Apparent Temperature (Heat Index)
How hot the air actually feels once humidity and wind are taken into account.
Formula
Variables
Example
High humidity can make 32°C feel like 41°C
How it works
This formula estimates how warm or uncomfortable the air feels to a person by combining three effects: the actual air temperature, the cooling effect of moving air, and the extra warmth caused by water vapour in the air. Humid air slows sweat evaporation, so the body loses heat less effectively, while wind removes heat from the skin and makes conditions feel cooler. The constants in the equation come from an empirical model designed to match human perception under typical weather conditions.
When to use it
Use this when you want a practical estimate of perceived heat in weather reports, outdoor safety planning, or comfort analysis. It is especially useful in warm, humid, or breezy conditions where the thermometer alone does not describe how hot it feels.
Worked example, step by step
Step 1: Write down the values: Tₐ = 32°C, e = 25 hPa, wₛ = 1.5 m/s Step 2: Substitute into the formula: AT = Tₐ + 0.33e - 0.70wₛ - 4.00 Step 3: AT = 32 + 0.33(25) - 0.70(1.5) - 4.00 Step 4: AT = 32 + 8.25 - 1.05 - 4.00 Step 5: AT = 35.20°C Final answer: The apparent temperature is about 35.2°C.
Common mistakes to avoid
- Using relative humidity instead of water-vapour pressure e
- Forgetting that wind speed must be in m/s
- Mixing up the sign of the wind term and adding it instead of subtracting it
Did You Know?
Humid heat is far more dangerous than dry heat because sweat can’t evaporate to cool you.
Share this formula
More in Weather & Calamities
View allSaturation Vapour Pressure
IntermediateThe maximum water-vapour pressure air can hold at a given temperature (Tetens formula).
Beaufort Wind Scale
IntermediateConverts a Beaufort force number into an approximate wind speed.
Wind Pressure (Dynamic)
IntermediateThe pressure a moving airmass exerts on a surface such as a wall or roof.
Radiative Forcing from CO₂
AdvancedExtra energy trapped by the atmosphere as carbon-dioxide concentration rises.
Planetary Equilibrium Temperature
AdvancedThe temperature a planet would settle at from sunlight alone, before greenhouse warming.
Global Warming Potential (GWP)
AdvancedWarming caused by a gas over a time horizon, relative to the same mass of CO₂.