How Humidity Is Defined

Air holds a maximum amount of water vapor that grows sharply with temperature. Relative humidity expresses how full the air is: RH = e(Td)/eₛ(T) × 100, computed here with the Magnus formula for saturation vapor pressure, accurate to a fraction of a percent across the −40 to +50 °C range.

The Dew Point Tells the Truth

Because RH is relative to temperature, it can mislead: 50% at 30 °C is more moisture than 60% at 15 °C. The dew point — the temperature where the air would saturate — removes the temperature illusion. It is the number forecasters and HVAC engineers actually design around.

Why Humidity Matters

Humidity drives heat stress (sweat can't evaporate in muggy air), condensation and mold (dew point crossing surface temperature), static discharge in dry air, and even how crisp audio and paper products feel. Understanding it is the first step to comfort in every climate.