A three year seeing measurement campaign was conducted at the 4.2-m William Herschel Telescope (WHT) using a Shack- Hartmann (S-H) wavefront sensor. An analysis of the spatio- temporal autocorrelation of the S-H centroid motion across the WHT pupil demonstrates the partial validity of Taylor's hypothesis, but at the same time highlights its limitations in the context of ground-based interferometry. So-called wavefront `boiling' is shown to play a major, perhaps dominant role in the evolution of phase errors on the timescales of practical interest. This conclusion is reached by comparing the coherence times measured from the fluctuation of reconstructed wavefronts with those inferred from Taylor's hypothesis using the wind speed determined from the S-H spatio-temporal autocorrelation. Nevertheless it appears that knowledge of the wind speed can be a reliable indicator of coherence time, provided a suitable calibration is established. The practical problem of measuring the turbulence coherent time using portable equipment is explored. The promising method of centroid velocity variance is investigated and the wind speeds measured this way are seen to agree with those determined from the S-H spatio-temporal autocorrelation.