The dew point is the temperature at which air must be cooled, at constant barometric pressure, for water vapor to condense into water. The dew point temperature is a saturation temperature, which means the relative humidity of the gas (air) is 100%. The further addition of water vapor or cooling of air leads to the appearance of condensed water. For positive temperatures, it is dew, and for negative temperatures, it is ice or snow.
For example, if you bring something to a warm place immediately after it has been exposed to frost, the air close to the surface is cooled below the dew point, and condensed water appears. Then, after this thing is warmed, the condensed water becomes vapor again. This is why it is advised not to turn on electronic devices immediately after transportation during cold weather.
The formula1 used to calculate the dew point is as follows:
where a = 17.27, b = 237.7, ln is the natural logarithm, RH is the relative humidity of the air (from 0 to 1), and Tp is the dew point.
It is worth noting that the formula has an error of approximately 0.4°C in the range of 0°C to 60°C, according to Wikipedia.