Diana Walsh, age 12, of Des Moines, Iowa, for her question:

How does a barometer help in weather forcasting?

A multitude of factors work together to create the changeable weather: And one of them is atmospheric pressure    the weight of the airy atmosphere pressing down upon the earth's land and sea. At sea level, the weight of this average pressure equals 14 1%2 pounds above every square inch. But the air is so restless that conditions are rarely average. Arise in temperature makes it expand, thin out and become lighter. A drop in temperature causes sir sasses to contract and become heavier. The barometer is the instrument used to measure these changing air pressures.

Heavy air presses down and causes the mercury to rise high in the barometer's glass tube. It forms a high pressure cell. Warm, light air lets the mercury fall lower in the tube. It forms a low pressure cell. Highs and lows cause our major weather events. Highs bring calm, sunny spells and lows bring cloudy storms. These weather events tend to approach us from the west. When meteorologists have barometer reports from many scattered areas, they know whether to expect a high or low pressure cell. Each line on a weather map is an isobar, drawn through points of equal pressure. A number of isobars reveal the pattern of the approaching high or low    and lots of barometer data is needed to assemble this picture of an approaching weather event.