Apparent Temperature (Heat Index) Calculator

Overview

This apparent temperature calculator shows how hot or cold the air actually feels, using the official formulas of the Korea Meteorological Administration (KMA). For summer (May–September) it applies the wet-bulb-based formula adopted on 2 June 2022; for winter (October–April) it applies the wind chill formula. Along with the number, it classifies the result against Korea's four heat-impact stages (31℃ watch, 33℃ caution, 35℃ warning, 38℃ danger) and the cold-impact apparent-temperature bands. Planning outdoor work, sports, or a commute around apparent temperature rather than air temperature is a far better guard against heat illness and hypothermia.

Formula

[Summer apparent temperature] (KMA, revised 2 June 2022)
AT = −0.2442 + 0.55399×Tw + 0.45535×Ta − 0.0022×Tw² + 0.00278×Tw×Ta + 3.0

Wet-bulb temperature Tw (Stull approximation)
Tw = Ta×atan[0.151977×(RH+8.313659)^0.5] + atan(Ta+RH) − atan(RH−1.676331) + 0.00391838×RH^1.5×atan(0.023101×RH) − 4.686035

[Winter apparent temperature] (wind chill index)
AT = 13.12 + 0.6215×Ta − 11.37×V^0.16 + 0.3965×Ta×V^0.16

Ta = air temperature (℃), RH = relative humidity (%), V = wind speed (km/h); atan returns radians.
The winter formula applies only when Ta ≤ 10℃ and wind speed ≥ 1.3 m/s.

How to Use

1. 1Pick the mode that matches the season: summer (humidity-based) for May–September, winter (wind-based) for October–April.
2. 2Enter the air temperature in ℃, straight from your forecast or weather app.
3. 3Enter relative humidity (%) in summer mode, or wind speed in winter mode. Wind speed can be entered in m/s or km/h.
4. 4Read the apparent temperature and how far it sits above or below the air temperature; the wet-bulb temperature (summer) or converted wind speed (winter) is shown too.
5. 5Check which heat- or cold-impact stage you are in and how many degrees remain to the next stage, then plan breaks or extra layers accordingly.

Examples

Background

On 2 June 2022 the Korea Meteorological Administration replaced its summer apparent-temperature formula with one built on wet-bulb temperature, so that humidity is weighted more realistically. The wet bulb itself comes from the approximation Roland Stull published in the Journal of Applied Meteorology and Climatology in 2011, valid for −20℃ ≤ Ta ≤ 50℃ and 5% ≤ RH ≤ 99%. The winter figure uses the standard North American wind chill index with 10-minute mean wind speed in km/h. Since 2020, Korean heat advisories have been issued on daily maximum apparent temperature rather than air temperature: an advisory at 33℃ and a warning at 35℃ when the level is expected to persist for two days or more. Korea's occupational heat-illness rules use the same scale, with required measures at 31℃, 33℃, 35℃ and 38℃.

Tips

• ✔At the same 33℃, 40% humidity feels like about 32℃ while 85% humidity feels close to 36℃. In summer, check humidity before temperature.
• ✔Korean forecasts give wind speed in m/s but the formula needs km/h, so multiply by 3.6. This calculator converts automatically once you pick a unit.
• ✔Apparent temperature assumes shade on flat ground. Direct sun, asphalt, and construction sites feel considerably hotter than the computed value.
• ✔From the 33℃ 'caution' stage, the rule of thumb is at least 20 minutes of shaded rest every two hours; from 35℃ 'warning', outdoor work stops between 14:00 and 17:00.
• ✔The winter formula is only valid at 10℃ or below with wind of at least 1.3 m/s — outside that range the KMA does not publish an apparent temperature either.

FAQ