What causes dew, and what makes it...

Air can hold only a certain amount of water vapor. This amount varies according to the temperature of the air: The warmer the air, the more water vapor it can hold. When the air reaches its saturation point, the water vapor begins to condense — that is, form a liquid. If the condensation occurs at ground level, the small droplets that are formed are called dew. When surface temperatures are below freezing (32 degrees Fahrenheit), the water vapor turns into ice crystals. If dew has formed before the drop in temperature, the water droplets simply freeze. Both types of ice formation are called frost. It is the latter type that is so dangerous for new plant life.