| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1931 | 47.9 | 52.8 | 50.9 | 61.3 | 66.5 | 78.5 | 83.0 | 78.3 | 79.7 | 71.0 | 61.5 | 52.2 | 65.3 |
| 1932 | 52.0 | 58.9 | 53.3 | 65.8 | 71.1 | 78.3 | 82.8 | 80.6 | 74.6 | 62.7 | 49.1 | 44.8 | 64.5 |
| 1933 | 52.1 | 46.8 | 57.2 | 61.7 | 72.9 | 76.4 | 80.0 | 80.5 | 79.3 | 67.7 | 57.5 | 56.9 | 65.9 |
| 1934 | 49.4 | 52.4 | 56.4 | 67.8 | 71.7 | 81.0 | 83.7 | 84.5 | 75.7 | 71.4 | 59.9 | 47.9 | 66.9 |
| 1935 | 50.9 | 50.4 | 64.3 | 64.8 | 70.8 | 78.0 | 82.3 | 83.4 | 75.4 | 70.1 | 56.2 | 46.3 | 66.1 |
| 1936 | 46.9 | 46.9 | 62.0 | 63.3 | 72.1 | 80.8 | 80.5 | 82.9 | 78.4 | 63.8 | 53.3 | 51.4 | 65.3 |
| 1937 | 51.7 | 52.1 | 53.5 | 63.0 | 72.3 | 80.0 | 81.6 | 82.9 | 75.8 | 65.0 | 51.8 | 48.5 | 64.9 |
| 1938 | 48.5 | 57.5 | 64.5 | 64.0 | 71.8 | 78.8 | 81.2 | 81.1 | 76.8 | 70.2 | 55.3 | 49.8 | 66.7 |
| 1939 | 51.4 | 51.9 | 59.9 | 64.3 | 71.8 | 79.7 | 84.0 | 83.2 | 80.6 | 69.1 | 53.7 | 53.2 | 67.0 |
| 1940 | 36.5 | 49.0 | 59.1 | 64.4 | 69.9 | 75.5 | 80.3 | 78.3 | 72.8 | 67.7 | 55.9 | 53.9 | 63.6 |
| 1941 | 51.4 | 47.2 | 53.0 | 66.3 | 72.5 | 77.9 | 80.8 | 82.0 | 78.5 | 72.5 | 53.7 | 51.8 | 65.7 |
| 1942 | 45.6 | 48.4 | 56.9 | 66.0 | 71.3 | 78.8 | 80.7 | 81.9 | 74.2 | 67.3 | 60.5 | 50.7 | 65.3 |
| 1943 | 48.4 | 54.6 | 55.5 | 67.3 | 74.9 | 81.4 | 82.5 | 82.9 | 74.0 | 64.6 | 53.2 | 47.6 | 65.6 |
| 1944 | 47.3 | 56.9 | 58.4 | 65.2 | 70.9 | 80.0 | 82.6 | 83.2 | 76.4 | 67.3 | 57.2 | 45.6 | 65.9 |
| 1945 | 46.3 | 55.9 | 65.4 | 67.1 | 71.7 | 80.4 | 81.4 | 82.1 | 79.1 | 65.5 | 62.3 | 48.3 | 67.2 |
| 1946 | 47.9 | 53.3 | 62.3 | 69.8 | 72.5 | 77.7 | 81.8 | 81.6 | 76.2 | 68.7 | 60.6 | 55.2 | 67.4 |
| 1947 | 48.7 | 45.6 | 53.1 | 68.0 | 72.0 | 79.8 | 81.2 | 84.8 | 80.4 | 75.0 | 55.5 | 52.3 | 66.5 |
| 1948 | 42.5 | 51.7 | 60.1 | 69.6 | 73.8 | 80.9 | 83.1 | 83.8 | 76.1 | 66.3 | 55.0 | 52.3 | 66.3 |
| 1949 | 46.6 | 53.1 | 56.9 | 62.2 | 74.9 | 79.4 | 81.4 | 78.6 | 75.2 | 66.6 | 57.0 | 51.2 | 65.3 |
| 1950 | 55.2 | 54.3 | 55.6 | 62.5 | 73.9 | 77.5 | 79.3 | 80.1 | 74.4 | 68.8 | 53.9 | 47.7 | 65.3 |
| 1951 | 48.4 | 50.0 | 58.3 | 63.9 | 72.7 | 79.9 | 83.6 | 85.7 | 77.2 | 69.0 | 53.7 | 51.4 | 66.2 |
| 1952 | 56.7 | 53.7 | 55.5 | 62.1 | 71.3 | 80.9 | 81.7 | 83.5 | 76.3 | 62.0 | 55.7 | 47.4 | 65.6 |
| 1953 | 51.7 | 50.2 | 63.7 | 63.7 | 73.9 | 83.1 | 80.7 | 80.9 | 77.1 | 69.5 | 54.1 | 45.5 | 66.3 |
| 1954 | 49.5 | 56.6 | 55.5 | 68.4 | 68.3 | 79.3 | 85.2 | 85.4 | 80.1 | 70.4 | 54.6 | 49.7 | 67.0 |
| 1955 | 46.8 | 49.6 | 60.3 | 68.9 | 75.6 | 77.6 | 83.2 | 81.7 | 80.2 | 66.6 | 55.5 | 49.7 | 66.4 |
| 1956 | 48.2 | 53.1 | 57.1 | 63.9 | 75.8 | 79.4 | 85.0 | 84.8 | 77.6 | 70.2 | 53.8 | 53.5 | 66.9 |
| 1957 | 48.7 | 58.1 | 55.4 | 65.4 | 73.9 | 79.7 | 84.2 | 82.9 | 75.3 | 63.6 | 55.4 | 51.7 | 66.2 |
| 1958 | 44.8 | 44.5 | 52.1 | 64.0 | 73.6 | 80.2 | 82.8 | 82.7 | 76.7 | 66.2 | 57.6 | 46.2 | 64.4 |
| 1959 | 43.4 | 51.7 | 55.0 | 62.9 | 76.2 | 79.9 | 82.4 | 82.6 | 78.2 | 67.8 | 50.3 | 50.4 | 65.2 |
| 1960 | 46.3 | 44.5 | 50.0 | 66.7 | 72.1 | 80.1 | 83.6 | 83.3 | 77.6 | 70.5 | 58.2 | 45.1 | 64.9 |