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Delhi feels the heat: How does India measure heatwave and why temperatures that define it are so high – India Today

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The United Kingdom on Monday raised the temperature at which forecasters will declare a heatwave in an area as climate change makes these extreme weather events more frequent. The UK Meteorological office raised the thresholds in some counties from 27 degrees Celsius to 28 degrees Celsius threshold and in others from 26 degrees Celsius to 27 degrees Celsius followed by 25 degrees Celsius to 26 degrees Celsius in one.
The decision comes in the backdrop of global warming intensifying the occurrence of these weather-related events. As the world grapples with heat waves, India is no different. The India Meteorological Department on Tuesday predicted a heatwave spell over Northwest, Central & West India for the coming four to five days.
The IMD in a statement said that there could be a gradual rise in maximum temperatures by about 2 degrees Celsius over many parts of Central and East India during the next three days. The weather agency warned that heatwave could lead to moderate health concerns for vulnerable people and there is an increased likelihood of heat illness symptoms in people who are either exposed to the sun for a prolonged period or doing heavy work.
A heatwave is defined when a region experiences predominantly high temperatures for a sustained period of time. The India Meteorological Department defines a heatwave as a condition of air temperature which becomes fatal to the human body when exposed. “It is defined based on the temperature thresholds over a region in terms of actual temperature or its departure from normal. In certain countries, it is defined in terms of the heat index based on temperature and humidity or based on the extreme percentile of the temperatures.”
IMD considers an area experiencing a heatwave if the maximum temperature of a station reaches at least 40 degrees Celsius or more for the plains and at least 30 degrees Celcius or more for hilly regions. A heatwave is also characterised if the temperature goes at least 4.5 to 6.4 notches up and if ts exceeds over 6.4 degrees Celcius above normal, the IMD classifies it as a severe heatwave.
Meanwhile, when it comes to coastal regions, a heatwave is announced if the maximum temperature is 4.5 degrees Celsius or more than the normal temperature experienced in the area.
IMD predicts heatwave based on synoptic analysis of various meteorological parameters and from the consensus guidance from various regional & global numerical prediction models.
The heatwave threshold in India is high compared to what the United Kingdom has deemed fit to declare a heatwave, since India has tropical weather conditions where temperatures are generally high.
Senior IMD scientist R K Jenamani told that heatwave thresholds differ across the world based on the climatic factors experienced in those regions. “Different countries have different climatic conditions, scoping capabilities and therefore, while in some countries it’s high, in others it’s low. The Indian sub-continent has a tropical climate with high temperatures. Therefore, in countries like India, Bangladesh, and Sri Lanka heatwave is declared at higher temperatures compared to that of the United Kingdom.”
The senior scientist denied any plans for the weather agency to alter the heatwave threshold for the country in the coming times, even as he acknowledged that climate change is leading to extreme weather events across the world.
The IMD in its latest projections has predicted a gradual rise in maximum temperatures by about 2 degrees Celsius very likely over many parts of Central and East India during the next three days. A rise in maximum temperatures by 2-4 degrees Celsius is very likely over Maharashtra during the next five days. The weather agency has also predicted severe heatwave conditions in most parts very likely over West Rajasthan, in some parts over East Rajasthan & West Madhya Pradesh on March 29, 30.
While parts of North India and Central India, northeast India is likely to experience light to moderate rainfall during
the next five days with a possibility of isolated heavy falls over Assam-Meghalaya and Arunachal Pradesh on March 31 and April 1.
Maximum temperatures have been rising in Delhi and parts of the national capital region in the last couple of days, hovering between 39-41 degrees Celsius. However, the IMD said that for it to declare a heatwave or severe heatwave the temperatures need to be consistently above normal by 4.5 degrees.
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