Notes

For the water cycle to work, water has to get from the Earth's surface back up into the skies so it can rain back down and ruin your parade or water your crops or yard. It is the invisible process of evaporation that changes liquid and frozen water into water-vapor gas, which then floats up into the skies to become clouds.

The need for heat: As usual, you can thank the sun for keeping evaporation going. The sun's energy breaks the bonds that hold liquid water's molecules together. Thus, it makes sense that water evaporates more easily in hot conditions, such as in the desert, rather than on a cloudy winter day.

Humidity: The amount of water vapor in the air is known as "humidity", often mentioned in the weather reports on the local news. When warm air containing a lot of water vapor (high humidity) moves into colder temperatures (either high in the atmosphere or even on the outside of your glass of iced tea), the colder temperatures cause water vapor to condense into a liquid. The colder temperatures cause water vapor to condense into water droplets and can result in fog.