Christi Ventura, age 14, of Johnson City, Tenn., for her question:

HOW DOES DEW FORM?

Dew forms when moist air is cooled by direct contact with cold objects out in the open.

Such things as blades of grass, leaves, or outside wires receive heat from the sun during the day by direct radiation. The heat also evaporates moisture into the air. These objects lose the heat again at night, also through radiation.

Since radiation is most effective on clear nights, objects in the open cool down faster when the sky is clear than when it is cloudy. As the objects cool, the air next to them cools too. When this air reaches the dew point, it can no longer hold all the moisture present. It deposits this excess moisture as dew.

When the temperature falls below the freezing point, frost forms' instead of dew.

When the wind is blowing, the air cannot stay in contact with cool objects long enough to cool to the dew point, so dew forms best on still nights.

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