Why does humidity make heat so much worse?

Evaporating sweat is the body's main cooling route above about 95 F. When the air already holds a lot of water, sweat sits on the skin instead of evaporating, so the cooling mechanism stalls.

What is wet bulb temperature and how does it differ?

Wet bulb is the lowest temperature achievable by evaporation alone. A wet bulb of 35 C is considered the survival limit for a healthy person at rest, while heat index is a comfort and risk scale rather than a physical limit.

Does a fan help in extreme heat?

Below roughly 95 F a fan helps by speeding evaporation. Above that, moving very hot air over skin can add heat faster than it removes it, so air conditioning or wetting the skin becomes necessary.

Heat Index Calculator

Calculate apparent temperature from heat and humidity

Temperature (°F)

Relative Humidity (%)

Heat Index (Feels Like)

100°F

37.6°C

Extreme Caution — possible heat cramps

Quick Facts

•90°F + 60% humidity = feels like ~100°F
•Heat exhaustion risk above 105°F heat index
•Drink extra water when heat index exceeds 90°F

Understanding Heat Index

The heat index combines air temperature and relative humidity to determine what the temperature 'feels like.' High humidity prevents sweat from evaporating, making you feel hotter. At 90°F and 60% humidity, the heat index is about 100°F. Heat exhaustion can occur above 105°F heat index.

Understanding the heat index calculator

Heat index combines air temperature and relative humidity into how hot it actually feels. Humidity matters because sweat only cools you when it evaporates, and humid air slows evaporation. At 90 F, going from 40 to 70 percent humidity adds about 15 degrees to the apparent temperature and moves the day from caution into danger.

Worked examples

Heat index at 92 F with 65 percent humidity

1The Rothfusz regression combines temperature and humidity terms.
2Reading the standard table at 92 F and 65 percent gives about 111 F.
3That falls in the danger band, where heat cramps and exhaustion are likely.
4Outdoor work should shift to early morning with hourly shaded breaks.

An apparent temperature near 111 F, which is in the danger category.

Same temperature in dry air

1At 92 F and 20 percent humidity, the table gives about 89 F.
2The apparent temperature is now below the air temperature.
3Sweat evaporates freely, so cooling works normally.

Roughly 89 F apparent, a 22 degree difference from the humid case.

Heat index in Fahrenheit

Relative humidity across the top, air temperature down the side.

Temp	40%	55%	70%	85%
80 F	80	81	83	85
85 F	85	88	93	99
90 F	91	97	106	117
95 F	99	110	124	141
100 F	109	127	146	n/a
105 F	123	148	n/a	n/a

Risk categories

Heat index	Category	What to expect
80 to 90 F	Caution	Fatigue with prolonged activity
90 to 103 F	Extreme caution	Heat cramps and exhaustion possible
103 to 124 F	Danger	Heat exhaustion likely, heat stroke possible
125 F and above	Extreme danger	Heat stroke highly likely

Practical tips

•Heat index assumes shade. Direct sun can add up to 15 degrees to the effective value.
•Acclimatisation takes one to two weeks. The first hot day of summer is the most dangerous one.
•Drink to a schedule rather than to thirst during heavy work, since thirst lags behind fluid loss.

Frequently asked questions

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This calculator is for educational and reference purposes only. Always verify critical calculations independently.