Maurice Lane, Jr., age 13, of Providence, R.I., for his question:

WHAT IS HUMIDITY?

Humidity is a term that describes the amount of water vapor in the air. When the air contains large amounts of moisture, the humidity is said to be high. It is said to be low when the air contains only a little moisture.

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 temperature. 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. Therefore, the lower the humidity, the lower the wet bulb thermometer will read.