Heatwave in India: What is wet bulb temperature, why is it important and how to measure it

• There is an ongoing heatwave in India, forcing millions of Indians indoors amidst the worst March in 122 years.
• Coal shortage and power cuts across the country have also worsened things for millions across the country.
• This is where wet bulb temperature comes in and here’s why it is important.

There is an ongoing heatwave in India that has forced millions of people indoors, with air conditioner sales nearly doubling when compared to the previous year. This heatwave has also brought the wet bulb temperature concept into mainstream conversation.

There are multiple reasons behind this – while the current heatwave is not India’s worst, yet, we endured the hottest March in 122 years. Coal shortages and a drastic surge in power demand have led to power cuts in several parts of the country – some as long as eight hours. In fact, there is a power cut right now as I write this story.

Global warming is expected to worsen this situation with each passing year. And while the temperature and humidity levels give us a fair amount of idea about the weather outside, they do not tell us exactly how bad it can be for our bodies.

This is where the wet bulb temperature is useful.

What is wet bulb temperature?

In simpler terms, wet bulb temperature tells us at what level our bodies will be unable to cool themselves down by sweating. In this case, the threat of a heat stroke rises dramatically.

Wet bulb temperature combines heat and humidity to indicate how much evaporation can be absorbed into the air. It measures the lowest temperatures that our bodies can reach when we are in hotter environments, by sweating.

Our bodies cool themselves down in hot temperatures by sweating – when the sweat evaporates from the surface of the skin, the temperature of our bodies comes down.

This evaporation is easier when you are in a drier zone with lower humidity levels. For instance, 35-degree Celsius in a coastal Indian city like Chennai is a lot more unbearable than in places like Hyderabad, Indore or any other non-coastal city in the country.

This is because the humidity levels are much higher in coastal areas than those far away from the seas. When the air around you is already humid, it becomes difficult for the sweat on your skin to evaporate.

When the sweat does not evaporate quickly enough, your body does not cool down and you start feeling uneasy.

This is exactly what wet bulb temperature measures.

Why is wet bulb temperature important?

Dry temperature – or the temperature that we see in daily weather forecasts – does not tell you the full story.

Wet bulb temperature, especially in times of a heatwave, tells us how habitable a place is for the human body.

A wet bulb temperature of 32-degree Celsius is usually the maximum that a human body can endure and carry out normal outdoor activities in. This is equivalent to a dry temperature of 55-degree Celsius.

The theoretical maximum wet bulb temperature is 35-degree Celsius – most humans, even with unlimited water supply, are likely to suffer heat strokes at this level, likely leading to death.

How to calculate wet bulb temperature?

Now that you know what wet bulb temperature is, and the maximum limit that you can survive in, you might want to know how to calculate it.

This handy calculator helps you calculate the current wet bulb temperature.

For this, you will need two details – the current dry temperature and relative humidity in the air, which will be available in weather forecasts.

For instance, the dry temperature in Hyderabad at 10:30 a.m. today is 31-degree Celsius with 55% humidity. The current wet bulb temperature is 24-degree Celsius.

In Chennai, the current temperature is 34-degree Celsius with 73% humidity, resulting in a wet bulb temperature of 29.8-degree Celsius.

Notice the massive difference in the temperatures due to marginally high dry temperature, but a much higher level of humidity.

Here’s a quick comparison for some other Indian cities:



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