India’s hidden heat: Why 42°C isn’t really 42°C

When the mercury reads 42°C, many assume that's the full measure of the day's heat. Yet across large parts of India, people often experience conditions that feel closer to 50°C or even higher. The reason lies not just in soaring temperatures but in a deadly combination of humidity, moisture and the body's diminishing ability to cool itself.

As heatwaves become longer, more intense and increasingly frequent, meteorologists are urging people to look beyond the ordinary temperature displayed on weather apps. Understanding the difference between dry-bulb temperature and wet-bulb temperature is becoming essential — not just for comfort, but for survival.

Why 42°C can feel like 50°C

The temperature reported in daily weather forecasts is usually the dry-bulb temperature — the actual air temperature measured by a standard thermometer kept in the shade and protected from direct sunlight.

However, the human body experiences heat very differently.

People cool themselves primarily by sweating. As sweat evaporates from the skin, it carries away body heat. But when humidity is high, the air is already saturated with moisture, making it difficult for sweat to evaporate. The body's natural cooling system begins to fail, causing people to feel far hotter than the actual air temperature.

This is why a city recording 42°C with 70–80 per cent humidity can feel closer to 48–50°C. Weather agencies describe this as the heat index or "feels-like" temperature, which combines air temperature with humidity to estimate how hot conditions actually feel to the human body.

What is dry-bulb temperature?

Dry-bulb temperature is the standard air temperature measured by an ordinary thermometer exposed to the air but shielded from direct sunlight and moisture.

It is the figure commonly reported by the India Meteorological Department (IMD) during weather bulletins and heatwave forecasts.

While it accurately measures the temperature of the air, it does not account for humidity, wind speed or solar radiation — all of which influence how humans actually experience heat.

What is wet-bulb temperature?

Wet-bulb temperature is a more critical measure of heat stress. It is recorded by wrapping a thermometer's bulb in a wet cloth and allowing water to evaporate.

As water evaporates, it cools the thermometer. The amount of cooling depends on the humidity in the air.

• Low humidity: Water evaporates quickly, producing a much lower wet-bulb temperature.

• High humidity: Evaporation slows dramatically, so the wet-bulb temperature remains close to the actual air temperature.

The smaller the difference between dry-bulb and wet-bulb temperatures, the more humid — and more dangerous — the conditions become.

Why wet-bulb temperature matters

Scientists consider wet-bulb temperature one of the best indicators of whether the human body can cool itself.

When wet-bulb temperatures climb above 30–32°C, the risk of heat exhaustion and heatstroke rises sharply, especially for outdoor workers, children and the elderly.

At around 35°C wet-bulb temperature, even healthy individuals resting in the shade with unlimited drinking water can struggle to survive for prolonged periods because sweat can no longer evaporate efficiently enough to cool the body.

Although such extreme conditions remain relatively rare, climate scientists warn that they are becoming more frequent in parts of South Asia as global temperatures continue to rise.

Why India feels especially oppressive

Several factors combine to make Indian summers particularly uncomfortable:

• High humidity from the Arabian Sea and Bay of Bengal.

• Moist monsoon winds arriving before or during the southwest monsoon.

• Urban heat islands created by concrete, asphalt and limited green cover.

• High nighttime temperatures that prevent the body from recovering.

• Rising global temperatures driven by climate change.

Coastal cities such as Mumbai, Chennai, Kolkata and Kochi often record lower maximum temperatures than inland cities, yet many residents report feeling far hotter because humidity remains extremely high.

Staying safe during extreme heat

Health experts recommend drinking plenty of water, avoiding strenuous outdoor activity during the hottest hours of the day, wearing loose and light-coloured clothing, seeking shade or air-conditioned spaces whenever possible, and checking regularly on children, older adults and those with chronic illnesses.

As India experiences increasingly frequent and intense heatwaves, experts say the number on the thermometer tells only part of the story. The real danger lies in how heat and humidity combine to overwhelm the body's natural cooling system — turning an already scorching 42°C day into one that feels closer to 50°C.