The left panel shows the weighted standard deviation of angular offsets between HWM14 predicted anti-wind direction and observed wave direction for periods between 20–27 min as a function of altitude, while the right panel shows the angular offsets as a function of time and day of year for an altitude of 220 km, where the standard deviation is minimised. Negative offsets correspond to the wave being rotated anti-clockwise with respect to the anti-wind direction, and positive offsets correspond to clockwise rotation. The calculation of the standard deviation is weighted by the wind speed.