Humidity Calculator

Relative humidity compares how much moisture the air actually holds (fixed by the Dew Point) to how much it COULD hold at the current Temperature. Warmer air can hold exponentially more water vapor before saturating, so if the actual moisture content (dew point) doesn't change, a rising temperature makes the air's saturation capacity climb faster than its actual moisture content -- which is exactly why relative humidity falls on a hot, dry afternoon even though the absolute amount of moisture in the air hasn't changed.

The Magnus formula is a widely used meteorological approximation for saturation vapor pressure as a function of temperature: es = 6.112 x exp(17.67 x T / (T + 243.5)), with T in degrees Celsius. These specific coefficients come from David Bolton, "The Computation of Equivalent Potential Temperature," Monthly Weather Review, 1980 — the variant NOAA cites as its standard formulation, accurate to about 0.1% across the -30 to 35 degrees C range. This calculator evaluates it once at the entered temperature and once at the entered dew point, then divides the two results to get relative humidity. It's an approximation, not an exact physical law.

When Dew Point and Temperature are equal, the saturation vapor pressure formula evaluates to the same value for both, so Relative Humidity comes out to exactly 100% -- the air is fully saturated and can't hold any more moisture without water condensing out (which is exactly why dew, fog, or precipitation forms when the two converge).

This calculator applies two simple cutoffs to the calculated Relative Humidity: below 30% is labeled "Dry," above 60% is labeled "Humid," and anything in between is labeled "Comfortable." These are simplified round-number thresholds meant to give a quick read on a weather station or hygrometer value, not a detailed thermal-comfort model that also accounts for temperature, air movement, or activity level.