Height anomalies in tapping mode atomic force microscopy (AFM) in air are shown to be caused by adhesion. Depending on the damping of the oscillation the height of a sticking surface is reduced compared to less sticking surfaces. It is shown that the height artefacts result from a modulation of oscillatory movement of the cantilever. Damping and excitation of the cantilever by the driver continuously compete. As a consequence a severe modulation of the cantilever oscillation occurs, depending on the phase mismatch between the driver and the cantilever. Phase images of tapping a mode AFM show contrast which correlates with adhesion. Examples of a partially removed gold layer on mica, a Langmuir-Blodgett film and DNA show height artefacts ranging up to 10 nm.