Imagine stepping outside and the air feels like a furnace your skin prickles, sweat rolls down your back, and with every breath, you feel a little heavier. Extreme heat isn’t just uncomfortable; it’s a silent threat that can change your body in surprising and sometimes alarming ways. Every year, record-breaking temperatures sweep across cities, leaving people struggling to cope, and the impact on human health is far deeper than just a sunburn or a sweaty forehead……..Continue reading….
Source: weather fox
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A heat wave or heatwave, sometimes described as extreme heat, is a period of abnormally hot weather. Definitions vary but are similar. A heat wave is usually measured relative to the usual climate in the area and to normal temperatures for the season. Temperatures that humans from a hotter climate consider normal, can be regarded as a heat wave in a cooler area. This would be the case if the warm temperatures are outside the normal climate pattern for that area.
High humidity often occurs during heat waves as well. This is especially the case in oceanic climate countries. Heat waves have become more frequent, and more intense over land, across almost every area on Earth since the 1950s, the increase in frequency and duration being caused by climate change. Heat waves form when a high-pressure area in the upper atmosphere strengthens and remains over a region for several days up to several weeks.
They can lead to widespread electricity outages because more air conditioning is used. A heat wave counts as extreme weather. It poses danger to human health, because heat and sunlight overwhelm the thermoregulation in humans. There are several definitions of heat waves: The IPCC defines heatwave as “a period of abnormally hot weather, often defined with reference to a relative temperature threshold, lasting from two days to months.
A definition based on the 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 5 °C (9 °F), the normal period being 1961–1990. The same definition is used by the World Meteorological Organization. A definition from the Glossary of Meteorology is: “A period of abnormally and uncomfortably hot and usually humid weather.”
Denmark defines a national heat wave (hedebølge) as a period of at least 3 consecutive days in which the average maximum temperature across more than half the country exceeds 28 °C (82.4 °F). The Danish Meteorological Institute also has a definition for a “warmth wave” (varmebølge). It defines this using the same criteria for a 25 °C (77.0 °F) temperature.[15] Sweden defines a heat wave as at least five days in a row with a daily high exceeding 25 °C.
In Greece, the Hellenic National Meteorological Service defines a heat wave as occurring over three consecutive days with temperatures at 39 °C (102 °F) or higher. In the same period the minimum temperature is 26 °C (79 °F) or more. During this period, there are either no winds or only weak winds. These conditions occur in a broad area. The Netherlands defines a heat wave as a period of at least five consecutive days in which the maximum temperature in De Bilt exceeds 25 °C (77 °F).
During this period the maximum temperature in De Bilt must exceed 30 °C (86 °F) for at least three days. Belgium also uses this definition of a heat wave with Ukkel as a reference point. So does Luxembourg. In the United Kingdom, the Met Office operates a Heat Health Watch system. This places each Local Authority region into one of four levels. Heat wave conditions occur when the maximum daytime temperature and minimum nighttime temperature rise above the threshold for a particular region.
The length of time above that threshold determines the level. Level 1 represents normal summer conditions. Level 2 occurs 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 arises 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.
In the United States, definitions also vary by region. They usually involve a period of at least two or more days of excessively hot weather. In the Northeast, a heat wave typically when the temperature reaches or exceeds 90 °F (32.2 °C) for three consecutive days. This is not always the case. This is because the high temperature 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 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 it expects unusual periods of hot weather.
In Adelaide, South Australia, a heat wave is five consecutive days at or above 35 °C (95 °F), or three consecutive days at or over 40 °C (104 °F).The Australian Bureau of Meteorology defines a heat wave as three or more days of unusual maximum and minimum temperatures. Before this new Pilot Heatwave Forecast there was no national definition for heat waves or measures of heat wave severity.
New high temperature records have outpaced new low temperature records on a growing portion of Earth’s surface.Large increases in both the frequency and intensity of extreme weather events (for increasing degrees of global warming) are expected.Map of increasing heat wave trends (frequency and cumulative intensity) over the midlatitudes and Europe, July–August 1979–2020.
It is possible to compare heat waves in different regions of the world with different climates thanks to a general indicator that appeared in 2015. With these indicators, experts estimated heat waves at the global scale from 1901 to 2010. They found a substantial and sharp increase in the number of affected areas in the last two decades.
One study in 2021 investigated 13,115 cities. It found that extreme heat exposure of a wet bulb globe temperature above 30 Celsius tripled between 1983 and 2016, and if the effect of population growth (increasing the urban heat island effect) during those years is excluded, the exposure increased a further 50%. The researchers compiled a comprehensive list of past urban extreme heat events.
Heat waves form when a high pressure area at an altitude of 10,000–25,000 feet (3,000–7,600 metres) strengthens and remains over a region for several days and up to several weeks. This is common in summer in both the Northern and Southern Hemispheres. This is because the jet stream ‘follows the sun’. The high pressure area is on the equator side of the jet stream in the upper layers of the atmosphere.
Weather patterns are generally slower to change in summer than in winter. So, this upper level high pressure also moves slowly. Under high pressure, the air sinks toward the surface. It warms and dries adiabatically. This inhibits convection and prevents the formation of clouds. A reduction of clouds increases the shortwave radiation reaching the surface. A low pressure area at the surface leads to surface wind from lower latitudes that brings warm air, enhancing the warming.
The surface winds could also blow from the hot continental interior towards the coastal zone. This would lead to heat waves on the coast. They could also blow from high towards low elevations. This enhances the subsidence or sinking of the air and therefore the adiabatic warming. In the eastern regions of the 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.
Hot humid air masses form over the Gulf of Mexico and the Caribbean Sea. At the same time hot dry air masses form over the desert Southwest and northern Mexico. The southwest winds on the back side of the high continue to pump hot, humid Gulf air northeastwards. This results in a spell of hot and humid weather for much of the eastern United States and into southeastern Canada.
In the Western Cape Province of South Africa, a heat wave can occur when the low-pressure area offshore and the high-pressure area inland combine to form a bergwind. The air warms as it descends from the Karoo interior. The temperature will rise about 10 Celsius from the interior to the coast. Humidity is usually very low. The temperature can be over 40 Celsius in summer.
The highest temperature recorded in South Africa (51.5 Celsius) occurred one summer during a berg wind along the Eastern Cape coastline. The level of soil moisture can intensify heat waves in Europe. Low soil moisture leads to a number of complex feedback mechanisms. These in turn can result in increased surface temperatures. One of the main mechanisms is reduced evaporative cooling of the atmosphere.
When water evaporates, it consumes energy. So, it will lower the surrounding temperature. If the soil is very dry, then incoming radiation from the sun will warm the air. But there will be little or no cooling effect from moisture evaporating from the soil.
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