Relative Humidity Calculator (from Dew Point)

Because the T/Td pair carries strictly more information: dew point gives absolute moisture (fog, icing, carb-ice and cloud-base prediction), temperature gives density, and their spread gives saturation proximity — RH compresses all that into one ambiguous ratio. A 100% RH reading can't distinguish freezing fog at −2° from a muggy 24° drizzle; the pair can. Most professional meteorology works in T/Td and treats RH as a derived display value, exactly what this tool computes.

Clausius-Clapeyron: warm air's capacity to hold vapor grows near-exponentially, roughly doubling per 10°C. Overnight air at 15°C with a 14° dew point reads 94%; warm the same air to 28° and capacity has nearly tripled, so the unchanged moisture reads 54%. This tool's 'RH if it warms 5°' output makes the effect visible. The reverse is the fog mechanism: cool saturated-ish air a few degrees and RH crosses 100%.

Outdoor air at −5°C holding 80% RH carries about 2.6 g/m³ of water (dew point −7.7°C). Bring it inside, heat it to 21°C — same grams, but capacity is now 18 g/m³, so it reads 14% RH: Sahara-dry, courtesy of physics rather than your furnace 'removing' moisture. That's why humidifiers exist, why wooden instruments crack in January, and why the static shocks start when the heating season does.

Standards converge on 30–60%: ASHRAE comfort sits there, mold growth accelerates above ~65–70% on cool surfaces, dust mites collapse below 50%, and museums/archives hold 45–55% with tight tolerance because cycling humidity (not the level itself) does the mechanical damage to wood and paper. Below 30%, respiratory irritation and static rise. The dew point view sharpens all of this: condensation risk lives wherever surface temperatures dip under the dew point this page computes from your conditions.