| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1961 | 30.0 | 39.6 | 47.2 | 47.4 | 59.7 | 67.0 | 70.3 | 66.0 | 64.5 | 50.7 | 42.4 | 35.8 | 51.8 |
| 1962 | 27.9 | 43.2 | 39.3 | 50.8 | 62.7 | 67.9 | 71.6 | 70.5 | 65.5 | 56.6 | 41.9 | 35.4 | 52.8 |
| 1963 | 27.3 | 33.8 | 44.9 | 58.9 | 63.2 | 68.1 | 71.9 | 69.0 | 65.8 | 54.8 | 48.0 | 29.6 | 53.0 |
| 1964 | 33.3 | 33.9 | 44.8 | 57.1 | 64.3 | 69.8 | 71.2 | 73.1 | 66.8 | 49.0 | 46.8 | 37.6 | 54.0 |
| 1965 | 38.2 | 36.4 | 39.6 | 58.6 | 65.4 | 68.6 | 71.7 | 69.4 | 66.9 | 51.8 | 52.1 | 38.6 | 54.9 |
| 1966 | 34.4 | 35.4 | 44.9 | 54.6 | 62.7 | 66.5 | 73.6 | 70.3 | 64.2 | 49.8 | 46.6 | 35.6 | 53.3 |
| 1967 | 34.8 | 34.3 | 48.6 | 61.3 | 58.9 | 69.2 | 69.6 | 68.5 | 61.3 | 47.9 | 42.7 | 37.6 | 53.0 |
| 1968 | 36.9 | 32.3 | 42.7 | 55.2 | 61.1 | 67.9 | 70.3 | 71.1 | 61.3 | 53.3 | 41.3 | 35.3 | 52.5 |
| 1969 | 39.3 | 39.3 | 40.2 | 55.4 | 61.0 | 67.9 | 74.5 | 71.6 | 64.4 | 55.8 | 40.6 | 37.3 | 54.0 |
| 1970 | 32.1 | 36.9 | 43.7 | 57.2 | 60.4 | 68.1 | 71.2 | 72.7 | 69.6 | 52.9 | 39.0 | 44.9 | 54.2 |
| 1971 | 39.6 | 38.3 | 42.9 | 51.6 | 60.2 | 70.1 | 72.4 | 69.2 | 67.6 | 58.7 | 43.3 | 46.3 | 55.1 |
| 1972 | 39.6 | 39.1 | 47.8 | 55.4 | 60.0 | 68.5 | 69.7 | 70.6 | 69.2 | 54.3 | 42.3 | 33.8 | 54.2 |
| 1973 | 32.0 | 37.9 | 49.5 | 51.1 | 58.3 | 67.7 | 71.3 | 67.5 | 66.6 | 57.6 | 52.7 | 35.9 | 54.1 |
| 1974 | 39.4 | 40.4 | 53.8 | 54.1 | 65.4 | 65.6 | 69.7 | 69.0 | 61.5 | 54.8 | 44.8 | 38.3 | 54.8 |
| 1975 | 40.7 | 38.3 | 47.8 | 54.7 | 64.1 | 66.9 | 69.8 | 70.0 | 61.0 | 53.8 | 43.4 | 39.3 | 54.3 |
| 1976 | 33.9 | 45.6 | 49.5 | 54.5 | 57.2 | 65.3 | 70.0 | 67.2 | 63.2 | 46.3 | 37.9 | 34.6 | 52.1 |
| 1977 | 27.9 | 37.3 | 47.1 | 54.0 | 63.0 | 69.5 | 72.1 | 71.8 | 68.2 | 53.9 | 47.3 | 37.5 | 54.2 |
| 1978 | 28.5 | 30.9 | 42.8 | 55.8 | 62.7 | 67.6 | 71.3 | 70.9 | 68.6 | 49.7 | 38.1 | 53.5 | |
| 1979 | 29.9 | 37.9 | 55.7 | 67.6 | 71.8 | 69.7 | 62.8 | 54.7 | 39.3 | 37.6 | 52.9 | ||
| 1980 | 40.5 | 36.7 | 43.9 | 49.8 | 62.7 | 70.5 | 72.4 | 71.4 | 69.3 | 50.4 | 42.5 | 39.4 | 54.2 |
| 1981 | 35.0 | 39.3 | 45.0 | 60.3 | 60.2 | 70.7 | 72.7 | 70.2 | 62.3 | 55.0 | 45.4 | 38.0 | 54.6 |
| 1982 | 34.7 | 38.0 | 51.7 | 53.1 | 63.3 | 68.4 | 71.5 | 71.9 | 64.2 | 53.4 | 46.7 | 41.9 | 55.0 |
| 1983 | 37.2 | 39.9 | 43.5 | 50.2 | 58.8 | 66.4 | 70.9 | 71.1 | 63.5 | 54.9 | 46.1 | 29.5 | 52.7 |
| 1984 | 31.1 | 38.3 | 46.3 | 52.3 | 61.4 | 68.2 | 70.8 | 70.6 | 64.2 | 59.2 | 44.8 | 49.7 | 54.8 |
| 1985 | 31.0 | 37.9 | 52.2 | 56.2 | 61.7 | 68.8 | 70.9 | 71.1 | 65.4 | 60.1 | 52.8 | 32.7 | 55.1 |
| 1986 | 38.3 | 44.0 | 46.1 | 57.4 | 62.0 | 70.1 | 72.5 | 69.1 | 70.2 | 55.6 | 49.5 | 38.8 | 56.2 |
| 1987 | 35.5 | 43.5 | 44.0 | 50.2 | 63.7 | 68.2 | 71.2 | 73.0 | 63.2 | 51.7 | 47.5 | 40.5 | 54.4 |
| 1988 | 33.0 | 37.8 | 45.1 | 52.8 | 59.1 | 67.0 | 71.4 | 71.8 | 66.1 | 54.3 | 46.8 | 39.8 | 53.8 |
| 1989 | 43.4 | 38.1 | 47.5 | 56.0 | 64.2 | 66.0 | 70.1 | 70.1 | 61.6 | 50.8 | 44.7 | 25.5 | 53.2 |
| 1990 | 40.2 | 42.4 | 47.4 | 51.6 | 63.0 | 70.1 | 69.8 | 70.3 | 67.1 | 50.6 | 45.8 | 35.3 | 54.5 |