Mark Woodward, age 17, of Florence, S.C., for his question:

HOW IS THE WIND CHILL FACTOR DETERMINED?

Wind is caused by the uneven heating of the atmosphere by energy from the sun. The sun heats the surface of the earth unevenly and air above hot locations expands and rises. Air from cooler areas then flows in to take the place of the heated air in a process called circulation. This general circulation of the air can then develop into wind.

Wind is actually air moving across the earth's surface. Wind may blow so slowly and gently that it can hardly be noticed or it may blow so fast and hard that it can knock over tall trees or even buildings.

In winter, the wind can play an important part in determining how cold you feel. The wind chill factor is an estimate of how cold a combination of the movement of the air and its temperatures can make a person feel.

The faster the wind blows, the faster the body loses heat. Therefore, the feeling of cold increases as the speed of the wind increases.

When the temperature of the air is 10 degree Fahrenheit, and the wind is blowing at a speed of 10 miles per hour, the wind chill temperature is equal to nine degrees below zero, Fahrenheit.

This example means that with a 10 mile per hour wind at 10 degrees Fahrenheit, people lose as much heat and feel as cold as they do when the actual temperature is nine degrees below zero and the wind is calm.

Here is another example: If the air temperature is zero and the wind is blowing at a rate of 20 miles per hour, the wind chill temperature figures to be 40 degrees below zero.

Wind chill is not an exact measurement of cold because temperature and wind are not the only conditions that make people feel cold. A thin person or one wearing damp clothes will lose more heat and feel colder than  a heavier, well dressed person.

Wind chill measurements were developed from experiments performed in Antarctica in 1939. A table of temperatures has been published by the United States National Weather Service. That agency reports wind speeds in miles per hour and temperatures in degrees Fahrenheit.

Occasionally the air temperature can drop to 40 degrees below zero. If thee wind happens to be blowing at 20 miles per hour at this time, the wind chill temperature will be 96 degrees below zero.

If the air temperature is standing at 30 degrees Fahrenheit and the wind is blowing at five miles per hour, the wind chill temperature reading will be 27 degrees.