A heatwave is a prolonged period of anomalously high temperatures that exceeds normal values for a given region and season. There is no single universal definition: the World Meteorological Organization (WMO) suggests at least 5 consecutive days with maximum temperatures exceeding the normal by 5 °C or more, but national agencies set their own thresholds. Spain's AEMET defines a heatwave when temperatures exceed the 95th percentile for at least 3 days in a significant portion of weather stations.
Heatwaves are typically caused by persistent high-pressure ridges or blocking anticyclones that trap hot air over a region for days or weeks. The subsiding air suppresses cloud formation, allowing intense solar radiation to heat the surface further in a positive feedback loop. Urban areas suffer more due to the urban heat island effect, where concrete and asphalt absorb and re-radiate heat, keeping overnight temperatures dangerously high.
The health impacts are severe: heatwaves are the deadliest meteorological hazard in Europe, surpassing floods and storms combined. The 2003 European heatwave caused an estimated 70,000 excess deaths. Vulnerable groups — the elderly, children, outdoor workers, and those with chronic illnesses — face risks of heat exhaustion, heatstroke, and organ failure. Dew point temperatures above 24 °C compound the danger because the body cannot cool through sweating. Climate change is making heatwaves more frequent, more intense, and longer-lasting: events that were once-in-50-years are now occurring every decade.