The two reasons that this layer ( Troposphere ) will contain much water vapor are :
The troposphere contains so much of the water vapor present in the atmosphere due to gravity and the compressibility air. As we move higher above the earth surface, the density of the atmosphere decreases rapidly. causing the majority of water vapor to be retained at the troposphere. which is lowest part of the atmosphere closest to the earth surface.
Hence we can conclude that The two reasons that this layer ( Troposphere ) will contain much water vapor are : Altitude and Air compressibility.
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