Class 7 Geography
Rains are highly important for all life forms. How does the rain come? What are different types of rainfall? What is cyclone? How devastating a cyclone can be?
Evaporation causes formation of water vapor and condensation causes formation of droplets of water. Condensation occurs when the water vapour cools on reaching higher altitude.
Clouds are masses of such water droplets. When theses droplets become too heavy to float in the air, they come down as precipitation. Precipitation can be in the form of: Rain, Snow, Sleet or Hail.
Precipitation that comes down to the earth in the liquid form is called rain. Most of the groundwater comes from rain. A rain gauge measures the amount of rainfall. When trees are cut in the hilly areas, rain water flows down and floods the low lying areas. Now it is clear why we should not cut trees and strive to plant more and more of them.
On the basis of the mechanism, there are three types of rainfall, as shown below.
When a portion of the earth becomes heated more than its surrounding, it creates significant amount of evaporation. Convection currents then push up the moist air and cumulonimbus clouds are formed. These clouds result in convectional rainfall. Convectinoal rainfall is in the form of showers with rapidly changing intensity. This type of rainfall happens for a short duration because horizonatl expanse of a cumulonimbus cloud is small.
This type of rainfall is formed by frontal system surrounding extratropical cyclones. Cyclone is formed when warm air meets cold air. Stratiform rainfall usually occurs from nimbostratus cloud.
Orographic rainfall occurs when a mass of air is pushed up because of a structure with very high elevation, such as a mountain. The lifted air cools down and precipitation occurs. Rain shadow area is formed on the other side of the elevation.
Rainfall is essential for the mere survival of plants and animals. It brings fresh water to the earth's surface. If it is less, we face water scarcity and resultantly severe drought. In 2014 there was scantly rainfall in India and many areas were declared drought–hit. If rains are too much, floods take place which also causes severe devastation.
Orissa, located on the eastern seacoast of India is prone to cyclones that originate in the Bay of Bengal. In October 1999, cyclone hit five districts of the state. There was large scale devastation. The damages caused were mainly due to three factors: Wind velocity, Rain and Tidal surge
Wind Velocity: Winds of upto 260 km. per hour lasted for about 36 hours. These winds uprooted trees and damaged the kutcha houses. Power supply and telecom line snapped completely.
Rain: Heavy rain occurred under the influence of cyclone for three days and led to the flooding of major rivers.
Tidal Surge: The cyclonic winds caused tidal waves that swept 20 km inland and brought massive destruction to the coastal areas.
About 13 million people were affected. A large number of livestock were killed. Standing crops of paddy, vegetables and fruits were severely damaged. Large tracts of agricultural lands have turned infertile due to the salinisation caused by the surge. Large tracts of sal, bamboo and teak plantations have disappeared. The mangrove forests between Paradip and Konark vanished.
Global Warming: It is the increase in temperature when heat retention takes place due to an increase in the level of carbon dioxide in the atmosphere.
Loo: Hot and dry local winds of the northern planes of India.
There is no air and hence no air pressure on the moon.
Copyright © excellup 2014