A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. There is no universal definition of a heat wave; the term is relative to the usual weather in the area. Temperatures that people from a hotter climate consider normal can be termed a heat wave in a cooler area if they are outside the normal climate pattern for that area. The term is applied both to routine weather variations and to extraordinary spells of heat which may occur only once a century. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power outages due to increased use of air conditioning.

Definitions

A formal, peer-reviewed definition from the Glossary of Meteorology is:<

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 . More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region.

A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches for three or more consecutive days over a wide area (tens of thousands of square miles).

In the Netherlands, a heat wave is defined as period of at least 5 consecutive days in which the maximum temperature in De Bilt exceeds , provided that on at least 3 days in this period the maximum temperature in De Built exceeds . This definition of a heat wave is also used in Belgium and Luxembourg.

In Denmark a heat wave is defined as a period of at least 3 consecutive days of which period the average maximum temperature across more than fifty percent of the country exceeds 28 °C.

In the United States, definitions also vary by region; however, a heat wave is usually defined as a period of at least three consecutive days above . The National Weather Service criterion for the issuance of a heat advisory is when the heat index is expected to (or does) reach with a nighttime low temperature not below , whereas an excessive heat warning is issued when a maximum heat index exceeding with minimum is expected (or is occurring).

Incidence

Heat waves often occur during the Dog Days of summer; indeed the French term canicule, denoting the general phenomenon of a heat wave, derives from the Italian canicula applied to the star Sirius, also known as the "Dog Star."