Large Monthly Sunspot Numbers

Solar Cycle 22 is almost over after a peak smoothed sunspot number in July 1989 of 158.5. This makes the cycle one of the largest ever recorded, continuing the sequence of strong cycles which included Cycle 19 (peak yearly smoothed sunspot number of 201 in 1957) and Cycle 21 (peak yearly smoothed sunspot number of 164 in 1979). Cycle 18 (peak yearly smoothed sunspot number of 152 in 1947) was also stronger than the majority of cycles observed since records commenced. Only Cycle 20 was relatively weak, reaching a peak of 111 in 1968.

The highest monthly sunspot number in August 1990 was observed to be 199.9 and it is interesting to compare this value with those observed for months in previous solar cycles. The following table ranks, in order, the 20 single months from solar cycles 18-22 for which the sunspot number was at its highest monthly-averaged value.

Notes

1. In terms of sunspot number, August 1990 ranks number 12 in this list. If the months during the record Cycle 19 are excluded, then August ranks higher than any except May 1947.
2. The strength of the record Solar Cycle 19 is indicated by the fact that it provides a total of 15 out of the list of 20. Also, that cycle contributes all of the top seven months in the list.
3. Even though Cycle 18 was relatively weaker that Cycle 21, it contributes 3 entries to the list. Cycle 21 does not contribute a single entry to the list and the remaining 2 entries come from Cycle 22 (June 1989 and August 1990).

Material prepared by Richard Thompson