Feels-like temperature

How it works

In the heat, humidity hinders sweat evaporation and makes it feel hotter than it is: that's the heat index (used by NOAA and civil protection agencies). In the cold, wind strips heat from your skin: that's wind chill. The tool automatically picks the right formula for the conditions.

Examples

32 °C (90 °F) with 70% humidity feels like about 41 °C (106 °F). And −5 °C with a 30 km/h wind «bites» like −13 °C.

Note

The formulas apply in the shade: in direct sun it can feel even hotter. On extreme days, follow your health authorities' guidance.

Why humidity changes everything

The body cools itself by evaporating sweat: every gram that evaporates carries heat away from the skin. But air already saturated with vapour will not take any more, so the sweat just sits there, soaks your shirt and cools nothing. That is why thirty degrees at 30% humidity is a pleasant day and thirty degrees at 90% is a punishing one: the temperature is the same, the cooling is not.

It is also why the dry heat of a desert is easier to bear than the damp heat of a lowland August, and why a crowded gym becomes unbearable sooner than the thermometer suggests.

Why wind is only cold to skin

Wind chill does not lower the air temperature: that stays what it is, and indeed a glass of water left outside in the wind does not freeze any sooner. What the wind takes away is the film of warm air the body has heated around itself, which works as insulation. Remove it and the skin gives up heat far faster.

One practical consequence follows: the sensation depends on how well covered you are. A windproof jacket gives back much of those degrees, because it rebuilds the layer the wind was stealing. For the risk of frostbite on exposed skin, on the other hand, the number matters a great deal.

The formulas have a range of validity

The heat index is built for heat: below about 27 °C it has no meaning. Wind chill applies to cold with wind above 5 km/h: using it on a summer breeze would give nonsense. In the middle band, where temperate climates sit for much of the year, the felt temperature equals the measured one, and that is why the number here sometimes does not change.

The two formulas also come from a model of a standard person, in the shade, dressed a certain way and standing still. In direct sun several degrees of sensation are easily added; running or cycling, you make your own wind.

Who really needs to watch it

The number is not a meteorological curiosity: it is the measure civil protection and health services build heatwave warnings on. Above certain values the body can no longer shed heat, and older people, small children, those with chronic illness and outdoor workers are the first to pay for it. Drinking before you feel thirsty, moving physical effort to the cool hours and staying in the shade are obvious recommendations precisely because they work.

Nearby tools

To switch between Celsius and Fahrenheit there is Temperature converter; for condensation and mould at home Dew point. For drinking water during the heat How much water to drink a day.