We have studied the daily Dobson measurements of ozone column made between 1957 and 1975. The ozone column density measured at the seasonal minima in the northern hemisphere, shows a clear increase over these years. The amount of increase is approximately 4--5% between latitudes of 15N and 60N, and 11% in the data obtained between 60N and 90N. The seasonal maxima show no evidence for an increase during the same time period, in the northern hemisphere. We conclude that the secular increase of global ozone noted between approximately 1960 and 1970 by a number of authors, is caused by a physical mechanism that leads to an increase in the seasonal minima, while allowing the seasonal maxima to remain roughly constant.