A heat wave (or heatwave), sometimes known as extreme heat, is a period of abnormally hot weather. High humidity often accompanies heat waves. This is especially the case in oceanic climate countries. Definitions vary but are similar. We usually measure a heat wave relative to the usual climate in the area and to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be called 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. Heat waves have become more frequent, and more intense over land, almost everywhere since the 1950s. This is due to climate change.
Heat waves form when a high pressure area in the upper atmosphere strengthens and remains over a region for several days up to several weeks. This traps heat near the ground.
Heat waves often have complex effects on human economies. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure not suitable for extreme heat. Severe heat waves have caused catastrophic crop failures and thousands of deaths from hyperthermia. They have increased the risk of wildfires in areas with drought. They can lead to widespread power outages because people use more air conditioning. A heat wave counts as extreme weather. It poses danger to human health because heat and sunlight overwhelm the human body's cooling system. It is usually possible to detect heat waves by using forecasting instruments. This allows the authorities to issue a warning.
Definitions
There are several definitions of heat waves: The IPCC defines heatwave as "a period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months." A definition based on the Heat Wave Duration Index is that a heat wave occurs when the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by , the normal period being 1961–1990. The same definition is used by the World Meteorological Organization. A definition from the Glossary of Meteorology is: "A period of abnormally and uncomfortably hot and usually humid weather." We can use the term in two cases. One is variations in hot weather. The other is extraordinary spells of hot weather which may occur only once a century.
Definitions by country
Europe
Denmark defines a national heat wave (hedebølge) as a period of at least 3 consecutive days in which the average maximum temperature across more than half the country exceeds . The Danish Meteorological Institute also has a definition for a "warmth wave" (varmebølge). It defines this as the same criteria for a temperature. Sweden defines a heat wave as at least five days in a row with a daily high exceeding .
In Greece, the Hellenic National Meteorological Service defines a heat wave as three consecutive days at or more. In the same period the minimum temperature is or more. There are no winds or only weak winds. These conditions occur in a broad area.
The Netherlands defines a heat wave as a period of at least five consecutive days in which the maximum temperature in De Bilt exceeds . During this period the maximum temperature in De Bilt must exceed for at least three days. Belgium also uses this definition of a heat wave with Ukkel as a reference point. So does Luxembourg.
In the United Kingdom, the Met Office operates a Heat Health Watch system. This places each Local Authority region into one of four levels. Heat wave conditions occur when the maximum daytime temperature and minimum nighttime temperature rise above the threshold for a particular region. The length of time above that threshold determines the level. Level 1 is normal summer conditions. Level 2 occurs when there is a 60% or higher risk that the temperature will be above the threshold levels for two days and the intervening night. Level 3 arises when the temperature has been above the threshold for the preceding day and night, and there is a 90% or higher chance that it will stay above the threshold in the following day. Level 4 is triggered if conditions are more severe than those of the preceding three levels. Each of the first three levels gives rise to a particular state of readiness and response by the social and health services. Level 4 involves a more widespread response. The threshold for a heat wave occurs when there are at least three days above across much of the country. Greater London has a threshold of .