Heat waves are stretches of unusually hot days that can persist for a week or more.

The longer version

A heat wave is a stretch of unusually hot weather for a particular place, judged against that location's own normal range rather than an absolute temperature. The usual cause is a slow-moving area of high pressure, where sinking air warms by compression and cloud formation is largely suppressed.

Clear skies let the sun beat down day after day, and the ground dries out, so more of that energy goes into heating the air instead of evaporating water. The pattern can persist because the surrounding weather systems change only slowly, and the heat therefore builds up day after day.

Where this entry sits

Clouds, wind, rain, and storms shape daily life in ways that most people barely notice. This collection explains snow, lightning, rainbows, the turning seasons, temperature, the instruments forecasters rely on, and scraps of old weather lore. The full list sits on the Weather and Climate page, where every entry is listed in order.