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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, 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. 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. == Definitions == A definition based on Frich et al.'s 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 , 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 . More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region. In the Netherlands, a heat wave is defined as a period of at least 5 consecutive days in which the maximum temperature in De Bilt exceeds , provided that on at least 3 days in this period the maximum temperature in De Bilt exceeds . 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 – the Danish Meteorological Institute further defines a "warmth wave" (''varmebølge'') when the same criteria are met for a temperature, while in Sweden, a heat wave is defined as at least 5 days in a row with a daily high exceeding . 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. In the Northeast, a heat wave is typically defined as three consecutive days where the temperature reaches or exceeds , but not always as this 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 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 , or three consecutive days at or over . 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Heat wave」の詳細全文を読む スポンサード リンク
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