Heat wave

For other uses, see Heat wave (disambiguation).

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

A heat wave is a prolonged period of excessively hot weather, which may be accompanied by high humidity. There is no universal definition of a heat wave; the term is relative to the usual weather in the area. 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. 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.

Definitions

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.

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

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. More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region.

A heat storm is an extended heat wave. Heat storms occur when the temperature reaches 100 °F (38 °C) for three or more consecutive days over a wide area (tens of thousands of square miles).

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).[1]. This definition of a heat wave is also used in Belgium, Denmark, and Luxembourg.

In the United States, definitions also vary by region; however, a heat wave is usually defined as a period of at least three consecutive days above 90 °F (32.2 °C).[2] The National Weather Service criterion for the issuance of a heat advisory is when the heat index is expected to (or does) reach 105 °F (40.6 °C) with a nighttime low temperature not below 80 °F (26.7 °C), whereas an excessive heat warning is issued when a maximum heat index exceeding 115 °F (46.1 °C) with minimum 80 °F (26.7 °C) is expected (or is occurring).

Incidence

Heat waves often occur during the Dog Days of summer; indeed the French term canicule, denoting the general phenomenon of a heat wave, derives from the Italian canicula applied to the star Sirius, also known as the "Dog Star."

Some regions of the globe are more susceptible to heat waves than others, such as Mediterranean-type climates with a summer dry spell which becomes much hotter than usual during certain years.

How they occur

In the summer in warm climates, if there is an area of high pressure with little or no rain and clouds, there is nothing to protect the air and ground from being heated excessively. When large high pressure air is trapped, the ground and air will continue to heat and the heat wave will last longer.

The position of the jet stream is very important because the air on one side is hot while the other side is cold. Heat waves are far more likely and more severe when you are on the hot side. Sometimes the jet stream does not move far enough to make many areas that normally get hot on the hot side; but sometimes many cold areas are made hot abnormally. The effects of El Niño and La Niña (opposite reaction to El Niño) can severely disrupt the positions of the jet streams.

Large desert zones and dry areas are more likely to get extreme heat because there is rarely any high cloud cover with very low humidity, making it like a furnace.

The direction of the wind is a very important part of the reasons for heat waves, because this pushes hot air towards colder areas if they are going away from the deserts and equator. The higher the force of the wind as well as the distance they travel from the hotter to colder areas, generally means hotter weather.

Cities also get hotter because they generate heat from the use of power, usually by about 1-2 °C or so and experience very warm night time minimum temperatures putting further strain on energy demands.

Humidity also has a serious effect on how hot it feels but this does not make the actual temperature any hotter. Heat Waves in Eastern North America and the British Isles are usually accompanied by high humidity, aggravating the heat index (one's discomfort), as opposed to the drier heat typically experienced during heat waves in continental Europe, Western North America or cities in Australia.

Perhaps the most well-known heat-related illness is hyperthermia, also known as heat stroke. Classic heat stroke occurs during periods of sustained high temperature and humidity. Sweating is absent from 84%-100% 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.

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.

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 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.

Mortality

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. The average annual number of fatalities directly attributed to heat in the United States is about 400. 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. 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. 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." "Heat doesn't bother me much, but I worry about my neighbors," said one of his older respondents.

Underreporting and "Harvesting" effect

The number of heat fatalities is likely highly underreported due to lack of reports and misreports. 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".

Psychological and sociological effects

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

Power outage

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.

Wildfires

If a heat wave occurs during a drought, which dries out vegetation, it can contribute to wildfires. During the disastrous heat wave that struck Europe in 2003, fires raged through Portugal, destroying over 3010 km² (740,000 acres) of forest and 440 km² (108,000 acres) of agricultural land and causing an estimated €1 billion worth of damage. High end farmlands have irrigation systems to back up crops with.

History

The European heat wave of 2003 killed around 35,000 people. Much of the heat was concentrated in France, where nearly 15,000 people died .

In early 2006, Adelaide, South Australia was hit by a dry heat wave with temperatures reaching 40+ °C for five days in a row, while Port Augusta experienced temperatures hovering around about mid 40s °C with one day recorded at approx 48 °C. Also in Sydney it reached 44.2 °C on the first day of 2006.

In July 2006, the United States experienced a massive heat wave, and almost all parts of the country had recorded temperatures above the average temperature for that time of year. Temperatures in some parts of South Dakota exceeded 115 °F (46 °C), causing many problems for the residents. Also, California experienced temperatures that were extraordinarily high, with records ranging from 100 to 130 °F (38 to 54 °C). On July 22, the County of Los Angeles recorded its highest temperature ever at 119 °F (48.33 °C).

The European heat wave of 2006 was the second massive heat wave to hit the continent in 4 years, with temperatures rising to 40 °C (104 °F) in Paris and even Ireland, which has a moderate maritime climate, reporting temperatures of over 32C (90 °F). Temperatures of 35 °C (95 °F) were reached in the Benelux and Germany, while the United Kingdom recorded 37C. Many heat records were broken (including the hottest ever July temperature in the UK) and many people who experienced the heat waves of 1976 and 2003 drew comparisons with them.

In January 2008, Alice Springs in Australia's Northern Territory recorded ten consecutive days of temperatures above 40°C with the average temperature for that month being 39.8°C.

In March 2008, Adelaide, South Australia experienced maximum temperatures of above 35°C for fifteen consecutive days, seven days more than the previous longest stretch of 35°C days. The March 2008 heat wave also included eleven consecutive days above 38°C. The heat wave was especially notable because it occurred in March, an autumn month, in which Adelaide averages only 2.3 days above 35°C.

The record for the longest heatwave in the world is generally accepted to have been set at Marble Bar in Australia, where from October 31, 1923 to April 07, 1924 the temperature broke the 37.8°C/100°F benchmark, setting the heat wave record at a scorching 160 days.

On May 13, 2008 to May 19, 2008, the bay area experienced a heat wave with a maximum temperature of 105°F.

Another heat wave is expected to develop and penetrate most of the eastern seaboard of the United States on June 7, 2008 and last through June 11, 2008. Temperatures are expected to climb to 90°F (32°C) and above in cities as far north as Montreal.

See also

• List of heat waves

Notes

• Klinenberg, Eric (2002). Heat Wave: A Social Autopsy of Disaster in Chicago. Chicago: University of Chicago Press. ISBN 0226443213.

External links

• FEMA: Extreme Heat
• Hot Weather Tips
• Marble Bar heatwave, 1923-1924
• WeatherBug Weather Wrap
• Social & Economic Costs of Temperature Extremes from "NOAA Socioeconomics" website initiative