Water vapour is one of the most variable gases in the atmosphere, which is present in small amounts, but is nonetheless very important. Water vapour is always present in some proportion in the lower atmosphere.
The water vapour content of air may vary from .02 percent by volume in a cold dry climate to nearly 4 percent in the humid tropics. The variations in this percentage over time and place are very important considerations climatically.
The significance of atmospheric moisture to all forms of life is a well-known fact and hardly needs any elaboration. Like carbon dioxide, water vapour plays a significant role in the insulating action of the atmosphere.
In addition, it absorbs not only the long wave terrestrial radiation, but also a part of the incoming solar radiation. Thus, it is an important control in regulating the energy transfer through the atmosphere.
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Water vapour is the source of all clouds and precipitation. Through the condensation of water vapour, vast amount of energy is released into the atmosphere in the form of latent heat of condensation, the ultimate driving force for most of the storms.
The most important as well as the most interesting thing about water vapour is that about 90 percent of it lies below 6 kilometers of the atmosphere. It is estimated that only less than 1 percent of the total atmospheric moisture in the atmosphere is found above 10 kilometers.
When the maximum possible quantity of water vapour that the air can hold at any given temperature and pressure is present, the air is said to be saturated. But the moisture holding capacity of the air varies in direct proportion to its temperature.
In other words, the higher the temperature of the air, the larger the capacity to hold moisture.