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As the monsoon continues to play truant over northwest India, including Delhi, the Capital is experiencing a mix of heatwave-like conditions and high humidity levels, which, forecasters say, will last till July 10 when the monsoon is expected to arrive. Scientists said that in the absence of rainfall, Delhi and parts of NCR are recording high wet-bulb temperatures, a combined measure of heat and humidity of a region.

On Tuesday, according to the India Meteorological Department (IMD), the maximum temperature in Delhi was 40.6 degrees Celsius, four degrees above what is considered normal for this time of year. The relative humidity was around 54%. This translates into a wet-bulb temperature of 32.28 degrees Celsius. That may not seem like much, but according to scientists, it is next to impossible to work outside when the wet-bulb temperature is over 32 degrees. Worse, on prolonged exposure to a wet-bulb temperature of 35 degrees or higher, even fit people may face significant discomfort, even collapse and require hospitalisation.

Scientists say the wet-bulb temperature is a good indicator of how hot we feel (as opposed to how hot it is). To be sure actual measures of heat perception involve other variables such as angle of sunlight and wind direction and speed, but, in general, the wet-bulb temperature itself is a good proxy for this. The formula for wet-bulb temperatures is complex although it involves only two variables -- temperature and relative humidity. It makes up for this by using inverse tangent (or arc tan) trigonometric functions.

Mahesh Palawat, vice-president (climate change and meteorology) at Skymet Weather, a private weather forecaster, explained that temperatures are high in Delhi on account of a delayed monsoon, but moisture carrying easterly winds that are intermittently blowing into the city are keeping the humidity levels also high. The result is high discomfort.

“Usually during this time of the year, Delhi receives a few spells of rainfall that brings the maximum temperature down. So, even if the humidity levels are high, the discomfort is not as much. But this time, the temperatures are high because of delayed monsoon and so are the humidity levels, this makes people sweat more profusely and makes staying outdoors extremely difficult,” Palawat explained.

In short, it’s almost as bad as it is in Chennai in May when temperatures in the low 40s and relative humidity in the high 80s combine to take wet-bulb temperatures dangerously high.

Climate Trends, a Delhi-based communications group focusing on environment and climate change, said that Delhi experiences 63 such days in a year (called wet-bulb days) , Mumbai , 47, Trivandrum, 113 and Chennai, 140.

India Meteorological Department (IMD) on Monday revised the dates for monsoon arrival in Delhi and parts of northwest and central India to July 10, which will make this the most delayed monsoons in at least the last 17 years.

“The human body can withstand dry heat, but as humidity levels increase along with heat, the ability to spend time outdoors slowly decreases,” said a senior IMD official.

IMD forecasts temperatures in the city over the next two days to be around 40 degrees Celsius, which will continue the spell of sweltering heat in the city.

Climatologists explained that this shift to recording felt heat is a global trend.

“People do not grasp the seriousness of the situation. An entire set of livelihoods depends on being outside. Imagine being a construction worker who can’t work for two months of the year,” said Camilo Mora, associate professor (department of geography and environment), University of Hawaii.

He added: “Your sweat doesn’t evaporate if it is very humid, so heat accumulates in your body. People can then suffer heat toxicity, which is like sunburn on the inside of your body. This is all an impact of years of neglect of the environment and it is now coming back to haunt us.”

Soumya Pillai covers environment and traffic in Delhi. A journalist for three years, she has grown up in and with Delhi, which is often reflected in the stories she does about life in the city. She also enjoys writing on social innovations. ...view detail

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