A heat wave is a prolonged period of extremely 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 believe 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 variation 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.
Heat waves often occur for the period of 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." Some regions of the globe are more susceptible to heat waves than others, such as Mediterranean-type climates with a summer dry spell which becomes much hotter than usual during certain years