| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1931 | 33.1 | 47.3 | 43.5 | 56.1 | 63.9 | 66.1 | 70.7 | 70.2 | 66.3 | 54.1 | 45.6 | 35.9 | 54.4 |
| 1932 | 37.3 | 43.2 | 40.5 | 50.7 | 56.1 | 67.2 | 72.1 | 67.6 | 67.3 | 58.8 | 51.5 | 44.3 | 54.8 |
| 1933 | 43.1 | 49.1 | 42.5 | 54.6 | 61.5 | 68.9 | 72.8 | 71.1 | 65.3 | 50.7 | 38.4 | 37.7 | 54.6 |
| 1934 | 42.5 | 42.8 | 45.8 | 57.3 | 61.5 | 70.3 | 72.0 | 73.3 | 64.4 | 58.9 | 49.5 | 39.9 | 56.6 |
| 1935 | 42.6 | 42.3 | 54.7 | 57.2 | 62.5 | 69.9 | 72.5 | 71.8 | 65.4 | 58.1 | 46.2 | 37.4 | 56.8 |
| 1936 | 34.5 | 36.2 | 50.5 | 51.2 | 63.3 | 69.7 | 71.5 | 71.6 | 68.3 | 52.8 | 42.1 | 42.3 | 54.6 |
| 1937 | 44.2 | 42.9 | 44.1 | 53.7 | 61.3 | 70.3 | 71.6 | 72.4 | 65.5 | 54.6 | 42.7 | 43.5 | 55.6 |
| 1938 | 41.9 | 48.7 | 55.3 | 55.6 | 62.5 | 69.1 | 71.7 | 71.2 | 63.8 | 56.2 | 43.6 | 40.1 | 56.7 |
| 1939 | 41.5 | 42.1 | 48.7 | 53.9 | 61.3 | 70.8 | 72.7 | 71.3 | 67.6 | 56.6 | 43.7 | 42.4 | 56.1 |
| 1940 | 27.5 | 40.0 | 48.4 | 55.4 | 60.2 | 67.5 | 71.2 | 69.3 | 62.3 | 54.7 | 47.4 | 45.4 | 54.1 |
| 1941 | 42.2 | 39.3 | 44.5 | 57.1 | 63.8 | 69.7 | 72.1 | 73.1 | 70.5 | 65.4 | 42.9 | 43.7 | 57.1 |
| 1942 | 35.5 | 40.4 | 46.3 | 57.3 | 62.9 | 70.6 | 71.6 | 73.0 | 65.1 | 55.5 | 49.8 | 40.9 | 55.8 |
| 1943 | 39.0 | 43.4 | 45.3 | 56.2 | 64.4 | 70.9 | 71.7 | 71.0 | 63.9 | 52.0 | 40.4 | 39.6 | 54.9 |
| 1944 | 38.9 | 49.0 | 48.9 | 55.7 | 61.8 | 70.2 | 71.4 | 71.8 | 66.1 | 52.5 | 48.3 | 36.6 | 55.9 |
| 1945 | 37.3 | 45.0 | 54.5 | 56.0 | 60.5 | 70.5 | 72.1 | 71.4 | 67.5 | 53.1 | 50.3 | 38.2 | 56.4 |
| 1946 | 39.6 | 42.1 | 50.8 | 58.3 | 63.0 | 68.2 | 71.8 | 71.0 | 65.9 | 57.1 | 51.1 | 44.8 | 57.1 |
| 1947 | 41.1 | 33.3 | 42.1 | 57.7 | 60.8 | 69.8 | 68.7 | 72.1 | 66.9 | 60.7 | 43.9 | 41.9 | 55.0 |
| 1948 | 23.2 | 41.8 | 48.4 | 57.8 | 63.4 | 69.4 | 71.7 | 69.9 | 62.7 | 51.5 | 42.4 | 40.3 | 54.3 |
| 1949 | 36.3 | 42.1 | 44.8 | 51.4 | 63.4 | 68.3 | 70.6 | 66.2 | 62.4 | 56.6 | 41.6 | 40.5 | 53.8 |
| 1950 | 46.3 | 43.4 | 43.0 | 51.5 | 62.9 | 67.5 | 69.0 | 67.3 | 63.3 | 55.8 | 39.8 | 35.7 | 53.9 |
| 1951 | 36.0 | 38.2 | 46.1 | 52.1 | 60.9 | 69.5 | 71.1 | 72.0 | 65.0 | 56.7 | 41.9 | 39.1 | 54.2 |
| 1952 | 45.8 | 42.6 | 43.3 | 50.2 | 60.4 | 69.6 | 70.7 | 71.0 | 61.9 | 45.3 | 44.0 | 36.5 | 53.5 |
| 1953 | 39.6 | 39.0 | 52.9 | 51.8 | 64.1 | 71.4 | 71.3 | 71.1 | 63.6 | 55.4 | 41.5 | 34.2 | 54.8 |
| 1954 | 38.0 | 41.8 | 42.8 | 57.5 | 57.8 | 67.3 | 72.5 | 71.4 | 63.1 | 56.5 | 38.4 | 35.0 | 53.6 |
| 1955 | 35.9 | 36.8 | 47.0 | 56.4 | 63.8 | 64.3 | 71.1 | 70.3 | 66.7 | 49.3 | 40.9 | 36.9 | 53.4 |
| 1956 | 33.3 | 40.6 | 42.5 | 50.1 | 63.5 | 67.3 | 71.5 | 69.9 | 60.0 | 52.2 | 37.4 | 39.4 | 52.3 |
| 1957 | 37.3 | 46.5 | 41.3 | 53.8 | 62.4 | 70.1 | 71.7 | 70.0 | 62.7 | 50.0 | 44.3 | 36.3 | 53.9 |
| 1958 | 30.5 | 32.4 | 36.7 | 50.3 | 60.5 | 68.4 | 71.2 | 69.6 | 66.8 | 53.1 | 43.5 | 32.1 | 51.4 |
| 1959 | 30.3 | 39.5 | 37.8 | 50.5 | 64.9 | 68.3 | 70.2 | 70.8 | 65.9 | 53.9 | 35.3 | 37.8 | 52.2 |
| 1960 | 35.5 | 30.8 | 37.6 | 53.1 | 58.5 | 67.9 | 72.0 | 73.0 | 64.2 | 57.6 | 46.1 | 35.1 | 52.7 |