Delhi on Tuesday recorded a ‘real feeling’ temperature of 53 degrees Celsius.
The highest temperature today on the thermometer was 37 degrees Celsius.
The difference between the two embodies the discomfort that comes with high humidity levels, a phenomenon that is uncommon in most parts of northern India.
Meteorologists say this moisture is the result of southwesterly winds off the Arabian Sea feeding moisture into northwest India, even though the monsoon, which was delayed from its normal start in Delhi on June 27, has not yet arrived.
Until it arrives, the combination of moisture and heat can only be broken by a scattered, short-lived respite of thunderstorms in the area.
Heat index
The “true feel” temperature or heat index (HI) is an estimate of how hot a person feels. This concept is based on the idea that high humidity slows the evaporation of sweat, which affects the human body’s ability to cool.
In the 1970s and early 1980s, American meteorologist Robert Steadman came up with a formula that quantified the physiological effects of heat and high humidity on humans. The US National Weather Service fitted a regression equation – called the Rothfus equation – to Steadman tables in 1990, and it is the formula used by most meteorological agencies until now.
Read also: Delhi records 55.6% rainfall deficit in June amid heatwave; Monsoon possible in 4 days
Why is it dangerous?
Although humidity levels are uncommon in the northern plains, humidity levels rise before the onset of the monsoon every year.
This poses a health risk unique to just hot, dry conditions, because high humidity disrupts the body’s main cooling mechanism: sweating.
As the body temperature rises, the brain causes sweating and widens of blood vessels in the skin that redirect blood from the body’s core to its surface, so heat can escape into the air. This only works when the humidity in the air is low enough for the moisture on the skin to evaporate easily. With high humidity, sweat simply collects on the skin and falls off without cooling anything at all.
As evaporation weakens, the body leans harder on its other channel, pumping more blood to the skin, making the heart work harder. That’s why cardiovascular stress, not heatstroke, is a more common result of exposure to humid heat.
Wet bulb temperature
A related but separate indicator is human humid body temperature, which actually measures the human body’s ability to cool when exposed to heat and humidity.
Between June 28 and 30, when the heat index in Delhi crossed the 50 degree Celsius mark, the humid temperatures approached 30 degrees Celsius, the Meteorological Department data showed.
In its initial form, this reading was taken using a thermometer bulb wrapped in a wick filled with water, then exposed to air and left to cool to the lowest temperature that evaporation could achieve.
When the wet bulb temperature is low, sweat evaporates easily. But when the skin temperature gets close to that, evaporation — the body’s main way to release heat — stops.
Scientists say the body cannot survive prolonged exposure to hot, humid weather when the wet-bulb temperature exceeds 35 degrees Celsius.
The threshold may be lower than that. Core body temperatures in healthy young people can start to rise uncontrollably after a wet bulb reading exceeds 31°C, putting them at risk of heat exhaustion and heatstroke, researchers have warned.
Read also: Delayed monsoon slows down paddy cultivation in Haryana
How successful are these standards in India?
The heat index is calibrated for an adult weighing approximately 67 kg walking at an easy pace in light, western-style summer clothing in the shade with a gentle breeze. This “normal” type is derived from the Western world, not the climate or dress of India.
Applying this to a country where a large number of people work outdoors in direct sunlight could reduce the risk of heat-related illness.
In 2023, researchers from IIT Delhi attempted to bridge this gap by constructing a separate India Heat Index (IHI). “All existing indicators were developed based on data from developed countries,” said Sajnik Dey, co-author of the study.
