| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1961 | 18.0 | 29.0 | 31.0 | 35.0 | 45.0 | 60.0 | 65.0 | 59.0 | 47.0 | 36.0 | 26.0 | 18.0 | 18.0 |
| 1962 | 5.0 | 29.0 | 25.0 | 27.0 | 45.0 | 62.0 | 68.0 | 60.0 | 54.0 | 39.0 | 30.0 | 16.0 | 5.0 |
| 1963 | 9.0 | 19.0 | 28.0 | 40.0 | 47.0 | 64.0 | 67.0 | 56.0 | 43.0 | 41.0 | 28.0 | 12.0 | 9.0 |
| 1964 | 10.0 | 22.0 | 29.0 | 39.0 | 49.0 | 54.0 | 65.0 | 68.0 | 57.0 | 38.0 | 27.0 | 22.0 | 10.0 |
| 1965 | 17.0 | 20.0 | 23.0 | 41.0 | 48.0 | 61.0 | 66.0 | 59.0 | 53.0 | 33.0 | 31.0 | 28.0 | 17.0 |
| 1966 | 13.0 | 22.0 | 26.0 | 36.0 | 54.0 | 55.0 | 67.0 | 57.0 | 52.0 | 35.0 | 24.0 | 18.0 | 13.0 |
| 1967 | 20.0 | 20.0 | 25.0 | 50.0 | 46.0 | 58.0 | 54.0 | 54.0 | 38.0 | 37.0 | 29.0 | 23.0 | 20.0 |
| 1968 | 20.0 | 23.0 | 22.0 | 39.0 | 46.0 | 57.0 | 58.0 | 61.0 | 51.0 | 38.0 | 26.0 | 23.0 | 20.0 |
| 1969 | 17.0 | 26.0 | 25.0 | 45.0 | 47.0 | 51.0 | 69.0 | 65.0 | 52.0 | 40.0 | 24.0 | 26.0 | 17.0 |
| 1970 | 14.0 | 23.0 | 30.0 | 33.0 | 44.0 | 54.0 | 60.0 | 61.0 | 52.0 | 36.0 | 22.0 | 24.0 | 14.0 |
| 1971 | 23.0 | 20.0 | 22.0 | 30.0 | 41.0 | 62.0 | 64.0 | 62.0 | 55.0 | 42.0 | 31.0 | 35.0 | 20.0 |
| 1972 | 18.0 | 19.0 | 30.0 | 38.0 | 52.0 | 53.0 | 58.0 | 63.0 | 51.0 | 43.0 | 32.0 | 17.0 | 17.0 |
| 1973 | 15.0 | 18.0 | 35.0 | 31.0 | 43.0 | 59.0 | 66. | 60.0 | 55.0 | 38.0 | 35.0 | 19.0 | 15.0 |
| 1974 | 21.0 | 23.0 | 32.0 | 39.0 | 51.0 | 54.0 | 62.0 | 63.0 | 48.0 | 39.0 | 28.0 | 22.0 | 21.0 |
| 1975 | 15.0 | 19.0 | 27.0 | 32.0 | 56.0 | 52.0 | 61.0 | 64.0 | 45.0 | 38.0 | 22.0 | 18.0 | 15.0 |
| 1976 | 13.0 | 25.0 | 31.0 | 41.0 | 43.0 | 57.0 | 65.0 | 60.0 | 48.0 | 31.0 | 15.0 | 17.0 | 13.0 |
| 1977 | 10.0 | 24.0 | 34.0 | 42.0 | 52.0 | 57.0 | 69.0 | 68.0 | 60.0 | 38.0 | 28.0 | 21.0 | 10.0 |
| 1978 | 18.0 | 17.0 | 26.0 | 41.0 | 45.0 | 62.0 | 64.0 | 66.0 | 57.0 | 34.0 | 15.0 | 15.0 | |
| 1979 | 10.0 | 17.0 | 38.0 | 54.0 | 63.0 | 65.0 | 50.0 | 41.0 | 21.0 | 18.0 | 10.0 | ||
| 1980 | 28.0 | 19.0 | 13.0 | 35.0 | 49.0 | 58.0 | 67.0 | 64.0 | 63.0 | 33.0 | 27.0 | 24.0 | 13.0 |
| 1981 | 23.0 | 12.0 | 30.0 | 46.0 | 44.0 | 65.0 | 67.0 | 62.0 | 46.0 | 36.0 | 29.0 | 20.0 | 12.0 |
| 1982 | 5.0 | 21.0 | 26.0 | 35.0 | 45.0 | 58.0 | 68.0 | 69.0 | 47.0 | 35.0 | 30.0 | 22.0 | 5.0 |
| 1983 | 23.0 | 30.0 | 28.0 | 36.0 | 44.0 | 55.0 | 62.0 | 69.0 | 42.0 | 40.0 | 25.0 | 6.0 | 6.0 |
| 1984 | 15.0 | 19.0 | 26.0 | 35.0 | 43.0 | 50.0 | 65.0 | 65.0 | 49.0 | 38.0 | 27.0 | 21.0 | 15.0 |
| 1985 | 10.0 | 18.0 | 39.0 | 40.0 | 50.0 | 58.0 | 65.0 | 63.0 | 45.0 | 42.0 | 38.0 | 18.0 | 10.0 |
| 1986 | 26.0 | 24.0 | 25.0 | 42.0 | 50.0 | 64.0 | 69.0 | 55.0 | 65.0 | 42.0 | 29.0 | 31.0 | 24.0 |
| 1987 | 22.0 | 27.0 | 25.0 | 29.0 | 56.0 | 62.0 | 65.0 | 69.0 | 52.0 | 40.0 | 28.0 | 24.0 | 22.0 |
| 1988 | 19.0 | 17.0 | 27.0 | 35.0 | 49.0 | 57.0 | 64.0 | 63.0 | 53.0 | 42.0 | 32.0 | 22.0 | 17.0 |
| 1989 | 26.0 | 17.0 | 22.0 | 30.0 | 53.0 | 55.0 | 63.0 | 57.0 | 45.0 | 27.0 | 28.0 | 3.0 | 3.0 |
| 1990 | 22.0 | 30.0 | 30.0 | 33.0 | 51.0 | 64.0 | 60.0 | 62.0 | 47.0 | 38.0 | 27.0 | 12.0 | 12.0 |