How this calculator works
Dew point is the temperature at which the current water-vapour pressure equals saturation pressure. Cooling air to it initiates condensation on an appropriate surface. The Bolton liquid-water Magnus fit supplies an invertible saturation relation. All temperature and dew-point values must remain between -40 and 60 C; below freezing this is a supercooled-water dew point rather than frost point. Zero RH has no finite dew point in this model, so inverse temperature and dew-point calculations require positive RH. Vapour pressure and absolute humidity are also shown.
Formula
e_s(t) = 611.2 exp[17.67 t/(243.5+t)] Pa. RH = e_s(Td)/e_s(T). Let a = ln(e/611.2); Td = 243.5 a/(17.67-a) C.
Worked example
At 20 C and 50% RH, dew point is about 9.2701 C. When dew point equals air temperature, RH is 100%. At 0 C and saturation the vapour pressure is 611.2 Pa.
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Frequently asked questions
Does 100% RH mean the air is all water?
No. It means vapour pressure equals saturation pressure at that temperature. Most of the gas can still be dry air.
Does dew point change when I simply warm the air?
At unchanged vapour partial pressure, dew point stays the same while relative humidity falls.
Is dew point below freezing the frost point?
Not here. This fit uses liquid-water saturation. Saturation over ice gives a different frost point.