Rafed English

The Formation of Rainfall

Besides moderating the temperature,winds also displace the humidity resulted from the seas and damp areas.In order for these particles of steam to return to the Earth,they should turn into liquid and in the form of heavy drops,so that they can get out of the clouds and then fall to the ground.This requires a drop in the temperature and also hypersaturation of dampness in the air.The drop in the temperature is usually brought about as a result of the climb of the currents of air,or because of the neighbouring and mixing with cold currents of air.The accumulation and piling up of damp currents also cause the state of hypersaturation of dampness in a region. Of course,althogh the drop in temperature and high humidity are essential for the rainfall to happen they are not enough because in such a case no clouds could ever evolve since liquid drops would immediately separate from the air and fall to the ground,and being light enough,the fine drops would also remain suspended in the air may be for days and vaporize after being heated again.Another important factor is the existence of nuclei of distilation.Wind and waves cause the sea water to spray into the air in fine droplets. After the evaporation of these droplets fine particles of salt remain suspended in the air.These salt particles can absorb humidity, and are infact centers for the accumulation of humidity and the formation of rain drops. To some extent dust and soot particles also play the same role.The contact and the mixing of these particles also accelerate the process of the development of raindrops. Bergeron the transfer of water vapor to the solid surface of ice particles is also another factor which causes the formation and the acceleration in the development of raindrops.

Rains are classified on the basis of such criteria as the type of cloud they originate from the kind of effective winds and the diameter of drops.

For example,on the basis of the winds affecting the rainfall,three types of rain are recognized,

1- when the climb of the wind is resulted from it contact with ground ridges,that is highmountains, the rain evolved is called mountainous rain,which may be of two kinds: simple or beach mountainous.

2- whenthe rain forms following the contact between the two cold and warm fronts of wind , the possible rainfall is called frontal rain.

3- when the warm area and the humid warm air cause a disturbance leading to rainstorms, the rain is called storm rain.

There has also been a classification made on the basis of the type of cloud originating rain and the diameter of raindrops.For example,drizzle evolves from nimbus clouds which are near the ground and in a quiet sky.Rainstorm is produced from cumulonimbus clouds which are in high altitudes in this kind of rain the diameter of the drops gets bigger in the course of rainfall.Very heavy rains known as showers fall briefly and intermitenty,but an abundant amount of water gets to the ground.

Other forms of precipitation such as snow and hail might evolve depending on the condition under which the rain falls.If the temperature of the land on which,it rains drops a certian amount because of a current of cold air or elevation ,on their way the rain drops freeze and turn into small solid crystalized particles which one generally hexagonal and are know as snow.Hail forms when falling raindrops pass through a very cold air stratum and hence freeze.

Adapted from the book : "Water"

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