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The air contains water vapor in addition to the gases we have mentioned. This will vary from a very small amount up to a maximum of four or five percent. The air gets this moisture by evaporation from bodies of water and from vegetation. The capacity of air for holding water vapor is directly related to the temperature of the air; the warmer the air, the more water vapor it can hold. When the air contains all the vapor it can for its temperature and pressure, it is said to be saturated. Weather forecasts provide us with the relative humidity which is the ratio of amount of water vapor which a sample of air holds to the amount it can hold when saturated (see figure 2-5). It may be surprising to you that water vapor (composed mainly of hydrogen) is lighter than air (composed mainly of nitrogen). Thus, air which feels damp and heavy is actually lighter than dry air. Its density is also less.
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