of ice in hail are called Concentric Layers.

• These big heavy hailstones fall rapidly to the ground due to gravity and such precipitation of hails is called hail showers or hail-storms.
• Crops gets destroyed and loss of life and property also occur due to hail-storms.
• The phenomenon of hail-storms is not seen in equatorial region as the temperature is very hot there.
• The phenomenon of hail-storms is also not seen in cold latitudes as there are no vertical movement of air current.
• During the hot summer, hail-storms are normally seen in the region of South-east Asia, India and Africa.

Q. Why Hail-stones do not occur frequently.

Rain :

Three Types of rain

• Convectional rain
• Orographic rain
• Cyclonic rain

Convectional rain :

• In Equatorial region very high temperature generated due to a perpendicular sun rays throughout the year and land surface gets heated very fast.

Click here to View Figure-1 

• Due to very high temperature, the evaporation accelerates.
• Air becomes moist and light and moves upward in the atmosphere.
• At higher altitudes, the moisture holding capacity of the air decreases and this leads to condensation and rain falls.

This type of rain is known as convectional rain.

• In the equatorial region, the land gets heated very fast by the afternoon and convectional process begins. This results into heavy rainfall every day in the afternoon or evening.
• As compared to land surface, such convectional movement of air does not take place on sea surface as water gets heated very slow and therefore convectional rain does not fall on the sea areas of equatorial region.

Orographic rainfall :

When the phenomenon of rainfall occurs due to obstruction of the mountains, it is called as orographic rainfall.

• Moist air approaching from lake / sea / water body gets obstructed by high mountains. Such winds then move upwards along the slope of mountain. At higher altitude the condensation process takes place and windward side and mountain top receives abundant rain. This type of rain is known as orographic rain.

Rain Shadow region :

• Moist air approaching from water bodies towards land gives more rainfall on the windward side of mountain.
• After crossing the mountain-top, when such winds go to the other side of the mountain, winds are dry and their moisture holding capacity also increases.
• The leeward side of the mountain thus, gets less rainfall and such area is called a rain shadow region

Cyclonic rain :

Click here to View Figure-3 

Cyclone is formed when the air from high pressure areas moves in circular motion towards the centre of very low air pressure. As the air in circular motion moves upward very rapidly, at high altitude condensation process takes place followed by rain. Such rain is known as cyclonic rain.

Distribution of rainfall in the world :

Global distribution of rainfall is uneven. In general, tropical region receives more rainfall. However, local factors like coastal location and the regions under trade winds receive more rainfall.

Regions with high, moderate and less rainfall are as follows :

• Regions of high rainfall : Equatorial region, monsoon region of Southeast Asia, North Western Europe, Eastern Australia and East and West Coast of America receive high rainfall.
• Regions of moderate rainfall : Areas adjacent to high rainfall and coastal regions of the world receive moderate rainfall.
• Regions of less rainfall : Polar zones, deserts and all interiors of continents receive less rainfall.

Besides these areas, regional variations due to specific reasons are also prominent in rainfall distribution of the world.

Know This : 1 mm rainfall means, if we do not let the rain water to percolate, run off or even evaporate and if this water is spread on the flat surface uniformly, the height of this rain water is 1 mm. If the water of 1 millimetre rainfall on 1 square kilometre area is collected, it will give us 10,00,000 litres of water. Layer of 120 mm of ice is equivalent to 10mm of rainfall.

Fog, Dew and Frost :

When the condensation or solidification of water vapour in the atmosphere takes place near the surface, then one can see condensation in the form of fog, dew and frost.

• Fog : Due to less temperature near the surface, water vapour condenses into microscopic water particles and float in the air. Such microscopic water particles near the surface with very high density is called fog.
• Dew : When moist air near the earth’s surface comes in contact with a very cold object, condensation of water vapour takes place and turns into very small water droplets. These water droplets stick to such cold surface. This form of precipitation is called dew.
• Frost : If the temperature of the air is less than zero degree, water droplets stuck to the surface of cold objects freeze. Such frozen water droplets is called frost.
• Dew and Frost occur on a large scale in winters.

Acid rain :

• Precipitation of water with dissolved acids is called acid rain.
• Various gases from polluted air gets mixed in water vapour and reacts chemically to form nitric acid, sulphuric acid, etc.
• Acid rain is very harmful to living organisms as well as non-living objects.