The relative humidity (RH) and the dewpoint temperature ( t _d ) are two widely used indicators of the amount of moisture in air. The exact conversion from RH to t _d , as well as highly accurate approximations, are too complex to be done easily without the help of a calculator or computer. However, there is a very simple rule of thumb that can be very useful for approximating the conversion for moist air (RH > 50%), which does not appear to be widely known by the meteorological community: t _d decreases by about 1°C for every 5% decrease in RH (starting at t _d = t , the dry bulb temperature, when RH = 100%). This article examines the mathematical basis and accuracy of this and other relationships between the dewpoint and relative humidity. Several useful applications of the simple conversion are presented, in particular the computation of the cumulus cloud-base level (or lifting condensation level) as z _LCL >> (20 + t /5)(100 – RH), where z _LCL is in meters when t is in degrees Celcius and RH in percent. Finally, a historical perspective is given with anecdotes about some of the early work in this field.