The dew point is the temperature at which water vapour condenses into liquid water. It is important to know the dew point because it is the temperature at which clouds form. When air rises from the ground, it cools until it reaches its dew point and then clouds form.

To calculate dew point temperature we use a wet and dry bulb thermometer. This consists of two thermometers placed side by side. The bulb of one is covered by a wick that is always kept wet (wet bulb) and the other has no wick (dry bulb). The thermometer with the wet wick will cool as the water evaporates from the wick. The less water vapour in the air or the drier the air the more water will evaporate from the wet wick. The more water evaporates the lower the reading on the wet bulb thermometer. The dry bulb or regular thermometer is used as a reference to give us the current air temperature. The temperature difference between the two thermometers tells us how much moisture is in the air. The greater the difference the drier the air.

The formula for calculating dew point: Set x = (1 0.01 RH)
where RH is the relative humidity expressed as a percent (a number between 1 and 100). If the relative humidity is 38 percent, x = 0.62.

Then calculate: DPD = (14.55 + 0.114T)x + ((2.5 + 0.007T)x)^3 + (15.9 + 0.117T)x^14
where T is the temperature in degrees Celsius.

This calculation yields the difference between the temperature and dew point in degrees Celsius. Finally, compute the dew point TD = T DPD. The answer is in degrees Celsius.