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The Dew point is the temperature to which air must be cooled, at constant barometric pressure, for water vapor to condense into water. Dew point temperature is a saturation temperature, that is, relative humidity of gas (air) is 100%. Further addition of water vapor or cooling of air leads to appearing of condensed water. For plus temperatures it is a dew, for minus - ice or snow.
For example, if you bring something to the warm place right after the frost, the air close to the surface is cooled below dew point, and condensed water appears. Then, after this thing is warmed, condensed water becomes the vapour again. That's the advice not to turn on immediately electronic devices after transportation during cold weather.
Calculation of dew point is taken from here
where a = 17.27, b = 237.7, ln - natural logarithm, RH - relative humidity of air (from 0 to 1), Tp - dew point
According to wikipedia, in the range from 0 to 60C this formula has error about 0.4C