The selection of the quietest days (Q-days) and most disturbed days (D-days) of each month is deduced from the Kp indices on the basis of three criteria for each day:
According to each of these criteria, a relative order number is assigned to each day of the month, the three order numbers are averaged and the days with the lowest and the highest mean order numbers are selected as the five (respectively ten) quietest and the five most disturbed days.
It should be noted that these selection criteria give only a relative indication of the character of the selected days with respect to the other days of the same month. As the general disturbance level may be quite different for different years and also for different months of the same year, the selected quietest days of a month may sometimes be rather disturbed or vice versa.
In order to indicate such a situation, selected days which do not satisfy certain absolute criteria are marked as follows: