MIKE MOSS SAYS: Katy, The key word in your question is "necessarily," as simply changing the temperature of a given parcel of air does not affect the number of water vapor molecules present. However, if the air is nearly saturated, and you cool the air to its dew point temperature or below, then some of the water vapor will condense into liquid water droplets, reducing the amount of vapor remaining, and if seom of the droplets settle out of the air, reducing the total water in the parcel.
Note that warming the air (with no other changes) will not change the amount of water vapor, but will lower the "relative humidity." The dew point, on the other hand, will not change as long as the amount of water vapor remains the same. This is one reason we often prefer to use dew point as an indicator of how moist air is, as opposed to using relative humidity.
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