According to the International Research Institute for Climate and Society at Columbia University, dew point is defined as "...the temperature to which the air must be cooled at constant pressure in order for it to become saturated, i.e., the relative humidity becomes 100 percent." What that means, in simple terms, is that the dew point is the temperature at which the moisture in the air becomes liquid water. It is a complicated calculation to determine the dew point, but according to Mark G. Lawerence in a 2005 paper published through the American Meteorological Society, there is a relatively simple approximation that can be used.
Subtract the relative humidity from 100.
Divide that answer by 5 and jot down your result.
Subtract the answer that you jotted down from the current temperature in degrees Celsius. The result is a reasonable approximation of the dew point in degrees Celsius.
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There are online dew point calculators that will calculate dew point for you. See the Resources section.
Keep in mind that this procedure is an approximation, albeit a good one.
Be sure that the temperature you use is in Celsius.