Temperature difference in Europe from the average during a heat wave in 2003.

A heat wave is a period of unusually warm or very hot weather, and may be accompanied by high humidity. "Heat wave" is not an exact term: Any weather that feels warmer than people expect, might be called a heat wave. For example, temperatures normal for a warmer climate might be called a "heat wave" by people living in a place that is usually cooler. So a "heat wave" could mean a little warm weather, (like "several days as hot as summer, but in winter"), or temperatures so high they make history, (such as, "the hottest summer in a hundred years").