Humidity & Dew Point Calculator
Calculate dew point temperature, absolute humidity, wet-bulb temperature, and vapor pressure from air temperature and relative humidity using the Magnus formula and psychrometric equations.
Formulas Used
1. Saturation Vapor Pressure — Buck Equation:
es(T) = 6.1078 × exp( (17.625 × T) / (243.04 + T) ) [hPa]
2. Dew Point — Magnus Formula Inversion:
γ = ln(RH/100) + (17.625 × T) / (243.04 + T)
Td = 243.04 × γ / (17.625 − γ) [°C]
3. Absolute Humidity:
AH = 216.7 × ea / TK [g/m³]
where ea = (RH/100) × es, TK = T + 273.15
4. Specific Humidity:
q = 621.97 × ea / (P − 0.378 × ea) [g/kg]
5. Wet-Bulb Temperature — Stull (2011) Empirical:
Tw = T·atan(0.151977·√(RH+8.313659)) + atan(T+RH)
− atan(RH−1.676331) + 0.00391838·RH1.5·atan(0.023101·RH) − 4.686035
6. Vapor Pressure Deficit:
VPD = es − ea [hPa]
7. Humidex (Environment Canada):
Humidex = T + 0.5555 × (es(Td) − 10.0)
Assumptions & References
- Magnus / August-Roche-Magnus constants: a = 17.625, b = 243.04 °C — valid from −40 °C to +60 °C (Alduchov & Eskridge, 1996).
- Saturation vapor pressure uses the Buck (1981) simplified equation; error < 0.02% over the valid range.
- Dew point formula accuracy: ±0.35 °C for RH > 50% (Lawrence, 2005, BAMS).
- Wet-bulb formula: Stull, R. (2011). "Wet-Bulb Temperature from Relative Humidity and Air Temperature." Journal of Applied Meteorology and Climatology, 50(11), 2267–2269. Accuracy ±1 °C for 5–99% RH, −20 to 50 °C.
- Specific humidity uses the standard psychrometric approximation with the enhancement factor 0.378 for moist air (WMO-No. 8, 2018).
- Default atmospheric pressure is 1013.25 hPa (ISA sea-level standard). Adjust for altitude.
- Humidex is defined by Environment and Climate Change Canada; values above 40 are considered dangerous.
- Absolute humidity formula: WMO Technical Note No. 8 (2018), Annex 4.B.