The three weather comfort formulas and the temperature conversion, each recomputed in code and matching the calculators. They are published regressions with limited ranges, so the calculators refuse inputs outside them rather than extrapolate.
Formulas
| Measure | Formula | Example |
|---|---|---|
| Heat index (NWS Rothfusz regression) | HI = −42.379 + 2.049·T + 10.143·RH − 0.2248·T·RH − … (T in °F, RH in %) |
90 °F at 70% humidity feels like 106 °F. Valid from 80 °F and 40% humidity |
| Wind chill (NWS 2001) | WC = 35.74 + 0.6215·T − 35.75·V^0.16 + 0.4275·T·V^0.16 (T in °F, V in mph) |
20 °F with a 15 mph wind feels like 6 °F. Valid at 50 °F or below and 3 mph or more |
| Dew point (Magnus approximation) | α = ln(RH/100) + 17.62·T/(243.12 + T), Td = 243.12·α / (17.62 − α) (°C) |
25 °C at 60% humidity: 16.7 °C |
| Celsius and Fahrenheit | °F = °C × 9/5 + 32 |
25 °C = 77 °F; −6.7 °C = 20 °F |
The Heat Index Calculator, Wind Chill Calculator and Dew Point Calculator apply them. For the sun and moon use the Sunrise & Sunset Calculator and Moon Phase Calculator.
How to read them
- Dew point is a more direct measure of mugginess than relative humidity. Below about 55 °F (13 °C) the air feels dry or comfortable; above about 70 °F (21 °C) it feels oppressive.
- Heat index assumes shade and light wind. Direct sun can add up to 15 °F.
- Wind chill describes the loss of heat from exposed skin; it does not change the actual air temperature, and it does not apply indoors.