Heat wave

A heat wave or heatwave, sometimes described as extreme heat, is a period of abnormally hot weather that lasts for multiple days. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. The main difficulties with this broad definition emerge when one must quantify what the 'normal' temperature state is, and what the spatial extent of the event may or must be. 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. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s, the increase in frequency and duration being caused by 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 earth's surface. It is usually possible to forecast heat waves, thus allowing the authorities to issue a warning in advance.

Heat waves have an impact on the economy. They can reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure. 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 electricity outages because more air conditioning is used. A heat wave counts as extreme weather. It poses a danger to human health, because heat and sunlight overwhelm the thermoregulation in humans.

There are several definitions of heat waves:

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 28 °C (82.4 °F). The Danish Meteorological Institute also has a definition for a "warmth wave" (varmebølge). It defines this using the same criteria for a 25 °C (77.0 °F) temperature. Sweden defines a heat wave as at least five days in a row with a daily high exceeding 25 °C (77.0 °F).

In Greece, the Hellenic National Meteorological Service defines a heat wave as occurring over three consecutive days with temperatures at 39 °C (102 °F) or higher. In the same period the minimum temperature is 26 °C (79 °F) or more. During this period, there are either 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 25 °C (77 °F). During this period the maximum temperature in De Bilt must exceed 30 °C (86 °F) 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 represents 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 25 °C (77 °F) across much of the country. Greater London has a threshold of 28 °C (82 °F).