A heat wave, sometimes described as extreme heat, is a period of abnormally hot weather. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. Temperatures that humans from a hotter climate consider normal, can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area. High humidity often occurs during heat waves as well. This is especially the case in oceanic climate countries. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s. This is due to climate change.

ग्रीष्म लहर यह मुख्य रूप से समुद्री जलवायु वाले देशों में हो सकता है। उच्च आर्द्रता के साथ हो सकता है, जो जरूरत से ज्यादा गर्म मौसम, की लम्बी अवधि से बनता है।