Melissa Mulligan, age 13, of Helena, Mont., for her question:

WHAT CAUSES WIND?

Wind is caused by the uneven heating of the Earths atmosphere by energy from the sun. Air above hot areas expands and rises. Air from cooler areas then flows in to take the place of the heated air. This process is called circulation.

When there is general circulation over large sections of the Earth's surface, prevailing winds result. There are no prevailing winds for about 700 miles on either side of the equator because the air there rises instead of moving across the earth.

Secondary circulation is the motion of air around relatively small regions of high and low pressure in the atmosphere.