A heat wave is a period of excessively hot weather, which may be accompanied by high humidity, especially in tropical regions. Heat waves can cause death and injury from several mechanisms: dehydration, hyperthermia, sunstroke, and fires. Extreme heat waves have caused mass global extinctions of species (including hominids) and are often followed by regional climate cooling.
A definition based on temperature alone cannot be used to define a heat wave as it varies by region. For example, in Australia during the summer months (December-February), temperatures above 35 °C (95 °F) are not uncommon; however, they would not be considered a heat wave in other parts of the world such as Europe where 26 °C (79 °F) would be considered warm. In addition to temperature, humid conditions need to be present for a prolonged period of time for a true heatwave to occur.
There are two types ofheat waves based on their causative mechanism: those caused by high air temperatures (solar radiation) and those caused by high ground temperatures (advection). The latter type is much more common than the former type and is generally associated with desert regions. However, both types can occur simultaneously in different parts of the world. For example, North America can experience both types of heat waves – one associated with an upper-level ridge of high pressure over the western United States that advects warm air from the deserts southwestward into the Plains states; at the same time, another type can develop along the East Coast when strong southerly winds bring humid air northward from the Gulf Coast states resulting in very high dewpoints.
The main difference between these two types is that solar radiation heats only the air while advection heats both the air and ground surfaces. The result is that during an advective heat wave there is less nighttime relief fromthe high temperatures because not only is the air still warm but alsothe ground radiates back some of its daytime warmth at night. This effectis compounded if winds are light during an advective event since therewould then be little mixing of cooler air at lower levels thereby allowingthe build-upof even hotter conditions nearthe surface. On average, about 55%of all reported deaths due to excessive natural heat exposure worldwideeach year are relatedto advection events while 45%are attributedto solar radiation events; however, this percentage will likely increaseas global warming continuesand more areas become susceptibleto prolonged bouts of extremelyhot weather brought aboutby either mechanism mentioned above or a combination thereof .