There exists a close balance between atmospheric humidity and air temperature. At any given point of time the capacity of air for water vapour depends upon its temperature. The moisture-holding capacity of air increases with increasing temperature.
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It is also to be noted that at higher temperatures the capacity to hold moisture increases at a much faster of rate. The moisture holding capacity increases slowly at low temperatures, and more and more rapidly at higher temperatures.
It is true that the amount of water vapour in the atmosphere undergoes large fluctuations. However, there is an upper limit which the moisture content of atmosphere does not exceed.
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The amount of water vapour that a given parcel of air can hold at a given temperature is referred to as its capacity. When the air at a particular temperature has water vapour to its capacity, it is said to be saturated. The concept of saturation may be further explained with reference to vapour pressure.