ON A map on the IMD website, “human discomfort” across the country is mapped, showing Mumbai as a blue blip, Kolkata in green and Agra in yellow. The colours indicate each location’s Heat Index, a measure of the stress placed on humans by the climate. IMD officials believe that Heat Index (HI) is a single value that takes both temperature and humidity into account.
The experiential map further shows that Mumbai has a heat index of 30 and its comfort index is currently comfortable — in contrast to Kolkata and Agra, where the heat index is 37 and 42, respectively, and the corresponding comfort indices readings are ‘sultry’ and ‘uncomfortable sultry’.
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IMD’s latest figures disclose that 2016 was the hottest year in a over a century since 1901. Now, a study conducted to find the Heat Index, mainly in urban India, places Mumbai in the “very hot” category.
“Since Mumbai is a coastal city, the influence of the sea does not increase the maximum temperatures in the summers. The humidity though in the same period is extremely high. In addition, with the following monsoon months reporting a decrease in rainy days, the research has found that monsoons in Mumbai are hotter than than the summers; with fewer cloudy days and many sunny days. This combination makes Mumbaikars feel very uncomfortable over a span of at least seven months,” says Ashok Jaswal, the lead researcher with the IMD who retired ten months ago.
Jaswal’s research that was published on January 19 mainly concludes that erratic rainfall and rapid urbanisation are the primary reasons for making summers more humid and monsoons hotter. The research indicates that Mumbai’s Heat Index would be between 41-54, as the city’s climate could effect 18 million population from heat cramps, exhaustion and heatstroke.
While the HI across the country is increasing during summer and monsoon seasons at the rate of +0.56 degrees Celsius/decade and +0.32 degrees Celsius/ decade, respectively, for Mumbai, the study shows that +0.38 degrees per decade during the summers and +0.56 degrees per decade during the monsoons.
The India Meteorological Department (IMD) and the Indian Institute of Tropical Meteorology carried out the analysis of temperature and humidity levels across 283 weather stations between 1951 and 2010. The monthly mean maximum temperature and relative humidity records had been used to analyse HI during summer (March to May) and monsoon (June to September) seasons for the last 60 years.
The study finds that most of the mega cities showed significant increasing trends in HI during summer and monsoon seasons. The hilly regions, however, were not found to be affected by the varying temperatures.
“The human body normally cools itself by perspiration, or sweating, in which the water in the sweat evaporates and carries heat away from the body. However, when the relative humidity is high, the evaporation rate of sweat is reduced. This means heat is removed from the body at a lower rate…The risk of heat-related illness becomes greater as the weather gets hotter and more humid,” the recently published ‘Journal on Climate Change’ reads.
The data further reveals that 13 out of the 15 warmest years were during the last 15 years between 2002 and 2016. The year 2016 was +0.91 degrees more than normal, followed by 2015 (+0.72), 2010 (+0.81) and 2009 recorded (+0.84).
“Almost all regions of the country have experienced significant increase in HI, which is consistent with the reported trends in neighbouring Pakistan and Bangladesh. These (urban) cities (in India) will be exposed to climate change from greenhouse gas induced radiative forcing and localised effects from urbanisation,” the research reads.
The rise in heat index results in higher heat related mortality rate during summer. According to 2016 IMD figures, of the total deaths due to natural causes, 40 per cent were due to severe heatwave. More than 700 died due across the country. Telangana and Andhra Pradesh together recorded 400 deaths, while Maharashtra reported 43 such deaths last year.
The Intergovernmental Panel on Climate Change (IPCC), in its recent report, has said effects of climate change will lead to more extreme temperatures, which will include more hot days than cold and more intense summer heatwaves.