A heat wave is a prolonged period of abnormally high temperatures for a given region. The definition usually requires several consecutive days above a local threshold that is well above the seasonal average.

High pressure systems that stall and produce clear skies and sinking air are common causes. Urban areas can intensify the heat through the urban heat island effect. Nighttime temperatures that remain elevated prevent the body from recovering, increasing health risks.

Impacts include heat-related illness, increased energy demand for cooling, stressed infrastructure, and damage to agriculture. Warnings urge reduced outdoor activity, hydration, and checks on vulnerable neighbors. Some heat waves are linked to larger patterns such as blocking highs or amplified jet-stream waves.

Historical heat waves have produced significant excess mortality in major cities.