A formal, peer-reviewed definition from the Glossary of Meteorology is: . This definition of a heat wave is also used in Belgium and Luxembourg. 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. A heat storm is a Californian term for an extended heat wave.
The National Weather Service issues heat advisories and excessive heat warnings when unusual periods of hot weather are expected. 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.
DPR Korea: Heat Wave Emergency Plan of Action Operation Final Report DREF Operation n° MDRKP010
Level 1 is normal summer conditions. 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. A more general indicator that allows comparing heat waves in different regions of the World, characterized by different climates, has been recently developed. Heat waves form when high pressure aloft from 10,—25, feet 3,—7, metres strengthens and remains over a region for several days up to several weeks.
On the equator side of the jet stream, in the upper layers of the atmosphere, is the high pressure area.
Summertime weather patterns are generally slower to change than in winter. As a result, this upper level high pressure also moves slowly.
The heat wave has caused damage already
Under high pressure, the air subsides sinks toward the surface, warming and drying adiabatically. This warmer sinking air creates a high level inversion that acts as a dome capping the atmosphere, inhibiting convection, thereby trapping high humidity warm air below it.
Typically, convection is present along the periphery of the cap where the pressure becomes less. This peripheral convection, however, can add to the high pressure dome by ventilating the upper level outflow of the thunderstorms into it. The end result is a continual build-up of heat at the surface that people experience as a heat wave. 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. In the Western Cape Province of South Africa , a heat wave can occur when a low pressure offshore and high pressure inland air combine to form a Bergwind. The highest official temperatures recorded in South Africa Global warming boosts the probability of extreme weather events, like heat waves, far more than it boosts more moderate events.
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. 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. 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.
Heatwave sweeps across Europe
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 stockings and elevating the affected legs will 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. 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 in the United States.
Between and , deaths from excessive heat in the United States numbered 2,, compared with deaths from floods and from hurricanes.
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. According to the Agency for Health care Research and Quality, about 6, Americans are hospitalized each summer due to excessive heat, and those at highest risk are poor, uninsured or elderly. Our concern now is 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. The number of heat fatalities is likely highly underreported due to a lack of reports and misreports. 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 reductions in mortality suggest that heat affects especially those so ill that they "would have died in the short term anyway". Another explanation for underreporting is the social attenuation in most contexts of heat waves as a health risk.
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As shown by the deadly French heat wave in , heat wave dangers result from the intricate association of natural and social factors. 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. In every society, crime rates go up when temperatures go up, particularly violent crimes such as assault, murder, and rape.
The fall heat wave was gnarly
Furthermore, in politically unstable countries, high temperatures are an aggravating factor that lead toward civil wars. Additionally, high temperatures have a significant effect on income. A study of counties in the United States found that economic productivity of individual days declines by about 1.
Abnormally hot temperatures can cause electricity demand to increase during the peak summertime hours of 4 to 7 p. 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.