Delhi’s temperature was 40.9 degrees Celsius but felt like 45. Understanding the heat index

A simple search for Mumbai’s temperature today (15 May) on one’s phone and it will say it is 32 degrees Celsius but it feels like 40 degrees Celsius. On Sunday, in the national capital, mercury levels rose to a staggering high of 40.9 degrees Celsius. But a search online and the weatherman said that it felt more like 45 degrees Celsius.

Why two temperatures?, you ask, scratching your head. The answer is that while one is the actual temperature, the one that says feels like is the heat index.

But what does this mean? Why are two temperatures mentioned? What is the purpose of the heat index?

We take a closer look and give you the answers.

Heat index explained

The heat index, also known as the apparent temperature or reel feel, is what the temperature feels like to the human body when relative humidity is combined with the air temperature.

The heat index was developed in 1979 by Robert G Steadman. It helped bring a human element to the weather by making the temperature readings closer to what we people really feel, or at least close to it.

But one may ask what’s the connection between humidity and heat with the human body? Humidity and heat have significant effects on the human body. When the mercury starts to climb, the body begins to perspire or sweat to cool off. However, when the atmospheric moisture content – the humidity –is high, the rate of the evaporation of sweat from your skin slows down. Instead, the sweat just drips, which leaves you with a damp T-shirt and none of the cooling effect. When the humidity spikes, we effectively lose a key tool that would normally cool us down.

Also read: India’s Killer Summer: What a heatwave does to your body

This means that higher the relative humidity, the higher is the heat index and the hotter it feels to our bodies outside.

Weather experts have found that when the air temperature is 24 degrees Celsius and the relative humidity is zero per cent, one will feel like the temperature is 21 degrees Celsius. Similarly, if the temperature is 24 degrees Celsius, but the relative humidity is 100 per cent, we feel like it’s 27 degrees Celsius.

Calculating the heat index

Besides the relative humidity and the actual temperature, meteorologists use specific calculations and assumptions to determine the heat index for a specific time of a certain day. They make things a bit simpler by assuming certain values and measurements, like an average person’s weight, how much clothing they’re wearing, and how fast the breeze is at the time.

Based on these, weather experts have come out with a chart to show the heat index based on the temperature and humidity. While every person is different from another, the measurements are close enough to be considered a good enough estimate to be used in everyday life.

One thing important to note is that the heat index assumes that a person is standing in the shade: this is because it is natural for a person to stay in the shade rather out in the sun. The body will feel differently if standing in direct sunlight.

The National Weather Service has said that full exposure to sunlight can make it feel even hotter by as much as 15 degrees.

Heat index in India

While the practice of issuing a heat index has been in play in the US for long, it was on 27 March that the IMD began issuing a heat index. Speaking to Times of India, IMD officials said that it would be calculated on factors such as relative humidity, minimum and maximum temperatures, wind speed and the duration of a heatwave spell.

Also: It’s not only India: How Asia is battling the ‘worst April heatwave’

And based on this heat index, the officials would decide the warning levels – red, yellow, or orange and issue precautions that people can take to avoid heat strokes, heat exhaustion and heat-related sicknesses.

Importance of heat index

A study by IMD scientists have found that the number of heatwaves have increased by about 24 per cent during 2010-2019 as compared to 2000-2009 with associated mortality rates also increasing by about 27 per cent.

Even the World Bank in a report titled “Climate Investment Opportunities in India’s Cooling Sector” which was released last year said that the country is experiencing higher temperatures that arrive earlier and stay far longer. It also said that the country could become one of the first places in the world to experience heat waves that break the human survivability limit.

With the release of a heat index, officials will be able to issue warnings that could help prevent people from contracting a heatstroke or other heat-related illnesses.

Presently, the temperatures are soaring across central India and parts of the north, affecting tens of millions. The weatherman has also stated that Odisha will experience hot and uncomfortable weather for the next five days.

Moreover, heatwave conditions are likely over parts of Gujarat, Maharashtra, Bihar and West Bengal after 12 May. The highest maximum temperature in India this week was 44.8 degrees Celsius at Jalgaon in Maharashtra.

India also logged its hottest February this year since record-keeping began in 1901, according to the Indian Meteorological Department.

With inputs from agencies

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