What is the heat index?

The heat index ("feels like" temperature) combines air temperature and relative humidity to estimate how hot it actually feels to the human body. High humidity prevents sweat from evaporating efficiently, reducing the body's ability to cool itself.

NOAA heat index categories

Heat index	Category	Risk
80–90°F (27–32°C)	Caution	Fatigue possible with prolonged activity
91–103°F (33–39°C)	Extreme Caution	Heat cramps and exhaustion possible
103–124°F (39–51°C)	Danger	Heat cramps/exhaustion likely; heat stroke possible
>125°F (>52°C)	Extreme Danger	Heat stroke highly likely

Heat index formula

The Rothfusz regression equation is used by NOAA and combines air temperature (T, °F) and relative humidity (RH, %) through a multi-term polynomial:

HI = −42.379 + 2.049T + 10.143RH − 0.225T·RH − 0.006T² − 0.057RH² + 0.001T²·RH + 0.0008T·RH² − 0.000002T²·RH²

A simpler approximation valid for RH ≥ 40% and T ≥ 80°F is: HI ≈ 0.5 × (T + 61 + (T−68)×1.2 + RH×0.094). For most educational purposes this simplified version is accurate within 1–2°F.

Heat illness progression

Condition	Symptoms	First aid
Heat cramps	Painful muscle spasms, usually in legs or abdomen	Rest in cool area; drink electrolyte fluids
Heat exhaustion	Heavy sweating, weakness, cold/pale skin, fast weak pulse, nausea	Move to cool location; loosen clothing; apply cool wet cloths; sip water
Heat stroke ⚠️ Medical emergency	High body temp (103°F+), hot/red/dry skin, patient stops sweating, rapid strong pulse, possible unconsciousness	Call 911 immediately; cool rapidly with ice baths or cold water immersion

Humidity's role

Sweating is the body's primary cooling mechanism. As sweat evaporates, it carries heat away from the skin. High relative humidity reduces the rate of evaporation - at 100% humidity, sweat cannot evaporate at all. This is why a 95°F day at 20% humidity feels tolerable while a 90°F day at 80% humidity can be dangerous: the body cannot cool itself efficiently.