Heat index: humidity and sweating

For water to evaporate it needs a lot of energy – physicists call this the latent heat of vaporisation. When you sweat, this energy comes from your skin and the blood beneath it. That is why the evaporation of sweat is the body's most important way of cooling in heat and during exertion. When the air is warmer than the skin, it is almost the only one, because the body then gains heat from the air instead of giving it off.

How fast sweat evaporates depends on how much more water vapour the air can take up. Relative humidity tells you how “full” the air is compared to the most it can hold at that temperature. At low humidity sweat evaporates quickly, sometimes so quickly that you hardly notice it. At very high humidity it hardly evaporates at all.

Wind helps, because it carries away the moist air right above the skin and brings in drier air. Sun, on the other hand, adds extra heat. The US weather service's heat index combines air temperature and relative humidity into a feels-like temperature; its values generally apply to shade.

In hot weather, look not only at the temperature but also at the humidity, or directly at the feels-like temperature and the weather service's heat warnings. A muggy day calls for more caution than a dry day at the same temperature. Plan strenuous work or trips for the cooler morning and evening hours and take breaks in the shade during the midday heat.

Even if your sweat does not visibly evaporate, you are losing water and salt. So drink regularly, even if you do not feel sweaty. Loose, airy clothing lets air reach the skin. A fan or wind promotes evaporation, and moistening the skin, for example with a wet cloth, can provide extra cooling.

In direct sun the strain is higher than the heat index says. Shade is therefore doubly valuable: it removes the radiation, and the felt temperature comes closer to the index value.

In humid tropical regions, on hot summer days in the city and during heatwaves, both come together: high temperature and high humidity. In cities, buildings and roads store heat during the day and release it again at night. If the nights also stay warm and muggy, the body can hardly recover.

Another case is clothing that does not let air through, such as rainwear or protective suits. Underneath, a small climate of its own forms with very moist air – sweat hardly evaporates, even when the air outside is dry. When doing physical work in such clothing, frequent breaks are important.

This entry explains the physics; it does not replace a medical assessment. Confusion, reduced consciousness or a seizure in the heat are an emergency: call the local emergency number (112 in the EU) immediately. Until help arrives, move the person into the shade or somewhere cooler and protect them from further heat. More details are in the first-aid entry on heat illness.

Older people, small children, the chronically ill and people doing heavy physical work are at particular risk. Older people often sense thirst and heat less well. During heatwaves, check on relatives and neighbours who live alone.

In sport and occupational safety a different measure is often used: the wet-bulb globe temperature. Among other things, it uses a thermometer whose tip sits in a damp cloth. The drier the air, the more evaporation cools this thermometer. Sun and wind are taken into account as well.

The heat index is an approximation for average people in the shade. How much strain a person actually experiences also depends on fitness, acclimatisation to heat, clothing, health and medication. It is therefore a good guide but no guarantee.