(Also called hot wave, warm wave.) A period of abnormally and uncomfortably hot and usually humid weather. To be a heat wave such a period should last at least one day, but conventionally it lasts from several days to several weeks. In 1900, A. T. Burrows more rigidly defined a 'hot wave' as a spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90°F. More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region. In the eastern United States, heat waves generally build up with southerly winds on the western flank of an anticyclone centered over the southeastern states, the air being warmed by passage over a land surface heated by the sun.