dew point
Youve probably noticed dew on the grass on a summer morning. Why does dew form? Remember that the land heats up and cools down fairly readily. So when night comes, the land cools. Air that was warm and humid in the daytime also cools over night. As the air cools, it can hold less water vapor. Some of the water vapor condenses on the cool surfaces, such as blades of grass. The temperature at which water vapor condenses is called the dew point . If this temperature is below freezing, ice crystals of frost form instead of dew Figure below . The dew point occurs at 100 percent relative humidity. Can you explain why?.