Dew is the name given to the glistening beads of water that often appear on blades of grass, leaves, and car tops early on clear mornings. Dew forms when air near the ground cools to the point where it cannot hold all its water vapor. The excess water vapor then condenses (changes to liquid) on objects near the ground.
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During the day, objects absorb heat from the sun. At night, they lose this heat through a process known as thermal radiation. As objects near the ground cool, the temperature of the air immediately surrounding them is also reduced. Colder air cannot hold as much water vapor as warmer air can. If the air continues to cool, it will eventually reach the dew point. The dew point is the temperature at which the air contains as much water vapor as it possibly can hold (see Dew point). If the air cools further, some of the vapor condenses on the nearest available surface.
Dew forms best on calm, clear nights. When the wind is blowing, air cannot stay in contact with cool objects as long and it needs more time to cool to the dew point. When it is cloudy, objects cool more slowly because the clouds radiate heat back to earth. Dew also forms better when the humidity is high.
Dew evaporates as the sun rises. The sunshine heats the ground, which in turn warms the air. This warmer air is able to hold more water vapor, and dew evaporates into this air.
When ordinary dew forms at the dew point and then freezes, it is called frozen dew or white dew. Frost forms when the dew point is below freezing, causing excess water vapor to freeze directly onto objects near the ground.
