Heat wave

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For other uses, see Heat wave (disambiguation).

A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity, especially in oceanic climate countries. While definitions vary,[1] a heat wave is measured relative to the usual weather in the area and relative to normal temperatures for the season. Temperatures that people from a hotter climate consider normal can be termed a heat wave in a cooler area if they are outside the normal climate pattern for that area.[2]

The term is applied both to routine weather variations and to extraordinary spells of heat which may occur only once a century. Severe heat waves have caused catastrophic crop failures, thousands of deaths from hyperthermia, and widespread power outages due to increased use of air conditioning. A heat wave is considered extreme weather, and a danger because heat and sunlight may overheat the human body.


The definition recommended by the World Meteorological Organization is 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. Heat waves usually occur in September[3]

A formal, peer-reviewed definition from the Glossary of Meteorology is:[4]

A period of abnormally and uncomfortably hot and usually humid weather.
To be a heat wave such a period should last at least one day, but conventionally it lasts from several days to several weeks. In 1900, A. T. Burrows more rigidly defined a “hot wave” as a spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90 °F (32.2 °C). More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region.
Temperature anomalies, March to May 2007

In the Netherlands, a heat wave is defined as period of at least 5 consecutive days in which the maximum temperature in De Bilt exceeds 25 °C (77 °F), provided that on at least 3 days in this period the maximum temperature in De Bilt exceeds 30 °C (86 °F). This definition of a heat wave is also used in Belgium and Luxembourg.

In Denmark, a national heat wave (hedebølge) is defined as a period of at least 3 consecutive days of which period the average maximum temperature across more than fifty percent of the country exceeds 28 °C (82.4 °F) – the Danish Meteorological Institute further defines a "warmth wave" (varmebølge) when the same criteria are met for a 25 °C (77.0 °F) temperature,[5] while in Sweden, a heat wave is defined as at least 5 days in a row with a daily high exceeding 25 °C (77.0 °F).[6]

In the United States, definitions also vary by region; however, a heat wave is usually defined as a period of at least two or more days of excessively hot weather.[7] In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds 90 °F (32.2 °C), but not always as this ties in with humidity levels to determine a heat index threshold.[8] 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 unusual periods of hot weather are expected.

In Adelaide, South Australia, a heat wave is defined as five consecutive days at or above 35 °C (95 °F), or three consecutive days at or over 40 °C (104 °F).[9]

In England and Wales, the Met Office operates a Heat Health Watch system which places each Local Authority region into one of four levels. Heatwave conditions are defined by the maximum daytime temperature and minimum nighttime temperature rising above the threshold for a particular region. The length of time spent above that threshold determines the particular level. Level 1 is normal summer conditions. Level 2 is reached 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 is triggered 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 is associated with a particular state of readiness and response by the social and health services, and Level 4 is associated with more widespread response.[10]

How they occur[edit]

High pressure aloft traps heat near the ground, forming a heat wave

Heat waves form when high pressure aloft (from 10,000–25,000 feet (3,000–7,600 metres)) strengthens and remains over a region for several days up to several weeks. This is common in summer (in both Northern and Southern Hemispheres) as the jet stream 'follows the sun'. On the equator side of the jet stream, in the middle layers of the atmosphere, is the high pressure area.

Summertime weather patterns are generally slower to change than in winter. As a result, this mid-level high pressure also moves slowly. Under high pressure, the air subsides (sinks) toward the surface. This sinking air acts as a dome capping the atmosphere.

This cap helps to trap heat instead of allowing it to lift. Without the lift there is little or no convection and therefore little or no convective clouds (cumulus clouds) with minimal chances for rain. The end result is a continual build-up of heat at the surface that we experience as a heat wave.[11]

In the Eastern 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 (typically known as a Bermuda High). Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea while hot dry air masses form over the desert Southwest and northern Mexico. The SW winds on the back side of the High continue to pump hot, humid Gulf air northeastward resulting in a spell of hot and humid weather for much of the Eastern States.[12]

In the Western Cape Province of South Africa, a heat wave can occur when a low pressure offshore and high pressure inland combine to form a Bergwind. The air warms as it descends from the Karoo interior, and the temperature will rise about 10 °C from the interior to the coast. Humidities are usually very low, and the temperatures can be over 40 °C in summer. The highest official temperatures recorded in South Africa (51.5 °C) was recorded one summer during a bergwind occurring along the Eastern Cape coastline.[13][14]

Global warming boosts the probability of extreme weather events, like heat waves, far more than it boosts more moderate events.[15][16][17]

Health effects[edit]

NOAA national weather service: heat index
 temperature (°F)
Relative Humidity (%)
  Extreme Caution
  Extreme Danger

The heat index (as shown in the table above) is a measure of how hot it feels when relative humidity is factored with the actual air temperature. Hyperthermia, also known as heat stroke, becomes commonplace during periods of sustained high temperature and humidity. Sweating is absent from 84–100%[citation needed] of those affected. Older adults, very young children, and those who are sick or overweight are at a higher risk for heat-related illness. The chronically ill and elderly are often taking prescription medications (e.g., diuretics, anticholinergics, antipsychotics, and antihypertensives) that interfere with the body's ability to dissipate heat.[18]

Heat edema presents as a transient swelling of the hands, feet, and ankles and is generally secondary to increased aldosterone secretion, which enhances water retention. When combined with peripheral vasodilation and venous stasis, the excess fluid accumulates in the dependent areas of the extremities. The heat edema usually resolves within several days after the patient becomes acclimated to the warmer environment. No treatment is required, although wearing support stocking and elevating the affected legs with help minimize the edema.

Heat rash, also known as prickly heat, is a maculopapular rash accompanied by acute inflammation and blocked sweat ducts. The sweat ducts may become dilated and may eventually rupture, producing small pruritic vesicles on an erythematous base. Heat rash affects areas of the body covered by tight clothing. If this continues for a duration of time it can lead to the development of chronic dermatitis or a secondary bacterial infection. Prevention is the best therapy. It is also advised to wear loose-fitting clothing in the heat. However, once heat rash has developed, the initial treatment involves the application of chlorhexidine lotion to remove any desquamated skin. The associated itching may be treated with topical or systemic antihistamines. If infection occurs a regimen of antibiotics is required.

The 1936 North American heat wave. Record temperatures were based on 112 year records

Heat cramps are painful, often severe, involuntary spasms of the large muscle groups used in strenuous exercise. Heat cramps tend to occur after intense exertion. They usually develop in people performing heavy exercise while sweating profusely and replenishing fluid loss with non-electrolyte containing water. This is believed to lead to hyponatremia that induces cramping in stressed muscles. Rehydration with salt-containing fluids provides rapid relief. Patients with mild cramps can be given oral .2% salt solutions, while those with severe cramps require IV isotonic fluids. The many sport drinks on the market are a good source of electrolytes and are readily accessible.

Heat syncope is related to heat exposure that produces orthostatic hypotension. This hypotension can precipitate a near-syncopal episode. Heat syncope is believed to result from intense sweating, which leads to dehydration, followed by peripheral vasodilation and reduced venous blood return in the face of decreased vasomotor control. Management of heat syncope consists of cooling and rehydration of the patient using oral rehydration therapy (sport drinks) or isotonic IV fluids. People who experience heat syncope should avoid standing in the heat for long periods of time. They should move to a cooler environment and lie down if they recognize the initial symptoms. Wearing support stockings and engaging in deep knee-bending movements can help promote venous blood return.

Heat exhaustion is considered by experts to be the forerunner of heat stroke (hyperthermia). It may even resemble heat stroke, with the difference being that the neurologic function remains intact. Heat exhaustion is marked by excessive dehydration and electrolyte depletion. Symptoms may include diarrhea, headache, nausea and vomiting, dizziness, tachycardia, malaise, and myalgia. Definitive therapy includes removing patients from the heat and replenishing their fluids. Most patients will require fluid replacement with IV isotonic fluids at first. The salt content is adjusted as necessary once the electrolyte levels are known. After discharge from the hospital, patients are instructed to rest, drink plenty of fluids for 2–3 hours, and avoid the heat for several days. If this advice is not followed it may then lead to heat stroke.

One public health measure taken during heat waves is the setting-up of air-conditioned public cooling centers.


Heat waves are the most lethal type of weather phenomenon, overall. 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.[19] The average annual number of fatalities directly attributed to heat in the United States is about 400.[20] The 1995 Chicago heat wave, one of the worst in US history, led to approximately 600 heat-related deaths over a period of five days.[21] Eric Klinenberg has noted that in the United States, the loss of human life in hot spells in summer exceeds that caused by all other weather events combined, including lightning, rain, floods, hurricanes, and tornadoes.[22][23] Despite the dangers, Scott Sheridan, professor of geography at Kent State University, found that less than half of people 65 and older abide by heat-emergency recommendations like drinking lots of water. In his study of heat-wave behavior, focusing particularly on seniors in Philadelphia, Phoenix, Toronto, and Dayton, Ohio, he found that people over 65 "don't consider themselves seniors." One of his older respondents said: "Heat doesn't bother me much, but I worry about my neighbors."[24]

According to the Agency for Health care Research and Quality, about 6,200 Americans are hospitalized each summer due to excessive heat, and those at highest risk are poor, uninsured or elderly.[25] More than 70,000 Europeans died as a result of the 2003 European heat wave.[26] Concern is now focusing on predicting the future likelihood of heat waves and their severity. In addition, because in most of the world most of those suffering the impacts of a heat wave will be inside a building, and this will modify the temperatures they are exposed to, there is the need to link climate models to building models. This means producing example time series of future weather.[27][28] Other work has shown that future mortality due to heat waves could be reduced if buildings were better designed to modify the internal climate, or the occupants better educated about the issues, so they took action in time.[29][30]

Underreporting and "Harvesting" effect

The number of heat fatalities is likely highly underreported due to lack of reports and misreports.[20] Part of the mortality observed during a heat wave, however, can be attributed to a so-called "harvesting effect", a term for a short-term forward mortality displacement. It has been observed that for some heat waves, there is a compensatory decrease in overall mortality during the subsequent weeks after a heat wave. Such compensatory reduction in mortality suggests that heat affects especially those so ill that they "would have died in the short term anyway".[31]

Psychological and sociological effects[edit]

In addition to physical stress, excessive heat causes psychological stress, to a degree which affects performance, and is also associated with an increase in violent crime.[32]

Power outages[edit]

Abnormally hot temperatures cause electricity demand to increase during the peak summertime hours of 4 to 7 p.m. when air conditioners are straining to overcome the heat. If a hot spell extends to three days or more, however, nighttime temperatures do not cool down, and the thermal mass in homes and buildings retains the heat from previous days. This heat build-up causes air conditioners to turn on earlier and to stay on later in the day. As a result, available electricity supplies are challenged during a higher, wider, peak electricity consumption period.

Heat waves often lead to electricity spikes due to increased air conditioning use, which can create power outages, exacerbating the problem. 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.[33] The 2009 South Eastern Australia Heat Wave caused the city of Melbourne, Australia to experience some major power disruptions which left over half a million people without power as the heat wave blew transformers and overloaded a power grid.


If a heat wave occurs during a drought, which dries out vegetation, it can contribute to bushfires and wildfires. During the disastrous heat wave that struck Europe in 2003, fires raged through Portugal, destroying over 3,010 square kilometres (1,160 sq mi) or 301,000 hectares (740,000 acres) of forest and 440 square kilometres (170 sq mi) or 44,000 hectares (110,000 acres) of agricultural land and causing an estimated 1 billion worth of damage.[34] High end farmlands have irrigation systems to back up crops with.

Physical damage[edit]

Heat waves can and do cause roads and highways to buckle and melt,[35] water lines to burst, and power transformers to detonate, causing fires. See the 2006 North American heat wave article about heat waves causing physical damage.

In the 2013 heatwave in England, gritters (normally only seen in snow) were sent out to grit melting tarmac roads.


20th century[edit]

1923/1924 - During a period of 160 such days from 31 October 1923 to 7 April 1924, the Western Australian town of Marble Bar set a world record for the most consecutive days above 100 °F (38 °C).[36]

1936 - The 1936 North American heat wave during the Dust Bowl, followed one of the coldest winters on record—the 1936 North American cold wave. Massive heat waves across North America were persistent in the 1930s, many mid-Atlantic/Ohio valley states recorded their highest temperatures during July 1934. The longest continuous string of 100 °F (38 °C) or higher temperatures was reached for 101 days in Yuma, Arizona during 1937 and the highest temperatures ever reached in Canada were recorded in two locations in Saskatchewan in July 1937.

1950s - A prolonged severe drought and heat wave occurred in the early 1950s throughout the central and southern United States. In some areas it was drier than during the Dust Bowl and the heat wave in most areas was within the top five on record. The heat was particularly severe in 1954 with 22 days of temperatures exceeding 100 °F (38 °C) covering significant parts of eleven states. On 14 July, the thermometer reached 117 °F (47 °C) at East St. Louis, Illinois, which remains the record highest temperature for that state.[37][38][39]

1972 - The heat waves of 1972 in New York and Northeastern United States were significant. Almost 900 people perished; the heat conditions lasted almost 16 days, aggravated by very high humidity levels.

1976 - The 1976 United Kingdom heat wave was one of the hottest in living memory and was marked by constant blues skies from May until September when dramatic thunderstorms signalled the heat wave's end.

1980 - An estimated 10,000 people perished in the 1980 United States heat wave and drought, which impacted the central and eastern United States. Temperatures were highest in the southern plains. From June through September, temperatures remained above 90 °F (32 °C) all but two days in Kansas City, Missouri. The Dallas/Fort Worth area experienced 42 consecutive days with high temperatures above 100 °F (38 °C), with temperatures reaching 117 °F (47 °C) at Wichita Falls, Texas on 28 June. Economic losses were $20 billion (1980 dollars).[40]

1983 - During the Summer of 1983 temperatures over 100 °F (38 °C) were common across Iowa, Missouri, Illinois, Michigan, Wisconsin, Indiana, Ohio, Minnesota, Nebraska and certain parts of Kentucky; the summer of 1983 remains one of the hottest summers ever recorded in many of the states affected. The hundred-degree readings were accompanied by very dry conditions associated with drought affecting the Corn Belt States and Upper Midwest. The heat also affected the Southeastern U.S. and the Mid-Atlantic states as well that same summer. New York Times represented articles about the heat waves of 1983 affecting the central United States.[41]

Temperature difference in Europe from the average during the European heat wave of 2003

1988 - intense heat spells in combination with the drought of 1988, reminiscent of the dust bowl years caused deadly results across the United States. Some 5,000 to 10,000 people perished because of constant heat across the United States although-according to many estimates-total death reports run as high as next to 17,000 deaths.[42]

1995 - The 1995 Chicago heat wave produced record high dew point levels and heat indices in the Chicago area and Wisconsin. The lack of emergency cooling facilities and inadequate response from civic authorities to the senior population, particularly in lower income neighborhoods in Chicago and other Midwest cities, lead to many deaths.

1999 - a heat wave and drought in the eastern United States during the summer of 1999. Rainfall shortages resulted in worst drought on record for Maryland, Delaware, New Jersey, and Rhode Island. The state of West Viriginia was declared a disaster area. 3,810,000 acres (15,400 km2) were consumed by fire as of mid-August. Record heat throughout the country resulted in 502 deaths nationwide.[43][44]

2000 - in late Summer 2000, a heat wave occurred in the southern United States, breaking many cities' all-time maximum temperature records.

21st century[edit]

Shifting Distribution of Summer Temperature Anomalies2.png



Land surface temperatures of 8–15 March 2012. Land surface temperatures are distinct from the air temperatures that meteorological stations typically measure.

See also[edit]


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External links[edit]