About the heat index
The heat index (also called apparent temperature or "feels-like" temperature) blends air temperature and relative humidity into one figure that reflects how hot it truly feels. Your body cools itself mainly by sweating, and sweat cools you only when it evaporates. When humidity is high, the air is already near-saturated, so sweat lingers on the skin instead of evaporating, cooling slows dramatically, and the same thermometer reading feels far hotter. This tool turns temperature plus humidity into the official US National Weather Service heat index.
The science traces to Robert Steadman's 1979 paper on apparent temperature, which modelled a typical adult walking in the shade. The NWS later fitted a polynomial (the Rothfusz regression) to Steadman's tables so it could be computed directly. The result is widely used in weather forecasts, occupational-safety rules (OSHA references it for outdoor workers), and sports-event planning.
The key public-health point is that heat illness is driven by the heat index, not the raw temperature. A dry 100 F day in Phoenix can be safer than a humid 92 F day in Houston, because the humid air blocks the body's cooling. That is why the NWS issues Heat Advisories and Excessive Heat Warnings based on the index, not the thermometer alone.
How it works: the formula
For temperatures of 80 F and above, the tool applies the full NWS Rothfusz regression; below 80 F humidity has little effect, so the heat index equals the air temperature:
HI = -42.379 + 2.04901523 T + 10.14333127 R
- 0.22475541 T R - 0.00683783 T^2 - 0.05481717 R^2
+ 0.00122874 T^2 R + 0.00085282 T R^2 - 0.00000199 T^2 R^2
T = air temperature in F R = relative humidity in %
Celsius = (HI - 32) x 5 / 9
- T = dry-bulb air temperature in degrees Fahrenheit.
- R = relative humidity as a whole-number percent (60, not 0.60).
- The nine-term polynomial is an empirical fit, accurate to about plus or minus 1.3 F over its valid range.
- The model assumes shade and light wind; direct sun can add up to 15 F.
Worked example: a humid summer afternoon
Suppose it is 95 F with 70 percent relative humidity, a common Gulf Coast reading.
- Confirm the regime: 95 F is above 80 F, so the full polynomial applies.
- Plug in T = 95, R = 70 into the nine-term equation.
- Evaluate: the terms sum to roughly 124 F.
- Convert: (124 - 32) x 5 / 9 = about 51 C.
- Classify: 124 F falls in the NWS "danger" band, just below extreme danger.
NWS heat-index risk bands
| Heat index | Category | Health risk with prolonged exposure |
|---|---|---|
| 80-90 F | Caution | Fatigue possible |
| 90-103 F | Extreme caution | Heat cramps and heat exhaustion possible |
| 103-124 F | Danger | Heat exhaustion likely; heat stroke possible |
| 125 F and up | Extreme danger | Heat stroke highly likely |
Common pitfalls
- Reading it in the sun. The heat index assumes shade. In full sunlight, add up to 15 F to get the felt temperature before judging risk.
- Entering humidity as a decimal. Use 70 for 70 percent, not 0.70. A decimal collapses the polynomial and gives a meaningless near-air-temperature result.
- Confusing dew point with relative humidity. The formula needs relative humidity (percent), not dew point (degrees). Convert first if your weather source only gives dew point.
- Assuming wind always helps. A breeze aids evaporative cooling at moderate temperatures, but a hot, dry wind above skin temperature can actually increase heat gain.
- Ignoring personal risk factors. The index models a healthy adult. Infants, older adults, people on certain medications, and those exerting heavily are at risk well below the "danger" threshold.