I am absolutely sure that <span>the percent of the Sahara Desert is covered by sand sheets and dunes is definitely 25 %. Even though it might seem weird, it is actually correct and here are the pic from artcicle about Sahara which will pose as a proof : )
Do hope it will help!
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The correct answer is - (1) rising expanding cooling condensation.
At first there's the evaporation of water, since the water vapor is less dense than the air it rises up and starts to expend as it goes further upwards in the atmosphere. Once it reaches the upper layers of the troposphere than the cooling process starts because the temperature is decreasing with the altitude. Because of the cooling the process of condensation will start taking place and thus the formation of clouds were the water vapor returns to liquid form and creates rain. The cycle repeats its self all the time.
It is called a river basin
