Relative Humidity From Dew Point Calculator
Calculate relative humidity dew point with our free science calculator. Uses standard scientific formulas with unit conversions and explanations.
Reviewed by Daniel Agrici, Founder & Lead Developer
Formula
RH = (e / es) * 100
Where e is actual vapor pressure from dew point and es is saturation vapor pressure at air temperature. Both use Magnus formula: e = 6.112 * exp(17.67*T/(T+243.5)). RH is 100% when T equals dew point.
Worked Examples
Example 1: Summer Afternoon
Problem:Air temperature 30 C, dew point 22 C. Calculate RH.
Solution:es = 6.112*exp(17.67*30/273.5) = 42.43 hPa e = 6.112*exp(17.67*22/265.5) = 26.43 hPa RH = 26.43/42.43 * 100 = 62.3% Depression = 8 C
Result:RH: 62.3% | Depression: 8 C | Comfortable
Example 2: Foggy Morning
Problem:Temperature 10 C, dew point 9 C.
Solution:es = 12.27 hPa, e = 11.52 hPa RH = 11.52/12.27 * 100 = 93.9% Depression = 1 C - Near saturation
Result:RH: 93.9% | Depression: 1 C | Near Fog
Frequently Asked Questions
What is relative humidity and how is it calculated from dew point?
Relative humidity is the ratio of actual water vapor pressure to saturation vapor pressure expressed as a percentage. When you know both air temperature and dew point you can calculate RH using the Magnus formula. First compute saturation vapor pressure es = 6.112*exp(17.67*T/(T+243.5)) and actual vapor pressure e = 6.112*exp(17.67*Td/(Td+243.5)) then RH = (e/es)*100. When temperature equals dew point RH is 100 percent meaning the air is saturated.
What is the dew point depression and what does it indicate?
The dew point depression is the difference between air temperature and dew point temperature always zero or positive. A small depression indicates air near saturation with high relative humidity while a large depression indicates dry air. A depression of zero means 100 percent relative humidity and fog or clouds may be present. In weather observations depression is useful for quickly assessing moisture conditions. Typical dry continental air has depressions of 10 to 20 degrees while humid tropical air may have depressions of only 1 to 3 degrees.
Why is dew point considered a better moisture indicator than relative humidity?
Dew point directly measures the absolute amount of moisture in the air while relative humidity depends on both moisture and temperature. On a summer day RH drops from morning to afternoon even though moisture content stays the same simply because temperature rises. Dew point remains constant under these conditions making it more useful for assessing true moisture levels. A dew point of 20 C always feels muggy regardless of temperature while 50 percent RH can feel either dry or humid depending on the temperature. Dew point is preferred in aviation and severe weather forecasting.
How does relative humidity affect human comfort?
Human comfort depends strongly on relative humidity because our bodies cool through evaporation of sweat. At high humidity evaporation is impaired making hot temperatures feel even hotter. The ideal indoor RH range for comfort and health is 40 to 60 percent. Below 30 percent skin dries out and respiratory irritation increases. Above 70 percent mold growth accelerates and the environment feels oppressive. The heat index combines temperature and humidity to estimate apparent temperature. At 35 C and 70 percent RH the heat index exceeds 45 C which is dangerous for health.
References
Reviewed by Daniel Agrici, Founder & Lead Developer · Editorial policy