Why did the temperature in Delhi feel like 52° when thermometers said 39°?

Earlier this month, on the evening of July 8th, people in Delhi experienced unbearably sultry weather even as the maximum temperature recorded that day was 39 degrees Celsius, just two degrees above the season’s average. 

With relative humidity going as high as 74%, Delhi’s heat index, also known as ‘real feel’ or ‘feels like’ temperature, shot to 52 degrees.

‘Real feel’ tells us the impact of weather on people. It takes into account wind speed, cloud cover, dew point, humidity and the amount of sunlight.

As a result, ‘real feel’ temperature gives a better idea about how the weather will actually feel when one steps outdoors, where wind and humidity can make a big difference.

While easterly winds brought moisture from the Bay of Bengal to Delhi, the near absence of rains in the first week of July led to high humidity levels creating near intolerable wet-bulb temperatures.

As humidity increases, it becomes harder for people to cool down through sweating as water evaporates slowly in humid conditions. Such conditions are indicated by wet-bulb temperature, which is measured by wrapping a wet cloth over a thermometer. 

The wet-bulb temperature is based on two factors - temperature and relative humidity. It is the lowest temperature that can be achieved under current ambient conditions by the evaporation of water.

Even heat-acclimatised people cannot carry out normal outdoor activities in wet-bulb temperatures above 32 °C, which is the equivalent of a heat index of 55 degrees.

And Delhi recently saw wet-bulb temperatures exceeding this level - albeit briefly - on a few occasions. In normal times, it is lower than the normal air temperature.

As long as the wet-bulb temperature is well below the skin temperature, the body can release heat to your surroundings through sweating. But as the wet-bulb temperature approaches our core temperature, we lose the ability to cool down, hence the discomfort.

At wet-bulb temperatures above 35 degrees, even healthy people with unlimited water, shade and no physical activity will die of heatstroke after a few hours of exposure.

At such extremely high wet-bulb temperatures, there is so much moisture in the air that sweating becomes ineffective at removing the body’s excess heat. After about six hours or more, it can lead to organ failure and death in the absence of access to artificial cooling. And that is why people in Delhi felt the thermometer wasn’t reflecting reality.