How this works
On a hot day, your body cools itself by sweating: liquid evaporates from your skin and carries away latent heat. High humidity slows that evaporation, so the body cools less efficiently and the air feels hotter than the thermometer reads. The Heat Index (HI) quantifies this: at 90 °F (32 °C) and 40% humidity it feels like 90 °F (no penalty), but at 90 °F and 90% humidity it feels like 105 °F (41 °C) — a 15 °F / 9 °C apparent jump from humidity alone.
The NWS uses the Rothfusz regression — a polynomial fit to a heat-balance model assuming a 5'7" 147-lb adult in shorts and short sleeves walking at 3.1 mph in light shade with average ventilation. It's calibrated for T ≥ 27 °C / 80 °F and humidity ≥ 40%; below those, normal evaporative cooling keeps your apparent temperature ≈ actual temperature and the index isn't reported.
The practical reading is heat-stress risk. HI 27–32 °C / 80–90 °F: caution — fatigue with prolonged exposure or strenuous activity. HI 32–41 °C / 90–105 °F: extreme caution — heat exhaustion possible. HI 41–54 °C / 105–130 °F: danger — heat exhaustion likely, heat stroke possible. HI > 54 °C / 130 °F: extreme danger — heat stroke imminent. Direct sun adds another ~8 °C / 15 °F to the apparent temperature beyond what the formula captures.
The formula
T = ambient air temperature (°F internally), R = relative humidity (%). The formula assumes shade, light wind, and a moderately active adult. Direct sun adds 8–15 °F to the apparent temperature; still air or high exertion adds more. Children and the elderly experience effective HI several degrees higher than the formula reports.
Example calculation
- Air 32 °C (90 °F), humidity 70%.
- HI ≈ 105 °F ≈ 41 °C — extreme caution: heat cramps and heat exhaustion possible with prolonged exposure or activity.
Frequently asked questions
Why does humidity make heat feel so much worse?
Sweat is your primary high-power cooling mechanism — it can shed 600+ watts at full evaporation rate. When ambient humidity is high, the air is already nearly saturated, so sweat evaporates more slowly (or just runs off without cooling you). At 100% humidity, evaporative cooling drops to near zero and you can only shed heat by conduction (skin to cooler surfaces) and radiation — both far slower. That's why a 32 °C / 90% RH day is dangerous and a 38 °C / 25% RH desert day is uncomfortable but tolerable.
How much does direct sun add?
NWS estimates 8–15 °F / 4.5–8 °C beyond the calculated HI. Asphalt or sand surfaces add even more — radiative load from a hot pavement can push surface-level apparent temperature 20 °F / 11 °C higher than open shade. For outdoor sports or work, treat the calculated HI as a lower bound and add a comfortable margin. The "wet bulb globe temperature" (WBGT) is the gold standard for sports-medicine heat decisions because it includes radiation directly.
Can fans help in extreme heat?
Counterintuitively, no — and possibly the opposite. Once the ambient air is hotter than your skin (~35 °C / 95 °F), a fan blowing hot air actually adds heat through convection while doing nothing for evaporation if humidity is already high. The CDC recommends turning fans off and seeking air conditioning above 35 °C / 95 °F. Below that threshold, fans help by accelerating evaporation as long as humidity is below ~80%. Always pair fans with hydration; sweat-driven fan cooling fails fast if you're dehydrated.