Because the vapor in the air is condensed into clouds by the cold at the top of the mountain.
When is a cloud formed in the air?
When a warm, damp wind meets with a cold wind, its vapor is condensed and a cloud is formed.
Are clouds often formed in this way?
Yes; clouds are often quickly formed in this way.
When the cloud passes into a warmer current of air, what becomes of it?
It is changed back into vapor, so that it cannot be seen.
How then can a cloud in the sky disappear very quickly?
By passing into a warmer current of air the cloud is changed into a vapor, and may thus quickly disappear.
Are clouds of different heights?