Dew Point and Relative Humidity Calculator

What is the dew point?

The dew point is the temperature at which air becomes saturated when it is cooled without changing its water vapour content. If the air cools to this temperature or below, vapour condenses. A cold glass sweating, windows fogging in winter and moisture forming on steel before painting are the same thing: the surface is colder than the dew point of the air.

How is it calculated?

The saturation pressure p_ws at the air temperature is found first. The vapour pressure is p_w = φ × p_ws. The dew point is the temperature whose saturation pressure equals p_w. In reverse, when the dew point is known, relative humidity is φ = p_ws(dew point) / p_ws(air temperature). This calculator uses the Hyland and Wexler correlation from the ASHRAE Handbook of Fundamentals for saturation pressure.

Example

At 20 °C the saturation pressure is 2339 Pa. At 60 percent relative humidity the vapour pressure is 1403 Pa, which is the saturation pressure at 12.0 °C. So the dew point is 12.0 °C, and any surface in the room colder than 12 °C will collect water droplets.

Why does relative humidity drop when air warms up?

Relative humidity is the ratio of the vapour in the air to the most it could hold at that temperature. Warm air can hold more. If saturated outdoor air at 0 °C is heated to 20 °C without adding moisture, its relative humidity falls to about 26 percent. That is why heated rooms are dry in winter.

Use in painting and coating work

When coating steel, a common requirement is that the surface temperature be at least 3 °C above the dew point. Check the coating data sheet and the job specification for the limit that applies.

Limits

The result is only as good as your temperature and humidity readings. An error of 2 percent in relative humidity moves the dew point by about half a degree. For pressure dependent quantities such as humidity ratio and enthalpy use the psychrometric calculator.