A heatwave is a marked warming of the air, or the invasion of very warm air, over a large area; it usually lasts from a few days to a few weeks (WMO, 1992).
Heatwaves vary according to the location of a particular region and the time of year and there is no universal way of defining or measuring heatwaves. Heatwaves can exacerbate urban air pollution which can affect the elderly, pregnant women and children. The heat stress burden is dependent on local climate and a humidifying effect can erase the cooling benefits that would come from trees and vegetation. Consequently, in humid climates humans can adapt to a lower temperature than previously thought.