heat wave
A heat wave (or heatwave), sometimes known as extreme heat, is a period of abnormally hot weather.: 2911  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.: 2911  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.": 2911 
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 5 °C (9 °F), 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 28 °C (82.4 °F). The Danish Meteorological Institute also has a definition for a "warmth wave" (varmebølge). It defines this as 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 three consecutive days at 39 °C (102 °F) or more. In the same period the minimum temperature is 26 °C (79 °F) 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 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 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 25 °C (77 °F) across much of the country. Greater London has a threshold of 28 °C (82 °F).

Other regions
In the United States, definitions also vary by region. They usually involve a period of at least two or more days of excessively hot weather. In the Northeast, a heat wave typically when the temperature reaches or exceeds 90 °F (32.2 °C) for three consecutive days. This is not always the case. This is because the high temperature ties in with humidity levels to determine a heat index threshold. The same does not apply to drier climates. A heat storm is a Californian term for an extended heat wave. Heat storms occur when the temperature reaches 100 °F (37.8 °C) for three or more consecutive days over a wide area (tens of thousands of square miles). The National Weather Service issues heat advisories and excessive heat warnings when it expects unusual periods of hot weather.
In Adelaide, South Australia, a heat wave is five consecutive days at or above 35 °C (95 °F). It can also be three consecutive days at or over 40 °C (104 °F). The Australian Bureau of Meteorology defines a heat wave as three or more days of unusual maximum and minimum temperatures. Before this new Pilot Heatwave Forecast there was no national definition for heat waves or measures of heat wave severity.

Observations
It is possible to compare heat waves in different regions of the World with different climates thanks to a general indicator. This appeared in 2015. With these indicators, experts estimated heat waves at the global scale from 1901 to 2010. They found a substantial and sharp increase in the number of affected areas in the last two decades.In July 2023 the world hit a new record high temperature. Increased wildfires in places such as Spain and Greece can also be attributed to heat waves.The 2021 Western North America heat wave resulted in some of the highest temperatures ever recorded in the region. These included a record 49.6 °C (121.3 °F) for Canada.One study in 2021 investigated 13,115 cities. It found that extreme heat exposure of a wet bulb globe temperature above 30 °C tripled between 1983 and 2016. It increased by about 50% if you exclude the effect of population growth in these cities. Urban areas and living spaces are often significantly warmer than surrounding rural areas. This is partly due to the urban heat island effect. The researchers compiled a comprehensive list of past urban extreme heat events.

Causes
Heat waves form when a high pressure area at an altitude of 10,000–25,000 feet (3,000–7,600 metres) strengthens and remains over a region for several days and up to several weeks. This is common in summer in both the Northern and Southern Hemispheres. This is because the jet stream 'follows the sun'. The high pressure area is on the equator side of the jet stream in the upper layers of the atmosphere.
Weather patterns are generally slower to change in summer than in winter. So, this upper level high pressure also moves slowly. Under high pressure, the air sinks toward the surface. It warms and dries adiabatically. This inhibits convection and prevents the formation of clouds. A reduction of clouds increases the shortwave radiation reaching the surface. A low pressure area at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming. The surface winds could also blow from the hot continental interior towards the coastal zone. This would lead to heat waves on the coast. They could also blow from high towards low elevations. This enhances the subsidence or sinking of the air and therefore the adiabatic warming.In the eastern regions of the United States a heat wave can occur when a high pressure system originating in the Gulf of Mexico becomes stationary just off the Atlantic Seaboard. We usually call this a Bermuda High. Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea. At the same time hot dry air masses form over the desert Southwest and northern Mexico. The southwest winds on the back side of the high continue to pump hot, humid Gulf air northeastwards. This results in a spell of hot and humid weather for much of the eastern United States and into southeastern Canada.In the Western Cape Province of South Africa, a heat wave can occur when low pressure offshore and high pressure inland air combine to form a berg wind. The air warms as it descends from the Karoo interior. The temperature will rise about 10 °C from the interior to the coast. Humidity is usually very low. The temperature can be over 40 °C in summer. The highest temperature recorded in South Africa (51.5 °C) occurred one summer during a berg wind along the Eastern Cape coastline.The level of soil moisture can intensify heat waves in Europe. Low soil moisture leads to a number of complex feedback mechanisms. These in turn can result in increased surface temperatures. One of the main mechanisms is reduced evaporative cooling of the atmosphere. When water evaporates, it consumes energy. So, it will lower the surrounding temperature. If the soil is very dry, then incoming radiation from the sun will warm the air. But there will be little or no cooling effect from moisture evaporating from the soil.

Climate change
Impacts on human health
Heat-related health effects for vulnerable people
Mortality
Underreporting of fatalities
The number of heat fatalities is probably highly underreported. This is due to a lack of reports and to misreporting. If we factor in heat-related illnesses, actual death tolls linked to extreme heat may be six times as high as official figures. This is based on studies of California and Japan.Part of the mortality during a heat wave may be due to short-term forward mortality displacement. In some heat waves there is a decrease in overall mortality in the weeks after a heat wave. These compensatory reductions in mortality suggest that heat affects people who would have died anyway, and brings their deaths forward.Social institutions and structures influence the effects of risks. This factor can also help explain the underreporting of heat waves as a health risk. The deadly French heat wave in 2003 showed that heat wave dangers result from a combination of natural and social factors. Social invisibility is one such factor. Heat-related deaths can occur indoors, for instance among elderly people living alone. In these cases it can be challenging to assign heat as a contributing factor.

Heat index for temperature and relative humidity
The heat index in the table above is a measure of how hot it feels when relative humidity is factored with the actual air temperature.

Psychological and sociological effects
Excessive heat causes psychological stress as well as physical stress. This can affect performance. It may also lead to an increase in violent crime. High temperatures are associated with increased conflict between individuals and at the social level. In every society, crime rates go up when temperatures go up. This is particularly the case with violent crimes such as assault, murder and rape. In politically unstable countries, high temperatures can exacerbate factors that lead to civil war.High temperatures also have a significant effect on income. A study of counties in the United States found that the economic productivity of individual days declines by about 1.7% for each degree Celsius above 15 °C (59 °F).

Surface ozone (air pollution)
High temperatures also make the effects of ozone pollution in urban areas worse. This raises heat-related mortality during heat waves. During heat waves in urban areas, ground level ozone pollution can be 20% higher than usual.One study looked at fine particle concentrations and ozone concentrations from 1860 to 2000. It found that the global population-weighted fine particle concentrations increased by 5% due to climate change. Near-surface ozone concentrations rose by 2%.An investigation to assess the joint mortality effects of ozone and heat during the European heat waves in 2003 concluded that these appear to be reinforce each other and increase mortality when combined.

Other impacts
Reduced GDP
Calculations from 2022 suggest heat waves will shrink the global economy by about 1% decrease by the middle of the 21st century.Heat waves often have complex effects on economies. They reduce labour productivity, disrupt agricultural and industrial processes and damage infrastructure that is not suitable for extreme heat.

Reduced agricultural yields
heat waves are a big threat to agricultural production. In 2019, heat waves in the Mulanje region of Malawi involved temperatures as high as 40 °C (104 °F). This and a late rain season scorched tea leaves and reduced yields.

Wildfires
A heat wave occurring during a drought can contribute to bushfires and wildfires. This is because a drought dries out vegetation, so it is more likely to catch fire. During the disastrous heat wave that struck Europe in 2003, fires raged through Portugal. They destroyed over 3,010 square kilometres (1,160 sq mi) of forest and 440 square kilometres (170 sq mi) of agricultural land. They caused about €1 billion worth of damage. High end farmlands have irrigation systems to back up crops.

Floods
Heat waves can also contribute to flooding. The record-breaking heat wave that afflicted Pakistan beginning in May 2022 led to glacier melt and moisture flow. These were factors in the devastating floods that began in June and claimed over 1,100 lives.

Infrastructural damage
Heat waves cause roads and highways to buckle and melt, water lines to burst, and power transformers to detonate, causing fires. A heat wave can also damage railways, by buckling and kinking rails. This can slow down or delay traffic. It can even lead to cancellations of service when rails are too dangerous to traverse by trains.

Power outages
Heat waves often lead to spikes in electricity demand because there is more use of air conditioning. This can create power outages, making the problem worse. During the 2006 North American heat wave, thousands of homes and businesses went without power, especially in California. In Los Angeles, electrical transformers failed, leaving thousands without power for as long as five days.
The 2009 South Eastern Australia Heat Wave caused major power disruptions in the city of Melbourne. They left over half a million people without power as the heat wave blew transformers and overloaded a power grid.

Options for reducing impacts of heat waves on people
A possible public health measure during heat waves is to set up air-conditioned public cooling centres. There are novel designs for cooling systems that are relatively low-cost. They do not use electrical components, are off-grid and store solar energy chemically for use on demand.Adding air conditioning in schools provides a cooler work place. But it can result in additional greenhouse gas emissions unless solar energy is used.

Examples by country
United States
In July 2019, there were over 50 million people in the United States in jurisdictions with heat advisories. Scientists predicted that many records for highest low temperatures would be broken in the days following these warnings. This means the lowest temperature in a 24-hour period will be higher than any low temperature measured before.According to a 2022 study, 107 million people in the US will experience extremely dangerous heat in the year 2053.Heat waves are the most lethal type of weather phenomenon in the United States. Between 1992 and 2001, deaths from excessive heat in the United States numbered 2,190, compared with 880 deaths from floods and 150 from hurricanes. About 400 deaths a year on average are directly due to heat in the United States. The 1995 Chicago heat wave, one of the worst in US history, led to approximately 739 heat-related deaths over 5 days. In the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events. These include lightning, rain, floods, hurricanes, and tornadoes.About 6,200 Americans need hospital treatment each summer, according to data from 2008. This is due to excessive heat, and those at highest risk are poor, uninsured or elderly.The relationship between extreme temperature and mortality in the United States varies by location. Heat is more likely to increase the risk of death in cities in the northern part of the country than in southern regions. Unusually hot summertime temperatures in Chicago, Denver, or New York City lead to predictions of higher levels of illness and death. Parts of the country that are mild to hot all year have a lower public health risk from excessive heat. Residents of southern cities such as Miami, Tampa, Los Angeles, and Phoenix tend to be acclimatized to hot weather conditions. They are therefore less vulnerable to heat-related deaths. As a whole, people in the United States appear to be adapting to hotter temperatures further north each decade. This might be due to better infrastructure, more modern building design and better public awareness.

Society and culture
Policymakers, funders and researchers have created the Extreme Heat Resilience Alliance coalition under the Atlantic Council. This advocates for naming heat waves, measuring them, and ranking them to build better awareness of their impacts.

See also
Cold wave
List of heat waves
List of severe weather phenomena
Urban heat island

References
External links
Social & Economic Costs of Temperature Extremes from NOAA Socioeconomics website initiative, National Centers for Environmental InformationCurrent global map of extreme temperatures
The emergence of heat and humidity too severe for human tolerance, American Association for the Advancement of Science research paper
Statistical analysis of Met Office Hadley Centre HadISD datasets
Global Sub-daily Station HadISD Dataset