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.

Ranking         Month/Year       Solar Cycle     Sunspot Number
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   1          October 1957            19             253.8
   2          December 1957           19             239.4
   3          September 1957          19             235.8
   4          January 1959            19             217.4
   5          November 1957           19             210.9
   6          January 1958            19             202.5
   7          November 1956           19             201.3
   7          May 1947                18             201.3
   9          September 1958          19             201.2
  10          June 1957               19             200.7
  11          August 1958             19             200.2

  12          August 1990             22             199.9

  13          August 1959             19             199.6
  14          June 1989               22             196.0
  14          April 1958              19             196.0
  16          December 1956           19             192.1
  17          July 1958               19             191.4
  18          March 1958              19             190.7
  19          April 1948              18             189.7
  20          August 1947             18             188.8
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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. © Copyright IPS - Radio and Space Services.