Humidity and Dew Point Calculator for Delaware Climate
Calculate relative humidity and dew point temperature for Delaware's humid subtropical climate using the Magnus-Tetens formula. Enter temperature and wet-bulb or dew point readings to determine comfort and moisture levels.
Formulas Used
1. Saturation Vapor Pressure — Magnus-Tetens Formula:
es(T) = 6.1078 × exp(17.27 × T / (T + 237.3)) [hPa]
where T is temperature in °C.
2. Actual Vapor Pressure — Sprung's Psychrometric Formula:
e = es(Tw) − A × P × (Td − Tw)
where A = 0.000799 (Assmann psychrometer coefficient), P = station pressure (hPa),
Td = dry bulb (°C), Tw = wet bulb (°C).
3. Relative Humidity:
RH = (e / es(Td)) × 100 [%]
4. Dew Point — Magnus Inversion:
Tdp = (237.3 × ln(e / 6.1078)) / (17.27 − ln(e / 6.1078)) [°C]
5. Station Pressure — Barometric Formula:
P = 1013.25 × (1 − 2.25577×10⁻⁵ × h)5.25588 [hPa]
where h = elevation in meters.
6. Absolute Humidity:
AH = 216.7 × e / (T + 273.15) [g/m³]
7. Heat Index (NWS Rothfusz Regression, T ≥ 80°F, RH ≥ 40%):
HI = −42.379 + 2.04901523T + 10.14333127R − 0.22475541TR − 0.00683783T² − 0.05481717R²
+ 0.00122874T²R + 0.00085282TR² − 0.00000199T²R²
Assumptions & References
- Uses the Magnus-Tetens formula (Tetens, 1930; Murray, 1967) for saturation vapor pressure, accurate to ±0.1% for temperatures −40°C to 60°C.
- Psychrometric coefficient A = 0.000799 applies to a ventilated (Assmann) psychrometer per WMO-No. 8 (Guide to Meteorological Instruments).
- Station pressure is adjusted for elevation using the International Standard Atmosphere barometric formula.
- Heat Index uses the NWS Rothfusz (1990) regression equation, valid for T ≥ 80°F and RH ≥ 40%.
- Delaware climate data references: NOAA Climate Normals 1991–2020 for Wilmington Airport (KILG) and Dover AFB.
- Delaware's elevation ranges from sea level (coast) to ~450 ft (Piedmont); default 80 ft represents Wilmington.
- Delaware experiences a humid subtropical climate (Köppen Cfa) with hot, humid summers (avg dew points 65–70°F in July) and mild winters.
- Dew point comfort thresholds follow NWS and American Meteorological Society guidelines.
- This calculator is for meteorological and educational purposes; consult DEOS (Delaware Environmental Observing System) for official readings.