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A heat wave is 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.
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 Celsius degrees (9 Fahrenheit degrees), the normal period being 1961–1990.
A heat storm is a Californian term for 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). This definition of a heat wave is also used in Belgium and Luxembourg.
In Denmark a heat wave 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.
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. The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected.
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, typically inland desert, semidesert, and Mediterranean-type climates.
Heat waves are spans of extreme heat.
In the summer in warm climates, an area of high pressure with little or no rain or clouds, the air and ground easily heats to excess. A static high pressure area can impose a very persistent heat wave.
The position of the jet stream allows air on one side to be considerably warmer than the other side. Heat waves are far more common and more severe on the warm side and at times an unusual position of the jet stream places unusual warmth in an unusual place for hot weather, and imposes a heat wave. 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.
Winds from hot deserts typically push hot, dry air towards areas normally cooler than during a heat wave. During the summer an area that has no geographic features that might cool winds that originate in the hot deserts get little mitigation, especially near the summer solstice when long days and a high sun would create warm conditions even without the transport of hot air from other locations. Should such a hot air mass travel above a large body of water, as a sirocco of Saharan origin crossing the Mediterranean sea, it likely picks up much water vapor with a reduction in temperature but far greater humidity that makes the original desert air little less moderate as demonstrated in a high heat index. Heat waves can also come from air originating over tropical seas penetrating far into the middle latitudes, as often occurs in the eastern United States and southeastern Canada. The heat island effects of large cities only exacerbate heat in large cities that endure heat waves because of the weakness of night-time cooling.
Hyperthermia, also known as heat stroke, becomes commonplace 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.
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. 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.
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".
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.
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. The heat wave in Melbourne, Australia also caused major power disruptions leaving over half a million people without power as the heat wave blew transformers and overloaded the 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 (740,000 acres) of forest and 440 square kilometres (110,000 acres) of agricultural land and causing an estimated €1 billion worth of damage. High end farmlands have irrigation systems to back up crops with.
Heat waves can and do cause roads, highways to buckle, water lines to burst, power transformers to detonate, causing fires. See the 2006 North American heat wave article about heat waves causing physical damage.
The record for the longest heat wave in the world is generally accepted to have been set in Marble Bar in Australia, where from October 31, 1923 to April 7, 1924 the temperature broke the 37.8 °C (100.0 °F) benchmark, setting the heat wave record at 160 days.
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.
During another heat wave in 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; to this day the summer of 1983 remains on record as 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 Southeast and The mid atlantic region as well that same summer. New York Times represented articles about the heat waves of 1983 affecting the central United States.
During 1988 intense heat spells in combination with the drought of 1988 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.
The summer of 1999 saw a devastating heat wave and drought in the eastern United States which claimed the lives of 500–700 people.
A heat wave is a long period of extremely hot weather, which may be followed by high humidity. There is no definition of a heat wave that everyone agrees on; the term depends on the usual weather in the area. Temperatures that people from a hotter climate consider normal can be called a heat wave in a cooler area if they are outside the normal climate pattern for that area. The term is used on both common weather changes and to very hot spells which may occur only once a century.