# Dewpoint

The water vapour content of air is often quoted as dew point. This is the temperature to which the air must be cooled before dew condenses from it.

At this temperature the actual water vapour content of the air is equal to the saturation water vapour pressure. The dew point is usually calculated from the RH. First one calculates *p*~s~ , the saturation vapour pressure at the ambient temperature. The actual water vapour pressure, *p*~a~, is:

paps * RH% / 100

The next step is to calculate the temperature at which *p*~a~ would be the saturation vapour pressure. This means running backwards the equation given above for deriving saturation vapour pressure from temperature:

Let w ln ( pa/ 610.78 )

Dew point w * 238.3 / ( 17.294 – w )

This calculation is often used to judge the probability of condensation on windows and within walls and roofs of humidified buildings.

The dew point can also be measured directly by cooling a mirror until it fogs. The RH is then given by the ratio

RH 100 * *p*~s~ dewpoint / *p*~s~ ambient

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