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When as a result of the exchange of heat and/or upward movement and exapansion,the temperature of the air decreases so much that the relative humidity reaches %001, a portion of the steam molecules in the air becomes dense ;this densitiy emerges following cloud formation in this state water has no longer a gaseous form , but changes to liquid droplets or tiny suspended solid crystals.Cloud can be considered as a collection of liquid droplets or tiny crystals of ice immediately after being formed.The diameter of these droplets varies from 2 to 100 microns usually 10 to 25 microns ,and their number in one cubic centimeter amounts to 1000 to 2500.Almost all clouds are formed in the troposphere layer upto 12 km.from the ground level.

For the recognition and classification of clouds it must first be determined how clouds are formed in a specific time and also how they are transformed and scattered.In general all clouds are formed as a result of the occurance of density in the lower layers of the atmosphere.This happens because the humid air climbs to the upper and middle layers of troposphere . Different kinds of vertical movements which take place in the atmosphere are as follows :

1- mechanical disturbances,
2- convection ,
3- mountainous climb ,

4- soft graduall climb when the ascending current of air is quick,the clouds produced have a high vertical development and density . On the contrary , if the ascending current of air proceeds moderately and slowly , the clouds thus produced will form layers which depending on the amount of density , with thick or thin strata.

Meteorologists divide clouds into four groups usually on the basis of height :

1- upper clouds,
2- middle clouds,
3- lower clouds,

4- clouds which have vast vertical developments the height of these clouds in areas with average latitude is 5 to 13 km. from the ground level for upper clouds, 2 to 7 km. for middle clouds and atmost 2 km for lower clouds.

Among all major groups of clouds,two main ones are of immense importance,the group known as Stratus clouds,is evolved in the form of strate in middle troposphere,when the existing steam cools and becomes dense, the other group known as Cumulus forms following the quick ascending local current of humid air.In these clouds there is great distance between the upper and the lower strata.

The international cloud Atlas issued by World Meteorological Organization WMO contains detailed information regarding the classification of clouds.According to the ratification by WMO,clouds have been divided into 4 families and 10 independent typesthere has recently been some important changes in the first international cloud atlases such as the deletion of nimbuses and the addition of nimbostratus.

Generally,the main types of clouds are as follows :

1 - Cirrus , ci ,
2 - Cirrostratus , cs
3 - Cirrocumulus,cc ,
4- Altostratus ,as
5 - Altocumulus , ac ,
6 - Stratus , st
7- Stratocumulus ,sc,
8- Nimbostratus,ns
9 - Cumulus , cu ,
10 - Cumulonimbus,cb. At present cloud divisions have become very vast and complicated.The division of clouds is based on such criteria as factor affecting their formation , the shape and type of precipitation resulting from them.

Adapted from the book : "Water"

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