Temperature, humidity and how warm it feels

The official air temperature stated in a forecast is measured at 2 metres above the ground, in shade, in a ventilated instrument screen that shields the thermometer from direct sunlight but lets air pass through freely. It's a standardised measurement that makes temperatures comparable between different places and times – but it's rarely exactly the temperature you experience, since you're usually neither in shade at exactly 2 metres nor unaffected by wind, sun and moisture.

A thermometer left in direct sunlight without ventilation can show several, sometimes tens of, degrees more than the air temperature, since it then measures its own warming rather than the temperature of the air. That's the explanation for why a car thermometer on the dashboard can show 40 degrees on a day when the official temperature is 25.

Humidity's role in how warm it feels

The body cools itself mainly by sweating: as sweat evaporates from the skin, energy is used, and that energy is taken from the body's heat. In dry air, sweat evaporates quickly and the cooling works efficiently. In humid air, the air is already close to saturated with water vapour, evaporation is sluggish, and the sweat stays on the skin as an unpleasant, sticky film without cooling much at all.

That's why relative humidity is at least as important as temperature for how a warm day feels. The measure that tries to capture the whole picture is called the heat index, or apparent temperature, and it combines temperature and humidity into a single felt figure.

The table shows why 28 degrees with high humidity, common in Sweden during humid summer spells with southerly winds, can feel just as unpleasant as, or worse than, 33 dry degrees. Many who have visited both the Mediterranean and tropical countries recognise the pattern: the dry Mediterranean heat often feels more manageable than a lower but humid temperature in, for example, South-East Asia.

Tropical nights, and why nights don't always cool things down

In climatology, a night counts as a *tropical night* if the lowest temperature doesn't drop below 20 °C. Such nights occur in Sweden mainly in the south-eastern parts of the country during heatwaves, and they're taxing because the body normally needs to cool down to sleep well. If the air is also humid, evaporative cooling disappears altogether, which makes tropical nights with high humidity particularly uncomfortable even though the thermometer doesn't show extreme figures.

Ground temperature versus air temperature

The air temperature measured at 2 metres often differs considerably from the temperature right at the ground. On a sunny day, asphalt can become 20–30 degrees warmer than the air above it, which is why walking barefoot on a car park can be painful even when the air is only 25 degrees. Conversely, on a clear night the ground can cool considerably below the air temperature through radiation, which means ground frost can occur even though the forecast states a few degrees above freezing – a connection described in more detail in the fog guide, since the same nighttime cooling often produces both frost and fog.

The sun's influence on how it feels

Standing in direct sun can raise the felt temperature by 10–15 degrees or more compared with shade, even if the air temperature is exactly the same. That's because the body then receives radiant heat in addition to the usual heat from the air. That's why a day with 20 °C and strong sun in calm weather can feel like a summer's day, while the same temperature in cloudy, windy weather feels distinctly chilly. See the wind chill guide for how wind lowers the felt temperature in the equivalent way in the other direction.

The daily cycle

Air temperature normally reaches its lowest point just before sunrise, once the ground has had all night to radiate out heat, and its highest point a few hours after the sun's midday peak, since the ground needs time to warm up and then warm the air above it. Cloud cover, wind and humidity dampen the daily variation: a cloudy, windy, humid day might have only a few degrees' difference between night and day, while a clear, calm, dry day can vary by ten degrees or more between night and afternoon.

Why does it sometimes feel colder in the shade despite the same thermometer reading?

Without direct sunlight, the extra radiant heat that would otherwise warm the skin is missing, so the felt temperature drops even though the air temperature is unchanged.

Can humidity make cold weather more dangerous too?

Yes, indirectly. Damp clothing conducts heat away much faster than dry clothing, so high humidity combined with cold and wind increases the risk of hypothermia even though the air temperature itself isn't extreme.

Why does my watch show a different temperature than the forecast?

Wearable sensors often sit close to the body or in direct sunlight and lack the ventilation an official weather station has, which easily produces a discrepancy of a few degrees.

In the visualiser you can compare how different combinations of temperature, humidity and sun affect how a day feels, not just what the thermometer shows.