Relative Humidity From Dew Point
Calculator
Results
- Relative humidity (%)
- 57.444594
- Vapour pressure (hPa)
- 18.163507
- Saturation vapour pressure (hPa)
- 31.619175
Weather results
| Relative humidity (%) | 57.444594 |
| Vapour pressure (hPa) | 18.163507 |
| Saturation vapour pressure (hPa) | 31.619175 |
formula-map diagram
- Relative humidity (%)
- 57.444594
- Vapour pressure (hPa)
- 18.163507
- Saturation vapour pressure (hPa)
- 31.619175
Weather relationship
Formula
RH = 100 × e(Td) ÷ es(T), with e(T) = 6.112×exp(17.27×T ÷ (237.7+T)) hPa= 57.44459423824
Note
This is a simplified model: it applies the published standard formula to the numbers you entered and ignores local terrain, radiation, precipitation type and instrument error. Wind chill is the 2001 NWS/Environment Canada regression, valid at or below 10 °C with wind at or above 4.8 km/h; the heat index is the Rothfusz regression, reliable above about 27 °C; dew point uses the Magnus approximation (a = 17.27, b = 237.7 °C) over liquid water; wet bulb uses Stull's approximation at standard sea-level pressure; pressure and density altitude assume the International Standard Atmosphere and dry air. Do not use these results for aviation, structural or safety decisions without an official forecast or a qualified professional.
More in Weather and climate
See all →Frequently asked questions
Why can relative humidity be calculated from dew point and air temperature alone?+
Relative humidity is the ratio of the air's actual water vapor content to the maximum it could hold at the current temperature; dew point already encodes the actual water vapor content, and a standard vapor pressure formula gives the maximum for any given air temperature, so the two together fully determine relative humidity.
What happens to relative humidity as air temperature rises with dew point held constant?+
Relative humidity drops, because warmer air can hold more water vapor at saturation, so the same absolute moisture content (same dew point) represents a smaller fraction of the air's total capacity as temperature increases.
Why do dew point and air temperature converging mean humidity near 100%?+
When air temperature equals the dew point, the air is exactly at its saturation point for that amount of moisture, which by definition is 100% relative humidity. The closer the two values are, the higher the relative humidity reads.
Is this calculation just an approximation, or is it exact?+
It relies on the Magnus or Clausius-Clapeyron approximation for saturation vapor pressure, which is very accurate across common atmospheric temperature ranges but is an approximation rather than a perfectly exact physical formula, particularly at extreme temperatures.
Why would I calculate relative humidity this way instead of measuring it directly?+
Many weather stations and instruments measure dew point (or wet-bulb temperature) more reliably than they measure relative humidity directly, and historical or reported weather data is often given as dew point and temperature, so this conversion lets you derive relative humidity from commonly available data.