Relative Humidity from Dew Point

Relative humidity from dew point

Relative humidity is just the ratio of the partial vapor pressure e to the saturation vapor pressure es at the current temperature: RH = e/es · 100. With the Magnus-Tetens form you have es(T) = 6.112·exp(17.625·T/(243.04+T)) in hPa, and since e = es(Td), that leaves RH = es(Td)/es(T)·100. The catch is that saturation pressure climbs exponentially with temperature: about 6.1 hPa at 0 °C, 31.7 hPa at 25 °C, and 73.8 hPa at 40 °C. As a check, T = 25 °C with RH = 60% gives Td ≈ 16.7 °C, and going the other way, Td = 16.7 °C at T = 25 °C lands back on RH ≈ 60%.

Applications

It shows up in air conditioning and HVAC work, where you watch for condensation on cooling coils, and in aviation METARs, which report T and Td side by side every 30 min. Farmers track it for frost risk, since the danger climbs as ambient temperature drops toward Td. Libraries and museums hold RH between 45-55% to protect paper, wood and pigments, and on the comfort side, RH below 30% dries out the mucous membranes.

FAQ

Why does RH change throughout the day if moisture doesn't? Because es grows with T. When the air warms, the same vapor mass becomes a smaller slice of saturation, so RH drops. After dark it rises again.

Can RH exceed 100%? Only for a moment. Supersaturated air condenses the extra vapor right away into mist, fog or dew, which holds the observed RH at 100%.

Is dew point or RH a better moisture metric? Td is an absolute number that compares cleanly across temperatures. RH is relative and can mislead you whenever T moves around. That is why meteorologists lean on Td for trend analysis.