Why humid heat feels worse than dry heat
Your body's main cooling mechanism is sweat evaporation — as sweat turns to vapor, it carries heat away from your skin. That process depends on the surrounding air having room to absorb more moisture. Humid air is already close to saturated, so sweat evaporates more slowly and your body sheds less heat, even though the thermometer reads the same number as a dry day.
The heat index — also called apparent temperature — puts a single number on that effect, estimating what the air temperature would need to be, at a more typical humidity, to feel exactly this uncomfortable.
The Rothfusz equation behind every forecast
This calculator uses the same regression the U.S. National Weather Service built its heat index charts from — a formula fitted to a physiological heat-stress model, not just a rough estimate:
HI = -42.379 + 2.049×T + 10.143×RH − 0.225×T×RH − 0.00684×T² − 0.0548×RH² + 0.00123×T²×RH + 0.000853×T×RH² − 0.00000199×T²×RH²
T is temperature in °F and RH is relative humidity as a whole number (70, not 0.70). Two small corrections get layered on top: a downward adjustment when humidity is very low (under 13%) with temperatures between 80-112°F, and an upward adjustment when humidity is very high (over 85%) with temperatures between 80-87°F — both refinements the NWS added after comparing the base formula against real conditions.
Because the equation only holds up in the range it was fitted to, the heat index is only defined at or above 80°F (27°C) — below that, normal thermoregulation handles things well enough that a separate feels-like figure isn't meaningful.
NWS heat index risk categories
| Heat index | Category | General effect |
|---|---|---|
| Below 80°F | Safe | No significant heat stress for most people |
| 80-89°F | Caution | Fatigue possible with prolonged exposure or activity |
| 90-102°F | Extreme Caution | Heat cramps or heat exhaustion possible |
| 103-124°F | Danger | Heat cramps and exhaustion likely, heatstroke possible |
| 125°F and above | Extreme Danger | Heatstroke highly likely with continued exposure |
These categories assume shade and light wind — direct sunlight can add up to another 15°F to the effective heat index.
Who faces the highest risk
Young children, older adults, outdoor workers, athletes, and people with heart or lung conditions all tolerate heat stress less well than a healthy adult at rest, and certain medications can further impair the body's ability to regulate temperature. None of that shows up in the raw heat index number, which is exactly why the same reading can be merely uncomfortable for one person and genuinely dangerous for another.
Heat index vs. wet-bulb temperature vs. wind chill
Heat index models ordinary human comfort in the shade with a light breeze — it's built for everyday forecasts, not extreme physiology. Wet-bulb temperature is a stricter physical measurement of the evaporative cooling limit itself, used in occupational heat-stress research and increasingly in climate science, since a wet-bulb temperature near human body temperature makes evaporative cooling essentially stop working altogether.
Heat index is the summer counterpart to wind chill, which does the same job for cold, windy conditions instead — see the Wind Chill Calculator for that side of the year. And for humidity in absolute rather than "feels like" terms, the Dew Point Calculator is the more direct measurement.
Sources and further reading
This page is for general reference only and is not medical advice. Seek shade, hydration, and medical attention as needed during periods of extreme heat.