Competitive Exams: Types of Rainfall

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Conventional Rainfall

Occur due to thermal convention currents caused due to insolational heating of ground surface.

Prevalent especially in equatorial areas;

  • Warm air rises up and expands, then reaches at a cooler layer and saturates and then condenses mainly in the form of cumulus or cumulonimbus clouds and normally precipitation takes place in the second half of the noon;

  • Also rains in the tropical, subtropical and little in temperate regions;

  • But there must be two pre conditions:

  • Abundant supply of moisture through evaporation to air so that relative humidity becomes high;

  • Intense heating of ground surface through incoming shortwave electromagnetic solar radiations.

Features of Conventional rainfall

  1. It occurs daily in the afternoon in the equatorial region.

  2. It is for very short duration but occurs in the form of heavy showers.

  3. They make Cumulonimbus clouds.

  4. In hot deserts it is not regular, but is irregular and sudden.

Cyclonic or frontal rainfall

  • Occur due to upward movement of air caused by convergence of extensive air masses

  • It happens due to the convergence of two different air masses with different temperature. The worm air rises over cold air and cyclonic rain occurs;

  • Cold air pushes up warm air and the sky is clear again.

Orographic Rainfall

Orographic Rainfall occurs due to ascent of air forced by mountain barrier. The preconditions are:

  • there should be mountain barrier across the wind direction, so that the moist air is forced on obstruction to move upward e.g. Aravali in Rajasthan is parallel to Arabian Sea and thus forms rain shadow area;

  • there should be sufficient moisture in the air;

  • the height of the mountain also affects rainfall; and

  • if the height is more but more distance from sea, lesser rainfall; if the height is less but nearer to sea, more rainfall.

Features of Orographic rainfall

  • The windward slope. E.g. Mangalore is located in the western windward slope and receives 2,000 mm of rainfall, whereas Bangalore is in rain shadow area and hardly receives 500 mm. rainfall. Similarly Coast Ranges of North America receives 2,000 mm. but eastern slope doesn't.

  • The maximum rainfall occurs near the mountain slope and decreases away from the foothills. E.g. in Shimla, 1520 mm.; Nainital, 2,000 mm. and Darjeeling receives 3150 mm. rainfall because Darjeeling is nearest to Himalayan slopes. Patna 1000 mm., Allahabad 1050 and Delhi 650 mm.

  • If mountain is of moderate height, the maximum rainfall doesn't occur at the top rather it occurs on the other side.

  • Cumulus clouds while the leeward side by Stratus clouds characterizes the windward slope of mountain at the time of rainfall.

  • The amount of rainfall increases with increasing height along the windward slope of mountain up to a certain height but the amount of rain decreases with increasing height because of marked decrease in the moisture content of air. This situation is called 'inversion of rainfall';

  • This type of rainfall may occur in any season. Inversion Point: maximum rainfall line is at 24,000 feet or 7,000 m. at the equator whereas in the Himalayas, it is 12,000 feet or 3600 m.; at Alps 21,000 or 6,300 m. and at Pyrenees mountain 18,00012,000 feet.

Distribution of Rainfall

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  • Rainfall is related with air temperature and atmospheric humidity, while humidity is closely related with temperature through the process of evaporation.

  • The regions having high temperature and abundance of water receive higher amount of rainfall e.g. Equatorial regions.

  • SubTropical regions also have the same conditions but the western parts receive least rainfall due to anticyclonic conditions.

  • Mean annual rainfall for the whole globe is 970 mm. but is unevenly distributed.

  • Some places receive less than 100 mm. of rainfall e.g. Hot deserts like Kalahari, Thar etc., while some receive more than 12,000 mm. like Cherrapunji in India.

  • The equatorial regions receive rainfall throughout the year, while the other areas have seasonal rainfall.

  • The Mediterranean region receives most of the annual rainfall during winters.

Other Forms of Precipitation

  1. Ice: If the temperature of the entire atmosphere is below 0 Equation , the condensation will lead to ice formation and snowfall.

  2. Snowfall: The fall of larger snowflakes from the clouds on the ground surface is called snowfall. It occurs when the freezing level is less than 300m from the ground surface. These crystals reach the ground , without being melted in a solid , form of precipitation as snow

  3. Sleet: In UK it refers to a mixture of snow and rain but in American terminology it means falling of small pellets of transparent and translucent ice having a diameter of 5 mm. or less.

  4. Hail: It consists of large pellets or spheres of ice. In fact hail is a form of solid precipitation wherein small balls or pieces of ice, Known as. Hail or stones, having a diameter of 550 mm fall downward as hail storms! They are very destructive and dreaded form of solid precipitation because they destroy agricultural crops and claim human and animal life. After condensation, if the temperature is below 0 degree C, than the water drops would take the form of hails.

  5. Drizzle: The fall of numerous uniform minute droplets of water having diameter of less than 0.5 mm. is called drizzle. They fall continuously from low stratus clouds but the total amount of water received on the ground surface is significantly.