How to use the apparent temperature calculator
- Choose the temperature unit, Celsius or Fahrenheit.
- Enter the air temperature, the relative humidity and the wind speed.
- Choose the wind unit: m/s, km/h or mph.
- Read the apparent temperature, how much it differs from the air temperature, the vapor pressure and the wind speed used.
Formula
Vapor pressure e (hPa) = relative humidity / 100 x 6.105 x exp(17.27 x T / (237.7 + T)), with T in C
Apparent temperature AT = T + 0.33 x e - 0.70 x wind speed - 4.00
Wind speed is in meters per second at about 10 m height
A result in Fahrenheit is converted from the Celsius value
This is the Steadman apparent temperature in its form that includes wind and humidity but not radiation. Humidity raises the value and wind lowers it.
30 C, 50 percent humidity and a 2 m/s wind
The vapor pressure is 0.50 x 6.105 x exp(17.27 x 30 / 267.7) = 21.14 hPa. The apparent temperature is 30 + 0.33 x 21.14 - 0.70 x 2 - 4.00 = 31.58 C, which is 1.58 degrees above the air temperature.
In dry air at the same temperature and no wind the value is lower, and at a high humidity it climbs well above the air temperature. In cold weather with a strong wind it falls below the air temperature, although the page is not meant as a wind chill tool for severe cold.
A single index for warm and cool days
Many comfort indexes work in one season only. The heat index ignores wind and the wind chill applies to cold and wind. The Steadman form on this page uses one formula over a wide range and includes the cooling effect of wind, so it gives a continuous estimate.
For a comparison, try the heat index calculator, the wind chill calculator and the humidex calculator. The numbers differ because the formulas differ.
How to use the result
The apparent temperature describes the weather in the shade for a person walking at an ordinary pace. Direct sun can add several degrees, and work or exercise raises the heat load further. Use it to compare days, not as a safety rating. For humidity, you can also work out the dew point of the air.
Assumptions and limits
- Wind is measured at about 10 m above the ground, as weather stations report it. Wind at a person is usually lower.
- The index excludes sun and radiation and assumes shade.
- Humidity is relative humidity between 0 and 100 percent.
- The result is an estimate of comfort, not a medical or safety limit.
Frequently asked questions
What is apparent temperature?
It is a temperature that describes how the air feels to a person, taking humidity and wind into account along with the air temperature.
How is it different from the heat index?
The heat index uses temperature and humidity only and is meant for hot weather. Apparent temperature also includes wind and uses a different formula.
Does wind cool you in hot weather?
In this formula it does, by 0.7 degrees for each meter per second. In very hot, dry air, strong wind can add heat load instead, which the formula does not capture.
Which units can I use?
Celsius or Fahrenheit for temperature, and m/s, km/h or mph for wind. The page converts as needed.