Heat index, not the thermometer, makes Delhi feel a punishing 51°C. What is it?

By midday on Sunday, June 28, Delhi's weather apps were telling two different stories at once. One said the temperature was 41.3 degrees Celsius. The other said it felt like 51.3 degrees Celsius, the highest reading this year. Both were right. The first was measured by a thermometer. The second was, in effect, measured by your own body.

That second number has a name. It is called the heat index, and it explains why this sticky spell feels so much crueller than the dry furnace of May.

WHAT IS THE HEAT INDEX?

The heat index, also called the apparent temperature or "feels like" temperature, is a single figure that tells you how hot the weather feels to a human being. It blends two ingredients: the air temperature, and the relative humidity.

Relative humidity is simply how full of moisture the air is. Picture the air as a sponge. Relative humidity tells you what fraction of that sponge is already soaked.

At 100 per cent, the air is completely saturated and can hold no more water, which is exactly why, on such days, your sweat has nowhere to go.

The heat index was worked out in 1979 by an American scientist, Robert Steadman, who modelled how heat and moisture act together on the human body. Weather agencies worldwide, including the United States National Weather Service, still use a version of his work today.

WHY DOES HUMIDITY MAKE DELHI FEEL HOTTER?

Your body cools itself mainly by sweating. As sweat evaporates off your skin, it carries warmth away. But evaporation only works if the surrounding air has room to absorb more moisture.

When humidity is high, the air is already crowded with water vapour, so the sweat simply sits on your skin. The cooling stalls, and your core temperature creeps up.

This is precisely Delhi's problem now. Southwesterly winds are dragging moisture inland from the Arabian Sea while the monsoon is still days away from the state.

Humidity on June 28 swung between 35 and 63 per cent. The result is sticky heat, not dry heat, and sticky heat is far harder for the body to shed.

HOW IS THE HEAT INDEX CALCULATED?

Here is the part that surprises most people. You cannot work out the heat index by simply adding a few degrees to account for the humidity. Heat and moisture do not take turns. They gang up.

Think of it this way. On a dry day, your sweat evaporates freely, so a rise in humidity barely troubles you. But on a day that is already hot and already damp, even a small extra dose of moisture lands hard, because your sweat has nowhere left to go. The same humidity that felt harmless in the morning becomes punishing by afternoon.

Scientists captured this teamwork in a formula. They did not dream it up from theory. They took thousands of real readings of temperature, humidity and how hot people actually felt, then drew a single curve through all of it. That curve is the heat index formula. The most widely used version, written by the American scientist Lans Rothfusz in 1990, reads like this:

HI = -42.379 + 2.04901523T + 10.14333127R - 0.22475541TR - 0.00683783T^2 - 0.05481717R^2 + 0.00122874T^2R + 0.00085282TR^2 - 0.00000199T^2R^2

The equation looks terrifying, but you only need to know two letters. T is the air temperature and R is the relative humidity. Everything else is just dials the scientists set, so the answer matches how a real body feels.

Now look at what the formula is quietly doing. Some parts use T on its own, which is the plain effect of heat. Some use R on its own, the plain effect of moisture. The interesting parts are the ones where T and R are multiplied together, like TR and T^2R.

That multiplication is mathematics' way of saying heat and humidity amplify each other rather than simply stacking up. When both are high at once, those combined terms grow fast and the felt temperature shoots up.

You can see the effect without doing any of the sums yourself. Take an air temperature of 39.5 degrees Celsius and 43 per cent humidity, close to Delhi's situation right now. The thermometer says one thing, but the formula returns 48.7 degrees Celsius.

That is almost ten extra degrees, conjured out of nothing but water vapour. The very same machinery is turning Delhi's 41.3 degrees into a brutal 51.3.

IS A HEAT INDEX OF 51 DEGREES DANGEROUS?

Yes. Once the heat index climbs past about 41 degrees Celsius, doctors warn of heat cramps and exhaustion. Beyond 51 degrees, the danger of heat stroke turns severe, especially for outdoor workers, the elderly, small children and anyone exerting themselves in the Sun.

Scientists studying the limits of human endurance also observe a sister measurement, the wet-bulb temperature.

In short, the heat index tells you how hot the weather feels to your body, while the wet-bulb temperature tells you whether your body can still cool itself at all, which is why scientists treat the wet-bulb figure as the more serious measure of survival.

A landmark 2010 study in the journal PNAS identified a 35-degree-Celsius wet-bulb as the point beyond which the body cannot cool itself at all, whatever the shade or water available.

A more recent paper in Science Advances found that brief crossings of dangerous wet-bulb thresholds are already happening in parts of the world, decades earlier than expected. Delhi is well short of that line, but humid episodes like this nudge towards it.

WHEN WILL DELHI'S HEAT EASE?

Soon, though not instantly. The IMD expects rain, thunderstorms and gusty winds from Monday, with a yellow alert on Tuesday and Wednesday.

Scattered showers are likely around July 2 or 3, with the monsoon forecast after July 4. Until the skies break, the number worth watching is not the one on the thermometer. It is the one your body already knows.