IN an earlier communication1 it was reported that the direction of horizontal movement of travelling ionospheric disturbances observed at Sydney, Australia, had a consistent diurnal variation during winter months in the years 1950–52 ; but no significant diurnal variation was then apparent in the summer. These conditions continued until 1955 ; but in the summer of 1955–56 a definite change appeared which was even more marked in 1956–57, the main feature being a marked change of direction from the south-east to the south-west quadrant about midday. The contrast is clearly evident in Fig. 1, which shows the mean diurnal variation in January for the years 1951–54 as compared with the variation for January 1957. The consistency of the results of 1951–54 is shown by the dotted lines, which indicate maximum deviation from the mean. It will be noted that the change from south-east to south-west quadrant in 1957 is quite sudden. This sequence is also found on a number of individual days, though not on all days. There was no significant change in mean speed over this time interval.