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
hypothermia, and widespread power outages due to increased use of air conditioning.
In North America, a heat wave is usually defined as a period of 3 or more consecutive days above 90°F (32.2°C)
Effects on health:
Severe heat waves can lead to deaths from dehydration, also known as heat stroke. Older adults, very
young children, and those who are sick or overweight are at a higher risk for heat-related illness. Heat waves are the most lethal type of weather phenomenon, overall. Between 1992 and 2001, deaths from excessive heat in the United States numbered 2,190. Continued hydration by drinking adequate amounts of fluids is necessary.
Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, further increasing the problem. During the 2006 North American heat wave, thousands of homes and businesses went without power, especially in California and the St. Louis, Missouri area. In Los Angeles, electrical
transformers failed, leaving tens of thousands without power for as long as five days.
Heat wave also cause droughts, which dry out vegetation and contribute to wildfires.