Heat Index Calculator
The heat index tells you how hot it actually feels when humidity is factored in with the air temperature. At 95°F with 55% humidity, it feels like 110°F because your body cannot cool itself as effectively through sweating when the air is already saturated with moisture. The NWS uses the heat index to issue heat advisories and warnings.
The Rothfusz regression equation is the standard NWS formula for computing heat index:
HI = -42.379 + 2.049T + 10.143RH - 0.2248T*RH - 0.00684T2 - 0.0548RH2 + 0.00123T2*RH + 0.000853T*RH2 - 0.00000199T2*RH2
Where T is temperature in °F and RH is relative humidity in percent. Additional adjustments apply when humidity is very low (below 13%) or very high (above 85%).
**NWS Danger Categories:** - **Caution (80-90°F HI):** Fatigue possible with prolonged exposure - **Extreme Caution (91-103°F HI):** Heat cramps and heat exhaustion possible - **Danger (104-125°F HI):** Heat cramps, exhaustion, and heat stroke likely - **Extreme Danger (126°F+ HI):** Heat stroke highly likelyIn the deadly 2023 Phoenix heat wave, air temperatures hit 119°F with low humidity, producing heat index values around 120°F. In more humid regions like the Gulf Coast, 95°F air temperature with 70% humidity produces a heat index of about 126°F, which is in the extreme danger zone.