Relative Humidity Calculator
Relative humidity tells you how close the air is to being fully saturated with water vapor. At 85°F with a dew point of 65°F, the relative humidity is about 51%. This calculator uses the Magnus formula to compute RH from air temperature and dew point, and it also shows absolute humidity and a comfort assessment.
The formula:
RH = (es(Td) / es(T)) × 100
Where es is the saturation vapor pressure, T is the air temperature, and Td is the dew point. The saturation vapor pressure is calculated using the Magnus formula: es = 6.1078 × exp((17.27 × T) / (237.7 + T)).
What relative humidity actually means:
At 50% RH, the air holds half the water vapor it could at that temperature. Warming the air increases its capacity, so the same amount of moisture in the morning (when it is cool and RH is high) gives a lower RH in the afternoon (when it is warm).
Relative vs. absolute humidity:
Relative humidity depends on temperature, which makes it confusing. Air at 90°F and 50% RH contains far more moisture than air at 40°F and 50% RH. Absolute humidity (grams of water per cubic meter) is a direct measure that does not change with temperature.
Fog prediction:
When the temperature-dew point spread narrows to 2-3°C (4-5°F), fog is likely. Pilots, drivers, and weather forecasters watch this spread closely during evening and early morning hours.
Comfortable humidity ranges:
- Indoor comfort: 30-50% RH is ideal
- Below 30%: Dry skin, static electricity, respiratory irritation
- Above 60%: Mold growth risk, musty conditions
- Above 70%: Condensation on windows, potential structural damage