How does temperature affects the amount of water vapor in the air?

Answer 1

The warmer the temperature, the more water vapor in the air. The colder the temperature, the less water vapor in the air.

Answer 2

Water vapor accounts for the majority of the "green house effect" for the atmosphere - accounting for about 95% of Earth's greenhouse effect. The impact is probably most notable when comparing daytime and night-time temperatures for desert areas (with low atmospheric water vapor levels) and very humid areas - such as near rivers, lakes and oceans (which have high atmospheric water vapor levels.) In Death Valley night temperatures will be about 20-30 degrees cooler than in the daytime. Monterey, CA, which is about 300 miles due west only has temperature swings half that size. At higher altitudes, the air is less dense and can hold less water vapor. That is one reason higher altitudes tend to be colder and have bigger temperature swings than lower altitudes. There is also the fairly obvious effect that when there is more water vapor, clouds are more likely to form, which keeps things cooler during the day and holds in more warmth at night after the sun goes down.