Mary Jean Smith, age 13, of Laconia, N.H., for her question:

WHAT IS DEW POINT?

Dew point is the temperature at which moisture in the air begins to condense. The dew point is either lower than the air temperature, or the same as the air temperature, when the relative humidity is 100 percent.

Dew forms when a thin film of air, in contact with the surface, is cooled to below the dew point. Cooling the air below dew point causes dew on the surface, or fog in the air, when the dew point is above the freezing temperature.

If the air temperature and dew point are below freezing, frost may form on the surface, or ice crystals may form in the air. Fog and clouds occur when large volumes of air are cooled to below the dew point.

In some places where rainfall is light, dew is important to plant growth. In Lima, Peru, for example, dew supplies more water than rain. Dew is often heavy in tropical areas where the air is moist and the nights cool.