| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1901 | 51.6 | 48.2 | 57.4 | 61.8 | 71.2 | 80.3 | 82.6 | 83.0 | 74.4 | 65.6 | 56.4 | 46.0 | 65.0 |
| 1902 | 44.8 | 46.9 | 59.0 | 68.7 | 76.0 | 79.9 | 79.9 | 83.3 | 74.1 | 66.3 | 61.8 | 48.5 | 65.9 |
| 1903 | 46.7 | 48.6 | 58.6 | 64.4 | 68.8 | 71.9 | 78.8 | 80.4 | 72.8 | 62.9 | 54.3 | 47.1 | 63.0 |
| 1904 | 44.3 | 55.3 | 62.7 | 63.1 | 71.5 | 78.3 | 79.0 | 81.2 | 78.1 | 68.4 | 56.3 | 49.5 | 65.7 |
| 1905 | 42.8 | 41.1 | 63.2 | 65.2 | 75.2 | 80.0 | 78.5 | 81.3 | 77.4 | 66.0 | 60.3 | 41.8 | 64.5 |
| 1906 | 48.3 | 48.2 | 54.3 | 66.6 | 71.9 | 78.7 | 79.4 | 79.1 | 77.6 | 60.6 | 57.9 | 53.9 | 64.8 |
| 1907 | 53.7 | 68.1 | 61.0 | 68.6 | 78.5 | 81.8 | 83.2 | 78.4 | 67.2 | 52.9 | 51.0 | 67.8 | |
| 1908 | 49.0 | 50.2 | 65.8 | 68.1 | 72.7 | 79.5 | 80.1 | 79.9 | 75.3 | 62.4 | 58.0 | 53.0 | 66.2 |
| 1909 | 52.2 | 53.8 | 59.2 | 64.4 | 70.6 | 78.6 | 83.6 | 83.8 | 77.7 | 67.5 | 64.1 | 43.3 | 66.6 |
| 1910 | 50.1 | 47.8 | 63.1 | 63.4 | 70.4 | 76.8 | 80.4 | 82.7 | 79.4 | 67.7 | 58.0 | 67.4 | |
| 1911 | 55.5 | 58.5 | 61.7 | 64.8 | 71.0 | 81.9 | 79.6 | 81.1 | 81.9 | 67.1 | 52.3 | 47.3 | 66.9 |
| 1912 | 44.4 | 45.0 | 53.1 | 64.9 | 72.0 | 75.2 | 82.4 | 80.6 | 77.0 | 67.6 | 53.4 | 45.6 | 63.5 |
| 1913 | 48.5 | 47.8 | 54.4 | 63.3 | 70.8 | 76.6 | 82.2 | 82.0 | 72.8 | 63.2 | 63.4 | 48.0 | 64.5 |
| 1914 | 52.4 | 47.0 | 54.7 | 64.6 | 71.9 | 81.6 | 84.6 | 80.3 | 76.2 | 65.4 | 55.2 | 42.0 | 64.7 |
| 1915 | 43.4 | 50.7 | 46.6 | 63.3 | 72.9 | 81.1 | 80.7 | 78.2 | 76.5 | 66.6 | 58.8 | 49.9 | 64.1 |
| 1916 | 52.6 | 50.3 | 60.1 | 62.8 | 71.7 | 79.3 | 82.5 | 82.0 | 76.3 | 66.5 | 55.3 | 50.6 | 65.9 |
| 1917 | 50.9 | 52.1 | 57.5 | 62.9 | 66.0 | 79.2 | 84.3 | 83.7 | 76.2 | 62.6 | 54.3 | 44.8 | 64.6 |
| 1918 | 39.6 | 55.2 | 64.4 | 64.1 | 74.0 | 83.1 | 83.4 | 81.8 | 72.8 | 68.8 | 54.7 | 54.2 | 66.4 |
| 1919 | 47.5 | 51.4 | 60.7 | 66.8 | 70.9 | 77.2 | 82.2 | 82.9 | 78.4 | 73.9 | 60.6 | 48.4 | 66.8 |
| 1920 | 54.5 | 58.8 | 65.6 | 75.9 | 78.6 | 82.5 | 79.9 | 79.6 | 66.1 | 52.2 | 47.2 | 67.4 | |
| 1921 | 52.7 | 52.2 | 63.7 | 60.5 | 70.8 | 79.2 | 81.8 | 83.0 | 81.1 | 66.1 | 63.1 | 55.5 | 67.6 |
| 1922 | 46.1 | 56.3 | 58.1 | 67.5 | 74.2 | 79.9 | 81.8 | 81.9 | 79.6 | 67.0 | 58.3 | 55.8 | 67.3 |
| 1923 | 56.6 | 50.6 | 52.7 | 64.1 | 70.4 | 79.3 | 81.9 | 83.3 | 76.5 | 65.6 | 53.7 | 53.6 | 65.8 |
| 1924 | 43.6 | 48.2 | 53.1 | 64.6 | 68.4 | 80.2 | 82.8 | 86.1 | 75.2 | 67.0 | 57.1 | 45.9 | 64.4 |
| 1925 | 43.9 | 52.7 | 57.6 | 66.4 | 68.7 | 81.3 | 83.7 | 81.8 | 79.6 | 64.5 | 53.7 | 44.3 | 64.9 |
| 1926 | 45.0 | 54.3 | 52.2 | 59.8 | 69.2 | 76.5 | 79.0 | 81.5 | 79.9 | 71.1 | 53.1 | 50.5 | 64.4 |
| 1927 | 50.9 | 56.2 | 56.4 | 68.0 | 74.6 | 77.0 | 80.2 | 82.4 | 77.6 | 68.7 | 64.8 | 45.7 | 66.9 |
| 1928 | 48.0 | 49.6 | 57.8 | 59.1 | 71.2 | 76.7 | 80.8 | 82.2 | 74.2 | 69.7 | 54.2 | 47.4 | 64.3 |
| 1929 | 49.3 | 42.3 | 60.4 | 68.2 | 69.9 | 77.5 | 79.9 | 81.5 | 77.0 | 67.3 | 49.7 | 48.4 | 64.4 |
| 1930 | 41.2 | 55.9 | 53.4 | 68.4 | 71.5 | 77.0 | 82.7 | 82.0 | 76.5 | 63.8 | 55.2 | 45.1 | 64.4 |