Eddie Tooley, age 11, of Providence, R.I., for his question:

WHAT IS HUMIDITY?

Humidity is a term that descries the amount of water vapor in the air. We say that the humidity is high when the air contains large amounts of moisture. We say it is low when the air contains only a little moisture.

When the air holds as much moisture as possible at a certain temperature and pressure, the air is saturated or at the dew point.

At saturation, the temperature and the dew point are the same.

The amount of humidity varies according to the temperature and location. The warmer the air, the more moisture it is able to hold. As an example, at normal atmospheric pressure, an increase in temperature of about 20 degrees Fahrenheit doubles the air's moisture capacity.

Saturated air at 65 degrees Fahrenheit contains twice as much moisture as saturated air at 45 degrees Fahrenheit.

The lower air over oceans is almost always saturated. But far inland and over deserts, the air may be very dry. Usually, air is not completely filled with vapor unless it is raining, snowing or foggy.

The amount of water vapor in the air compared to the amount required for saturation is called the "relative" humidity. If the air contains only half the amount of vapor that it can hold when saturated, the relative humidity is 50 percent.

The amount of moisture in the air can be determined in various ways, but generally a form of hygrometer is used. One kind of hygrometer uses two thermometers. One thermometer, called a dry bulb thermometer, gives the air pressure. The bulb of the other thermometer is wrapped in a piece of wet cloth. This is the wet bulb thermometer.

The wet bulb thermometer gives a lower reading than the dry bulb because evaporation of the water in the cloth cools it. More water evaporates when the humidity is low than when it is high.

The lower the humidity, the lower the wet bulb thermometer will read.

To determine relative humidity, before your test you must first allow the hygrometer to stand for 15 minutes in a breeze away from direct sunlight.

To continue the test, read both thermometers. If the dry bulb thermometer reads 70 degrees Fahrenheit, and the wet bulb reads 60 degrees, the relative humidity is 55 percent.

Humidity affects our comfort and health. A person feels "sticky" and uncomfortable when the humidity is high because the perspiration of the skin does not evaporate easily.

But too little humidity is bad for the health. Many homeowners use humidifiers to put water vapor into the dry air.