Janice Basta, age 11, of Monroe, Connecticut, for her question:

What causes a heat wave?

Usually a heat wave is a large and lazy mass of air centered around a region of high pressure. It may be large enough to cover the continent from the Rockies, eastward to the Atlantic. In late summer, after the earth has had time to absorb many long hours of sunshine, a major high pressure cell may bring a heat wave of sweltering, suffocating weather that lasts perhaps a week.

Normally, weathery pockets of high and low pressure are swept eastward with the prevailing westerly winds. Sometimes they are delayed or diverted by other weather fronts moving up from the south or down from the north. Extra heat may be added by warm moist air from the Gulf of Mexico    and more heat is absorbed as the great spiraling air mass lingers above the warm continent. These conditions help to build a large, lazy system of high pressure into a major heat wave.