Dew point vs relative humidity: which tells you how muggy it is?

Dew point is the better guide to how humid it feels. It is the temperature the air would have to cool to for its water vapour to start condensing, and it tracks the actual amount of moisture in the air. Relative humidity, by contrast, is a percentage that swings up and down with temperature even when the moisture stays the same. A summer dew point above about 18 °C (65 °F) feels muggy whatever the humidity percentage says. The dew point calculator converts between the two, and this guide explains why they behave so differently.

Two ways to measure moisture

Relative humidity (RH) compares the water vapour in the air with the most the air could hold at its current temperature. 50% means the air holds half of its maximum. Because warm air can hold much more vapour than cold air, the same moisture gives a lower RH on a warm afternoon than on a cool morning.

The dew point is the temperature at which that same air would reach 100% RH if cooled at constant pressure. At that point dew forms on grass, fog forms in the air and windows mist up. Air at 25 °C with 60% RH has a dew point of 16.7 °C: cool it to 16.7 °C and it is saturated. The gap between the air temperature and the dew point, called the spread, tells forecasters how close the air is to fog or cloud.

Why relative humidity misleads

Imagine a still summer day where the moisture in the air does not change. At dawn the air is 15 °C and the dew point is 15 °C, so RH is 100% and there is dew on the grass. By mid-afternoon the air has warmed to 30 °C. The dew point is still 15 °C, but RH has fallen to about 40%. Nothing about the moisture changed; only the temperature did. That is why a winter morning can show 90% humidity while feeling crisp, and a hot afternoon can show 50% while feeling sticky. The US National Weather Service makes the same point: relative humidity on its own can be misleading, and the dew point tells you how much moisture is really there.

Dew point and comfort

Your body cools itself by evaporating sweat, and evaporation slows when the air already holds a lot of water vapour. So comfort follows the dew point closely. The National Weather Service office at La Crosse gives these general summer comfort levels:

• 55 °F (about 13 °C) or lower: dry and comfortable;
• 55 to 65 °F (about 13 to 18 °C): becoming sticky, with muggy evenings;
• 65 °F (about 18 °C) or higher: lots of moisture in the air, becoming oppressive.

My calculator splits these into finer bands, from dry below 10 °C to oppressive in the low twenties, but the idea is the same. People who live in humid climates tend to tolerate higher dew points than visitors. For the combined effect of heat and humidity on the body, the feels-like temperature calculator applies the official heat index formula.

Converting between them

The two are linked through the saturation vapour pressure of water, which the Magnus formula approximates. The calculator uses the coefficients 17.625 and 243.04 °C published by Lawrence in 2005, which are accurate to a fraction of a degree in normal weather. For a quick mental estimate when RH is above about 50%, there is a rule of thumb: each 1 °C gap between the temperature and the dew point lowers RH by about 5 percentage points. So 30 °C air with a dew point of 27 °C sits at roughly 85%. If you work in Fahrenheit, the temperature converter handles the switch, and the calculator accepts either unit directly.

Condensation and damp at home

Indoors, dew point explains why windows stream in winter. A room at 20 °C and 50% RH has a dew point of about 9 °C, so any glass or wall surface colder than that collects condensation. Lowering indoor humidity lowers the dew point and dries those surfaces; ventilation after cooking and showers helps, and in persistently damp rooms a dehumidifier does the work. The dehumidifier size calculator estimates the capacity a space needs. For how weather is described in other ways, The Beaufort wind scale: what force 0 to 12 means covers wind strength.