The action of viscous forces on the motion of an atomic force microscope cantilever operating in resonant mode in air is modelized. We demonstrate that for most applications, the vibration of a V shaped cantilever in the air can be approximated to a simple damped oscillator. The damping factor is distance dependent and includes terms issued of the interaction of both cantilever and tip with the sample. Expressions for the various damping forces have been derived and related to the geometry of the tip-cantilever system. They lead to an expression which quantifies the variations in oscillation amplitude versus tip sample distance. ©1997 American Institute of Physics.