Stevie F. Beck, age 11, of Franklin, North Carolina, for his question:

Why are barometers so important to weather forecasting?

Forecasting from ground level depends mainly on wind, temperature and barometric data. Readings are taken at countless farflung weather stations and assembled to form a general picture over a large section of the globe. The most important weather information comes from the barometer. This instrument measures atmospheric pressure, the weight of air pressing down upon the surface of the earth. The gaseous molecules of the air are more fluid than water, and they can spread themselves thin or pile up into dense, heavy masses. The barometer records these light and heavy air masses as regions of low or high pressure.

A heavy air mass tends to blow outward and merge with the lighter air around it, On the other hand, the heavier air around a light sir mass tends to blow inward. These fluid pockets of mixing air are twisted around by the spinning earth to form circulating weather systems. Barometric readings can detect them from afar, and weather experts can predict their paths with the prevailing winds. Temperature, precipitation, and wind data merely add the details to the barometric picture of the advancing pocket of weather.