Step outside in Delhi this week and the air does something strange. It does not just feel hot. It feels heavy, sticky, almost solid, as though the heat has weight and is leaning against your skin.

On June 28, the India Meteorological Department (IMD) said the capital’s heat index, or “feels like” temperature, touched 51.3 degrees Celsius, the highest reading this year. Yet the thermometer outside read only 41.3 degrees Celsius. So which number is true? Both are. And the gap between them is one of the most important ideas in modern climate science.

IS DELHI REALLY 51 DEGREES CELSIUS?

No. The air temperature was 41.3 degrees Celsius, around 4 degrees above normal for this time of year. The 51.3 figure is the heat index, a number that tells you how hot the weather feels to your body once humidity is added in.

Humidity is simply the amount of water vapour floating invisibly in the air. The more there is, the harder your body has to work to stay cool. And that is where the real danger begins.

WHAT IS WET-BULB TEMPERATURE?

Your body cools itself by sweating. Sweat sits on your skin, evaporates, and carries heat away with it. This only works if the surrounding air is dry enough to soak up that moisture.

Scientists measure this cooling power using the wet-bulb temperature. The idea is beautifully simple. You wrap a wet cloth around a thermometer and blow air across it. As the water evaporates, it cools the bulb. In dry air, the reading drops sharply. In humid air, it barely moves, because the air is already so full of water that it cannot accept any more.

On June 28, Delhi’s wet-bulb temperature peaked at 29.77 degrees Celsius. That sounds modest, but it is not. At a wet-bulb reading of 32 degrees Celsius, even a fit, heat-trained adult cannot work outdoors for long. At 35 degrees Celsius, the top of the scale, sweat stops working altogether, and the body can no longer cool itself at all. Delhi, in other words, is edging towards a line nobody wants to cross.

WHY DOES DELHI FEEL SO MUGGY RIGHT NOW?

The culprit is moisture arriving from the wrong direction. Southwesterly winds from the Arabian Sea are feeding water vapour over northwestern India, said Mahesh Palawat, vice president of Skymet Weather. The monsoon, which normally brings cooling rain from the Bay of Bengal, is still some distance away.

Delhi’s summer heat is usually dry. Right now it is wet, and wet heat is the kind that kills.

WHEN WILL DELHI GET RELIEF FROM THE HEAT?

Soon, mercifully. The IMD expects temperatures to ease after Monday, with rain, thunderstorms and strong winds through the week, and a yellow alert for Tuesday and Wednesday. Nights could cool to around 22 degrees Celsius by Friday, and the monsoon is expected after July 4.

Until then, the lesson is worth carrying with you. It is not the heat. It is the humidity.