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Although, according to Met Office meteorologist John Hammond, there’s no official definition of a heatwave, these hot-weather phenomena take their toll not only on a population’s health, but also business and infrastructure – such as power, water and transport. A heatwave is a period of unusually hot or humid weather that lasts at least two or three days – and remaining hot throughout the nights – that affects large areas. Heatwaves are caused by a system of higher atmospheric pressure, whereby air from upper levels of the atmosphere descends and rotates out. As it descends, it compresses, increasing the temperature. The outward flow, meanwhile, makes it difficult for other systems to enter the area, and the large size and slow speed of the hot air causes the heatwave to remain for days or even weeks. The lack of clouds means that an affected area is struck with strong sunlight.
Hammond reveals that the hottest temperatures in the UK are likely to be over parts of central and southern England, away from immediate coastal areas, which are cooled by sea breezes. “Temperatures have exceeded 30ºC in the UK,” he explains, “[but around] Europe and the world, weather conditions can bring temperatures exceeding 40ºC. This has happened in Mediterranean regions, the Middle East and Australia among other areas.”
Heatwaves are relative to an area’s climate – temperatures that would constitute a heatwave in one area might not in another location – and the health effects on the individual are also relative to a range of risk factors. People adapt and become accustomed to their long-term temperature patterns, making a heatwave a relative experience.
The Met Office Heat Health Watch is a warning system that issues alerts – levels 1-4 – if a heatwave is imminent. “[We] can identify weather patterns that might bring hot temperatures to the UK several days in advance,” explains Hammond. “When high temperatures are expected, detailed advice will go to the relevant health organisations, so they can inform people affected by the heat. Met Office forecasts on TV, radio, newspapers and online will also provide temperature forecasts for the public.”

Australia Heatwave Breaks Records
This map of Australia captured by NASA’s Moderate-Resolution Imaging Spectroradiometer (MODIS) compares January-February 2009’s average land temperature with previous years. Red areas are warmer than previously while blue areas are cooler. The darker the colour the more extreme the temperature change. Really dark red or dark blue areas reveal where the difference in temperature is 10ºC higher or lower than previously.
The abnormally hot temperatures here – the highest recorded being 48.8°C in Hopetoun, Victoria – indicate a severe heatwave. Slow-moving high pressure lingered over the Tasman Sea and conditions conspired to cause hot tropical air to blow across south-east Australia. The extreme heat worsened the country’s alreadydangerous bushfi reseason, and led to the Black Saturday bushfires, which caused the deaths of 173 people on Saturday 7 February, also destroying homes and towns in the process.
South-east Australia’s January heatwave set the record for Melbourne’s highest recorded temperature of a blistering 46.4ºC according to the Bureau of Meteorology in Melbourne.

Effects on the body