Meridional winds can be determined from measurements of the peak height of the ionospheric F2 layer ( h m ), the peak density ( N m ), or the total electron content ( N t ). At night, ionospheric changes follow the wind with a time constant of 30–40 min. During the day, this increases to 50–100 min for h m , and 3–6 h for N m and N t . Thus, peak-height data are most suitable for the direct calculation of atmospheric winds, and daytime results should be advanced by 1 h. The wind calculations require an estimate of the peak height h 0 at zero wind. h 0 is close to the servo result ( h s) at night, if the servo constant c is increased by 25% to agree with current theory. During the day, however, the long time constants prevent the F layer from reaching equilibrium before sunset. h 0 is well below h s from sunrise until afternoon, giving serious errors in any results based on servo theory. Results from a full ionospheric modelling program are used to obtain analytic expressions which reproduce the true, zero-wind peak height with an accuracy of a few kilometres, for all times of the day, all seasons and solar flux, and all latitudes in the useful range of 20–60° geomagnetic. The wind W required to produce a given change in h m varies closely as sin 1.4 I, where I is the magnetic dip angle, and the variation of h m with W is accurately reproduced by a modified servo equation. Use of these results with accurate peak height data should give horizontal winds with an accuracy of about ±25 m/s. Peak heights derived from scaled ionospheric data (M3000F2 and foF2) have an accuracy of typically 10–20 km, giving overall errors of about 40 m/s in calculated winds.