|   | Mean Maximum | Mean Minimum | Overall Means |
| 1931 |   |   |   |
| January | 6.1 | 1.2 | 3.7 |
| February | 6.8 | 1.8 | 4.3 |
| March | 8.4 | 1.1 | 4.8 |
| April | 11.6 | 5.2 | 8.4 |
| May | 16.8 | 8.3 | 12.6 |
| June | 20.3 | 11.8 | 16.1 |
| July | 20.9 | 13.0 | 17.0 |
| August | 19.0 | 11.7 | 15.4 |
| September | 15.6 | 9.8 | 12.7 |
| October | 12.9 | 5.9 | 9.4 |
| November | 10.7 | 6.1 | 8.4 |
| December | 8.3 | 3.2 | 5.8 |
| 1932 |   |   |   |
| January | 6.6 | 3.8 | 5.2 |
| February | 5.1 | 1.2 | 3.2 |
| March | 10.7 | 1.5 | 6.1 |
| April | 10.8 | 3.9 | 7.4 |
| May | 17.1 | 7.8 | 12.5 |
| June | 20.3 | 10.7 | 15.5 |
| July | 20.1 | 17.6 | 18.9 |
| August | 21.9 | 18.7 | 20.3 |
| September | 18.1 | 13.9 | 16.0 |
| October | 13.3 | 9.4 | 11.4 |
| November | 8.9 | 7.1 | 8.0 |
| December | 8.6 | 6.0 | 7.3 |
| 1933 |   |   |   |
| January | 5.0 | 0.5 | 2.8 |
| February | 7.4 | 1.3 | 4.4 |
| March | 12.1 | 3.5 | 7.8 |
| April | 14.7 | 5.9 | 10.3 |
| May | 17.1 | 8.9 | 13.0 |
| June | 22.2 | 11.7 | 17.0 |
| July | 24.7 | 14.7 | 19.7 |
| August | 24.8 | 14.0 | 19.4 |
| September | 20.0 | 11.9 | 16.0 |
| October | 13.5 | 7.2 | 10.4 |
| November | 8.1 | 3.9 | 6.0 |
| December | 3.8 | 0.0 | 1.9 |
| 1934 |   |   |   |
| January | 7.2 | 1.7 | 4.5 |
| February | 7.0 | 0.9 | 4.0 |
| March | 9.0 | 2.3 | 5.7 |
| April | 13.0 | 5.4 | 9.2 |
| May | 18.0 | 7.7 | 12.9 |
| June | 21.5 | 11.6 | 16.6 |
| July | 25.2 | 13.9 | 19.6 |
| August | 21.4 | 12.0 | 16.7 |
| September | 19.8 | 11.2 | 15.5 |
| October | 14.1 | 8.0 | 11.1 |
| November | 9.0 | 4.6 | 6.8 |
| December | 10.3 | 6.4 | 8.4 |
| 1935 |   |   |   |
| January | 7.1 | 3.7 | 5.4 |
| February | 9.0 | 3.9 | 6.5 |
| March | 10.7 | 3.6 | 7.2 |
| April | 12.3 | 5.1 | 8.7 |
| May | 15.8 | 6.5 | 11.2 |
| June | 20.9 | 12.1 | 16.5 |
| July | 24.8 | 14.0 | 19.4 |
| August | 22.1 | 13.3 | 17.7 |
| September | 18.5 | 10.1 | 14.3 |
| October | 13.6 | 6.7 | 10.2 |
| November | 9.8 | 4.8 | 7.3 |
| December | 5.2 | 1.5 | 3.4 |
| 1936 |   |   |   |
| January | 7.1 | 1.9 | 4.5 |
| February | 9.0 | 0.6 | 4.8 |
| March | 10.7 | 4.8 | 7.8 |
| April | 12.3 | 3.9 | 8.1 |
| May | 15.8 | 7.9 | 11.9 |
| June | 20.9 | 11.3 | 16.1 |
| July | 24.8 | 12.7 | 18.8 |
| August | 22.1 | 12.7 | 17.4 |
| September | 18.5 | 12.0 | 15.3 |
| October | 13.6 | 6.6 | 10.1 |
| November | 9.8 | 3.1 | 6.5 |
| December | 5.2 | 2.4 | 3.8 |
| 1937 |   |   |   |
| January | 7.7 | 3.0 | 5.4 |
| February | 8.5 | 3.7 | 6.1 |
| March | 6.9 | 0.8 | 3.9 |
| April | 13.2 | 6.6 | 9.9 |
| May | 17.5 | 9.0 | 13.3 |
| June | 19.7 | 11.3 | 15.5 |
| July | 21.4 | 13.4 | 17.4 |
| August | 23.2 | 13.8 | 18.5 |
| September | 18.3 | 10.0 | 14.2 |
| October | 14.0 | 8.1 | 11.1 |
| November | 8.4 | 3.1 | 5.8 |
| December | 5.8 | 1.5 | 3.7 |
| 1938 |   |   |   |
| January | 8.7 | 3.6 | 6.2 |
| February | 8.0 | 2.9 | 5.5 |
| March | 14.6 | 5.3 | 10.0 |
| April | 12.9 | 4.1 | 8.5 |
| May | 16.3 | 7.6 | 12.0 |
| June | 20.6 | 11.5 | 16.1 |
| July | 21.0 | 12.5 | 16.8 |
| August | 21.6 | 13.2 | 17.4 |
| September | 18.4 | 10.9 | 14.7 |
| October | 13.8 | 7.6 | 10.7 |
| November | 12.3 | 7.1 | 9.7 |
| December | 6.6 | 2.0 | 4.3 |
| 1939 |   |   |   |
| January | 6.3 | 2.3 | 4.3 |
| February | 8.6 | 3.2 | 5.9 |
| March | 9.1 | 3.3 | 6.2 |
| April | 13.8 | 5.5 | 9.7 |
| May | 16.9 | 8.0 | 12.5 |
| June | 20.2 | 10.6 | 15.4 |
| July | 20.7 | 12.8 | 16.8 |
| August | 21.1 | 13.5 | 17.3 |
| September | 18.7 | 11.7 | 15.2 |
| October | 11.3 | 5.3 | 8.3 |
| November | 11.2 | 6.7 | 9.0 |
| December | 5.8 | 1.6 | 3.7 |
| 1940 |   |   |   |
| January | 1.7 | -1.2 | 0.3 |
| February | 4.5 | 0.7 | 2.6 |
| March | 9.7 | 2.8 | 6.3 |
| April | 13.7 | 5.5 | 9.6 |
| May | 19.4 | 7.7 | 13.6 |
| June | 22.5 | 12.4 | 17.5 |
| July | 20.7 | 11.9 | 16.3 |
| August | 21.9 | 12.0 | 17.0 |
| September | 18.8 | 8.7 | 13.8 |
| October | 13.2 | 6.8 | 10.0 |
| November | 9.2 | 4.0 | 6.6 |
| December | 6.4 | 1.1 | 3.8 |
| 1941 |   |   |   |
| January | 2.5 | -0.9 | 0.8 |
| February | 6.3 | 0.8 | 3.6 |
| March | 8.9 | 1.6 | 5.3 |
| April | 10.8 | 3.9 | 7.4 |
| May | 14.1 | 5.6 | 9.9 |
| June | 21.3 | 11.5 | 16.4 |
| July | 24.0 | 14.5 | 19.3 |
| August | 19.3 | 11.7 | 15.5 |
| September | 18.8 | 11.5 | 15.2 |
| October | 13.8 | 7.0 | 10.4 |
| November | 9.4 | 4.2 | 6.8 |
| December | 8.4 | 2.9 | 5.7 |
| 1942 |   |   |   |
| January | 3.0 | -1.6 | 0.7 |
| February | 2.3 | -1.9 | 0.2 |
| March | 8.7 | 1.6 | 5.2 |
| April | 14.6 | 5.4 | 10.0 |
| May | 17.3 | 7.1 | 12.2 |
| June | 21.5 | 10.6 | 16.1 |
| July | 20.7 | 13.0 | 16.9 |
| August | 22.5 | 13.4 | 18.0 |
| September | 18.8 | 11.2 | 15.0 |
| October | 14.3 | 7.7 | 11.0 |
| November | 8.5 | 2.6 | 5.6 |
| December | 9.5 | 5.0 | 7.3 |
| 1943 |   |   |   |
| January | 7.7 | 2.6 | 5.2 |
| February | 6.8 | 3.0 | 4.9 |
| March | 10.2 | 2.2 | 6.2 |
| April | 16.3 | 7.3 | 11.8 |
| May | 19.0 | 6.6 | 12.8 |
| June | 20.6 | 11.4 | 16.0 |
| July | 21.6 | 12.3 | 17.0 |
| August | 23.4 | 13.4 | 18.4 |
| September | 18.2 | 9.9 | 14.1 |
| October | 14.2 | 6.9 | 10.6 |
| November | 8.9 | 3.2 | 6.1 |
| December | 6.8 | 0.9 | 3.9 |
| 1944 |   |   |   |
| January | 9.4 | 3.1 | 6.3 |
| February | 7.0 | -4.0 | 1.5 |
| March | 9.7 | 2.3 | 6.0 |
| April | 15.7 | 6.5 | 11.1 |
| May | 18.5 | 8.1 | 13.3 |
| June | 19.6 | 10.2 | 14.9 |
| July | 22.3 | 13.6 | 18.0 |
| August | 23.3 | 13.8 | 18.6 |
| September | 16.7 | 9.4 | 13.1 |
| October | 12.8 | 7.0 | 9.9 |
| November | 9.5 | 4.0 | 6.8 |
| December | 6.1 | 1.2 | 3.7 |
| 1945 |   |   |   |
| January | 3.1 | -1.4 | 0.9 |
| February | 10.6 | 4.3 | 7.5 |
| March | 13.0 | 4.5 | 8.8 |
| April | 16.2 | 5.3 | 10.8 |
| May | 18.8 | 9.2 | 14.0 |
| June | 20.3 | 13.0 | 16.7 |
| July | 22.6 | 13.5 | 18.1 |
| August | 21.3 | 12.6 | 17.0 |
| September | 17.9 | 13.6 | 15.8 |
| October | 15.6 | 8.2 | 11.9 |
| November | 10.0 | 5.5 | 7.8 |
| December | 7.5 | 2.3 | 4.9 |
| 1946 |   |   |   |
| January | 6.2 | 0.7 | 3.5 |
| February | 9.3 | 3.3 | 6.3 |
| March | 9.3 | 2.0 | 5.6 |
| April | 16.3 | 5.7 | 11.0 |
| May | 16.1 | 6.3 | 11.2 |
| June | 18.9 | 9.9 | 14.4 |
| July | 23.0 | 13.4 | 18.2 |
| August | 20.0 | 11.4 | 15.7 |
| September | 18.1 | 11.0 | 14.5 |
| October | 13.4 | 7.1 | 10.2 |
| November | 11.3 | 5.4 | 8.4 |
| December | 5.6 | -0.1 | 2.7 |
| 1947 |   |   |   |
| January | 4.4 | -0.9 | 1.8 |
| February | -0.1 | -4.4 | -2.2 |
| March | 6.0 | 1.1 | 3.6 |
| April | 13.6 | 4.8 | 9.2 |
| May | 19.8 | 9.2 | 14.5 |
| June | 22.2 | 11.8 | 17.0 |
| July | 22.8 | 13.6 | 18.2 |
| August | 25.8 | 13.5 | 19.6 |
| September | 20.0 | 10.7 | 15.3 |
| October | 14.8 | 6.3 | 10.6 |
| November | 10.2 | 3.4 | 6.8 |
| December | 8.0 | 5.5 | 6.7 |
| 1948 |   |   |   |
| January | 8.4 | 2.6 | 5.5 |
| February | 4.3 | 4.8 | 4.6 |
| March | 13.0 | 3.7 | 8.3 |
| April | 15.0 | 4.6 | 9.8 |
| May | 18.1 | 6.9 | 12.5 |
| June | 19.0 | 9.8 | 14.4 |
| July | 21.2 | 12.0 | 16.6 |
| August | 19.9 | 11.0 | 15.5 |
| September | 19.0 | 10.1 | 14.5 |
| October | 13.5 | 4.5 | 9.0 |
| November | 9.9 | 1.3 | 5.6 |
| December | 9.1 | 2.2 | 5.6 |
| 1949 |   |   |   |
| January | 8.4 | 2.3 | 5.3 |
| February | 9.5 | 1.1 | 5.3 |
| March | 8.7 | 0.9 | 4.8 |
| April | 15.8 | 5.8 | 10.8 |
| May | 17.1 | 7.2 | 12.1 |
| June | 22.1 | 11.3 | 16.7 |
| July | 24.7 | 13.8 | 19.3 |
| August | 23.9 | 13.5 | 18.7 |
| September | 21.8 | 12.8 | 17.3 |
| October | 15.6 | 8.3 | 12.0 |
| November | 9.7 | 4.3 | 7.0 |
| December | 8.9 | 3.2 | 6.0 |
| 1950 |   |   |   |
| January | 6.9 | 2.5 | 4.7 |
| February | 9.9 | 3.3 | 6.6 |
| March | 11.8 | 4.5 | 8.2 |
| April | 12.4 | 4.2 | 8.3 |
| May | 16.3 | 7.7 | 12.0 |
| June | 16.3 | 7.7 | 12.0 |
| July | 22.1 | 13.2 | 17.6 |
| August | 21.9 | 12.6 | 17.2 |
| September | 17.6 | 10.1 | 13.8 |
| October | 13.7 | 6.6 | 10.2 |
| November | 9.1 | 3.9 | 6.5 |
| December | 3.6 | -1.1 | 1.2 |
| 1951 |   |   |   |
| January | 7.2 | 2.2 | 4.7 |
| February | 6.7 | 1.7 | 4.2 |
| March | 7.7 | 1.7 | 4.7 |
| April | 11.8 | 3.2 | 7.5 |
| May | 15.6 | 7.6 | 11.6 |
| June | 20.8 | 10.7 | 15.7 |
| July | 22.9 | 12.6 | 17.8 |
| August | 20.8 | 11.7 | 16.2 |
| September | 19.1 | 11.3 | 15.2 |
| October | 13.6 | 5.6 | 9.6 |
| November | 11.3 | 6.6 | 8.9 |
| December | 9.0 | 3.5 | 6.3 |
| 1952 |   |   |   |
| January | 5.8 | 0.5 | 3.2 |
| February | 6.2 | 0.2 | 3.2 |
| March | 9.6 | 3.8 | 6.7 |
| April | 15.3 | 6.5 | 10.9 |
| May | 18.0 | 10.1 | 14.0 |
| June | 20.9 | 11.0 | 16.0 |
| July | 23.2 | 13.1 | 18.1 |
| August | 23.1 | 12.6 | 17.8 |
| September | 15.9 | 7.4 | 11.6 |
| October | 12.8 | 5.5 | 9.1 |
| November | 6.9 | 0.9 | 3.9 |
| December | 6.0 | 0.3 | 3.1 |
| 1953 |   |   |   |
| January | 5.7 | 0.8 | 3.2 |
| February | 7.8 | 1.3 | 4.5 |
| March | 10.8 | 0.3 | 5.6 |
| April | 12.5 | 2.9 | 7.7 |
| May | 18.3 | 7.6 | 12.9 |
| June | 18.2 | 10.2 | 14.2 |
| July | 20.7 | 10.7 | 15.7 |
| August | 21.8 | 11.4 | 16.6 |
| September | 19.0 | 8.8 | 13.9 |
| October | 14.3 | 5.9 | 10.1 |
| November | 11.0 | 5.6 | 8.3 |
| December | 9.1 | 4.9 | 7.0 |
| 1954 |   |   |   |
| January | 5.6 | 3.0 | 4.3 |
| February | 6.1 | 2.9 | 4.5 |
| March | 9.7 | 6.1 | 7.9 |
| April | 12.7 | 7.6 | 10.2 |
| May | 16.4 | 11.8 | 14.1 |
| June | 17.8 | 14.1 | 15.9 |
| July | 18.5 | 14.9 | 16.7 |
| August | 19.2 | 15.2 | 17.2 |
| September | 17.2 | 13.0 | 15.1 |
| October | 15.6 | 12.1 | 13.8 |
| November | 13.3 | 8.3 | 10.8 |
| December | 9.3 | 6.6 | 7.9 |
| 1955 |   |   |   |
| January | 5.0 | 2.3 | 3.6 |
| February | 4.7 | 1.8 | 3.2 |
| March | 14.4 | 3.7 | 9.1 |
| April | 14.4 | 9.6 | 12.0 |
| May | 14.4 | 9.9 | 12.2 |
| June | 19.2 | 14.4 | 16.8 |
| July | 24.1 | 18.3 | 21.2 |
| August | 23.9 | 18.5 | 21.2 |
| September | 19.3 | 14.5 | 16.9 |
| October | 13.9 | 9.4 | 11.7 |
| November | 10.0 | 6.9 | 8.5 |
| December | 9.1 | 5.8 | 7.4 |
| 1956 |   |   |   |
| January | 6.3 | 3.3 | 4.8 |
| February | 2.8 | 0.1 | 1.4 |
| March | 11.1 | 6.6 | 8.9 |
| April | 11.9 | 7.0 | 9.4 |
| May | 18.5 | 12.7 | 15.6 |
| June | 17.6 | 13.3 | 15.4 |
| July | 20.8 | 16.4 | 18.6 |
| August | 18.1 | 14.0 | 16.0 |
| September | 18.7 | 14.7 | 16.7 |
| October | 18.3 | 9.4 | 13.8 |
| November | 8.9 | 6.0 | 7.4 |
| December | 7.8 | 5.6 | 6.7 |
| 1957 |   |   |   |
| January | 8.5 | 5.6 | 7.0 |
| February | 8.9 | 5.6 | 7.2 |
| March | 13.3 | 9.4 | 11.3 |
| April | 14.0 | 9.2 | 11.6 |
| May | 15.8 | 10.6 | 13.2 |
| June | 22.2 | 16.2 | 19.2 |
| July | 21.0 | 16.9 | 18.9 |
| August | 20.1 | 15.8 | 18.0 |
| September | 16.5 | 12.8 | 14.6 |
| October | 14.8 | 10.8 | 12.8 |
| November | 9.0 | 6.4 | 7.7 |
| December | 7.2 | 4.6 | 5.9 |
| 1958 |   |   |   |
| January |   |   |   |
| February |   |   |   |
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| 1959 |   |   |   |
| January |   |   |   |
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| 1960 |   |   |   |
| January |   |   |   |
| February |   |   |   |
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| May |   |   |   |
| June |   |   |   |
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| 1961 |   |   |   |
| January |   |   |   |
| February |   |   |   |
| March |   |   |   |
| April |   |   |   |
| May |   |   |   |
| June |   |   |   |
| July |   |   |   |
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| November |   |   |   |
| December |   |   |   |
| 1962 |   |   |   |
| January | 7.4 | 1.5 | 4.5 |
| February | 7.0 | 1.5 | 4.3 |
| March | 6.7 | -1.3 | 2.7 |
| April | 12.0 | 4.0 | 8.0 |
| May | 14.4 | 6.5 | 10.5 |
| June | 20.1 | 9.0 | 14.6 |
| July | 19.6 | 11.0 | 15.3 |
| August | 19.3 | 12.6 | 16.0 |
| September | 17.6 | 8.7 | 13.2 |
| October | 14.6 | 6.3 | 10.4 |
| November | 7.9 | 2.4 | 5.1 |
| December | 4.2 | -2.0 | 1.1 |
| 1963 |   |   |   |
| January | 0.6 | -4.9 | -2.1 |
| February | 2.5 | -3.6 | -0.6 |
| March | 9.5 | 2.7 | 6.1 |
| April | 13.2 | 4.2 | 8.7 |
| May | 16.0 | 6.3 | 11.1 |
| June | 20.8 | 10.7 | 15.8 |
| July | 21.1 | 10.6 | 15.8 |
| August | 18.6 | 10.8 | 14.7 |
| September | 17.7 | 8.3 | 13.0 |
| October | 14.9 | 7.1 | 11.0 |
| November | 11.1 | 4.6 | 7.9 |
| December | 5.3 | -0.1 | 2.6 |
| 1964 |   |   |   |
| January | 5.4 | 0.6 | 3.0 |
| February | 7.5 | 3.2 | 5.3 |
| March | 7.4 | 1.6 | 4.5 |
| April | 13.2 | 5.1 | 9.1 |
| May | 19.3 | 8.7 | 14.0 |
| June | 18.7 | 10.3 | 14.5 |
| July | 21.9 | 12.2 | 17.1 |
| August | 21.7 | 10.6 | 16.2 |
| September | 20.6 | 8.6 | 14.6 |
| October | 13.2 | 4.1 | 8.6 |
| November | 10.9 | 1.4 | 6.1 |
| December | 6.6 | -2.4 | 2.1 |
| 1965 |   |   |   |
| January | 6.7 | 0.6 | 3.6 |
| February | 6.6 | 0.6 | 3.6 |
| March | 10.7 | 0.6 | 5.6 |
| April | 13.3 | 3.9 | 8.6 |
| May | 15.7 | 7.4 | 11.6 |
| June | 19.7 | 10.1 | 14.9 |
| July | 18.4 | 10.2 | 14.3 |
| August | 20.2 | 10.2 | 15.2 |
| September | 16.3 | 8.4 | 12.4 |
| October | 14.9 | 6.1 | 10.5 |
| November | 7.5 | 1.3 | 4.4 |
| December | 8.3 | 1.7 | 5.0 |
| 1966 |   |   |   |
| January | 8.7 | 0.2 | 4.5 |
| February | 8.4 | 3.1 | 5.7 |
| March | 10.5 | 2.9 | 6.7 |
| April | 11.2 | 4.3 | 7.8 |
| May | 16.4 | 6.5 | 11.4 |
| June | 20.9 | 11.6 | 16.2 |
| July | 19.7 | 11.1 | 15.4 |
| August | 19.4 | 10.7 | 15.1 |
| September |   |   |   |
| October | 13.8 | 6.5 | 10.2 |
| November | 7.2 | 1.6 | 4.4 |
| December |   |   |   |
| 1967 |   |   |   |
| January |   |   |   |
| February |   |   |   |
| March | 10.8 | 4.3 | 7.6 |
| April | 11.4 | 4.9 | 8.2 |
| May | 14.6 | 7.5 | 11.0 |
| June | 19.1 | 10.6 | 14.9 |
| July | 22.5 | 14.9 | 18.7 |
| August | 19.9 | 12.8 | 16.3 |
| September | 17.1 | 10.8 | 13.9 |
| October | 12.6 | 8.4 | 10.5 |
| November | 8.5 | 3.1 | 5.8 |
| December | 6.2 | 2.8 | 4.5 |
| 1968 |   |   |   |
| January | 6.9 | 2.2 | 4.6 |
| February | 4.3 | -0.1 | 2.1 |
| March | 10.4 | 4.2 | 7.3 |
| April | 13.0 | 1.7 | 7.4 |
| May | 14.4 | 7.1 | 10.8 |
| June | 18.6 | 11.7 | 15.2 |
| July | 18.4 | 11.8 | 15.1 |
| August | 19.1 | 14.5 | 16.8 |
| September | 17.3 | 11.2 | 14.3 |
| October | 15.2 | 10.7 | 12.9 |
| November | 8.4 | 4.3 | 6.3 |
| December | 4.6 | 0.9 | 2.7 |
| 1969 |   |   |   |
| January | 8.2 | 3.9 | 6.1 |
| February | 3.4 | -1.3 | 1.1 |
| March | 6.4 | 0.9 | 3.7 |
| April | 12.6 | 3.4 | 8.0 |
| May | 15.6 | 8.9 | 12.3 |
| June | 18.9 | 10.5 | 14.7 |
| July | 21.2 | 14.6 | 17.9 |
| August | 19.9 | 13.6 | 16.8 |
| September | 17.4 | 11.1 | 14.3 |
| October | 16.5 | 10.3 | 13.4 |
| November | 8.7 | 2.3 | 5.5 |
| December | 5.7 | 1.6 | 3.6 |
| 1970 |   |   |   |
| January | 6.2 | 1.9 | 4.1 |
| February | 6.0 | 0.4 | 3.2 |
| March | 6.6 | 0.3 | 3.4 |
| April | 10.4 | 4.2 | 7.3 |
| May | 18.5 | 9.2 | 13.8 |
| June | 22.2 | 12.2 | 17.2 |
| July | 20.1 | 13.3 | 16.7 |
| August | 20.9 | 13.0 | 16.9 |
| September | 20.5 | 11.9 | 16.2 |
| October | 15.1 | 8.1 | 11.6 |
| November | 11.4 | 5.8 | 8.6 |
| December |   |   |   |
| 1971 |   |   |   |
| January | 7.2 | 2.8 | 5.0 |
| February | 7.8 | 2.2 | 5.0 |
| March | 8.9 | 3.3 | 6.1 |
| April | 12.2 | 4.8 | 8.5 |
| May | 17.2 | 8.9 | 13.1 |
| June | 16.1 | 10.0 | 13.1 |
| July | 22.8 | 14.4 | 18.6 |
| August | 21.1 | 13.9 | 17.5 |
| September | 19.1 | 10.6 | 14.9 |
| October | 14.4 | 7.9 | 11.2 |
| November | 10.0 | 3.3 | 6.7 |
| December | 8.9 | 5.0 | 7.0 |
| 1972 |   |   |   |
| January | 5.9 | 2.0 | 4.0 |
| February | 6.7 | 2.8 | 4.8 |
| March | 11.7 | 3.8 | 7.8 |
| April | 12.8 | 6.1 | 9.5 |
| May | 15.0 | 7.8 | 11.4 |
| June | 17.2 | 8.9 | 13.1 |
| July | 21.1 | 12.2 | 16.7 |
| August | 20.6 | 12.2 | 16.4 |
| September | 16.7 | 9.4 | 13.1 |
| October | 15.0 | 7.2 | 11.1 |
| November | 10.0 | 3.9 | 7.0 |
| December | 8.3 | 3.3 | 5.8 |
| 1973 |   |   |   |
| January | 7.2 | 2.8 | 5.0 |
| February | 8.3 | -3.3 | 2.5 |
| March | 11.1 | 2.8 | 7.0 |
| April | 12.8 | 4.5 | 8.7 |
| May | 16.7 | 8.3 | 12.5 |
| June | 21.4 | 12.2 | 16.8 |
| July | 21.1 | 11.7 | 16.4 |
| August | 22.2 | 11.9 | 17.1 |
| September | 21.1 | 11.1 | 16.1 |
| October | 13.3 | 6.1 | 9.7 |
| November | 10.0 | 2.8 | 6.4 |
| December | 5.0 | 0.0 | 2.5 |
| 1974 |   |   |   |
| January | 7.8 | 1.7 | 4.8 |
| February | 8.2 | 3.2 | 5.7 |
| March | 8.8 | 2.1 | 5.5 |
| April | 12.2 | 4.1 | 8.2 |
| May | 16.1 | 7.2 | 11.7 |
| June | 16.2 | 8.5 | 12.4 |
| July | 20.0 | 11.7 | 15.9 |
| August | 21.1 | 11.7 | 16.4 |
| September | 16.7 | 8.9 | 12.8 |
| October | 11.1 | 5.6 | 8.4 |
| November | 10.0 | 5.0 | 7.5 |
| December | 11.1 | 6.1 | 8.6 |
| 1975 |   |   |   |
| January |   |   |   |
| February |   |   |   |
| March |   |   |   |
| April |   |   |   |
| May |   |   |   |
| June |   |   |   |
| July |   |   |   |
| August |   |   |   |
| September |   |   |   |
| October |   |   |   |
| November |   |   |   |
| December |   |   |   |
| 1976 |   |   |   |
| January |   |   |   |
| February |   |   |   |
| March |   |   |   |
| April |   |   |   |
| May |   |   |   |
| June |   |   |   |
| July |   |   |   |
| August |   |   |   |
| September |   |   |   |
| October |   |   |   |
| November |   |   |   |
| December |   |   |   |
| 1977 |   |   |   |
| January |   |   |   |
| February |   |   |   |
| March |   |   |   |
| April |   |   |   |
| May |   |   |   |
| June |   |   |   |
| July |   |   |   |
| August |   |   |   |
| September |   |   |   |
| October |   |   |   |
| November |   |   |   |
| December |   |   |   |
| 1978 |   |   |   |
| January | 5.2 | -0.8 | 2.2 |
| February | 5.5 | -1.5 | 2.0 |
| March | 10.8 | 2.2 | 6.5 |
| April | 9.0 | 2.2 | 5.6 |
| May | 15.5 | 6.0 | 10.8 |
| June | 17.8 | 8.8 | 13.3 |
| July | 18.5 | 10.5 | 14.5 |
| August | 18.5 | 10.5 | 14.5 |
| September | 18.0 | 9.8 | 13.9 |
| October | 15.2 | 7.8 | 11.5 |
| November | 11.1 | 4.3 | 7.7 |
| December | 5.8 | 0.5 | 3.2 |
| 1979 |   |   |   |
| January | 2.2 | -2.2 | 0.0 |
| February | 3.9 | -0.6 | 1.7 |
| March | 8.9 | 1.7 | 5.3 |
| April | 12.2 | 4.4 | 8.3 |
| May | 16.1 | 7.8 | 12.0 |
| June | 19.0 | 11.1 | 15.1 |
| July | 22.2 | 13.3 | 17.8 |
| August | 19.4 | 11.1 | 15.3 |
| September | 18.9 | 9.4 | 14.2 |
| October | 15.6 | 7.2 | 11.4 |
| November | 10.6 | 3.9 | 7.3 |
| December | 9.4 | 4.4 | 6.9 |
| 1980 |   |   |   |
| January | 5.9 | 0.8 | 3.4 |
| February | 9.8 | 3.6 | 6.7 |
| March |   |   |   |
| April |   |   |   |
| May |   |   |   |
| June | 19.0 | 10.9 | 15.0 |
| July | 19.9 | 11.8 | 15.9 |
| August | 21.7 | 12.6 | 17.2 |
| September | 19.8 | 12.0 | 15.9 |
| October | 14.0 | 5.7 | 9.9 |
| November | 9.3 | 4.6 | 7.0 |
| December | 9.2 | 3.2 | 6.2 |
| 1981 |   |   |   |
| January | 8.2 | 2.5 | 5.4 |
| February | 6.7 | 1.2 | 4.0 |
| March | 11.7 | 6.2 | 9.0 |
| April | 12.5 | 4.8 | 8.7 |
| May | 17.3 | 9.0 | 13.2 |
| June | 19.3 | 11.4 | 15.4 |
| July | 21.0 | 12.6 | 16.8 |
| August | 22.4 | 14.1 | 18.3 |
| September | 19.2 | 11.6 | 15.4 |
| October | 12.9 | 5.3 | 9.1 |
| November | 11.6 | 4.6 | 8.1 |
| December | 4.1 | -2.1 | 1.0 |
| 1982 |   |   |   |
| January | 5.8 | -0.2 | 2.8 |
| February | 8.2 | 3.1 | 5.7 |
| March | 10.8 | 2.9 | 6.9 |
| April | 14.4 | 5.2 | 9.8 |
| May | 18.2 | 7.9 | 13.1 |
| June | 19.2 | 12.3 | 15.8 |
| July | 22.1 | 13.4 | 17.8 |
| August | 21.7 | 13.2 | 17.5 |
| September | 20.7 | 10.5 | 15.6 |
| October | 14.6 | 7.8 | 11.2 |
| November | 11.1 | 5.9 | 8.5 |
| December | 7.9 | 1.4 | 4.7 |
| 1983 |   |   |   |
| January | 9.7 | 4.4 | 7.1 |
| February | 4.7 | -0.6 | 2.1 |
| March |   |   |   |
| April | 10.9 | 1.7 | 6.3 |
| May | 13.6 | 7.9 | 10.8 |
| June | 19.9 | 10.9 | 15.4 |
| July | 25.4 | 15.3 | 20.4 |
| August | 24.1 |   | 24.1 |
| September | 18.6 | 11.3 | 15.0 |
| October | 14.9 | 8.1 | 11.5 |
| November | 11.3 | 5.7 | 8.5 |
| December | 8.5 | 4.8 | 6.7 |
| 1984 |   |   |   |
| January | 7.0 | 0.1 | 3.6 |
| February | 7.2 | -0.9 | 3.2 |
| March | 7.7 | 0.9 | 4.3 |
| April | 13.9 | 1.8 | 7.9 |
| May | 15.6 | 5.1 | 10.4 |
| June | 20.9 | 8.7 | 14.8 |
| July | 23.8 | 12.8 | 18.3 |
| August | 25.1 | 14.4 | 19.8 |
| September | 15.8 | 10.7 | 13.3 |
| October | 14.7 | 7.9 | 11.3 |
| November |   |   |   |
| December | 10.9 | 5.8 | 8.4 |
| 1985 |   |   |   |
| January | 4.1 | -0.8 | 1.7 |
| February | 7.2 | 0.0 | 3.6 |
| March | 9.3 | 4.1 | 6.7 |
| April | 13.2 | 5.5 | 9.4 |
| May | 15.6 | 7.9 | 11.8 |
| June | 17.3 | 10.1 | 13.7 |
| July | 22.4 |   | 22.4 |
| August | 19.8 | 11.7 | 15.8 |
| September | 20.4 |   | 20.4 |
| October | 15.4 | 9.1 | 12.3 |
| November | 8.4 | 2.9 | 5.7 |
| December | 10.5 | 5.3 | 7.9 |
| 1986 |   |   |   |
| January | 9.7 | 1.8 | 5.8 |
| February | 1.0 | -4.1 | -1.6 |
| March | 8.7 | 3.2 | 6.0 |
| April | 10.6 | 3.8 | 7.2 |
| May |   |   |   |
| June | 20.8 | 11.1 | 16.0 |
| July | 21.9 |   | 21.9 |
| August | 17.9 | 10.1 | 14.0 |
| September | 16.5 |   | 16.5 |
| October | 17.4 | 11.9 | 14.7 |
| November | 11.8 | 5.2 | 8.5 |
| December | 10.2 | 4.4 | 7.3 |
| 1987 |   |   |   |
| January | 3.8 | -1.2 | 1.3 |
| February | 7.8 | 1.1 | 4.5 |
| March | 7.3 | 1.7 | 4.5 |
| April | 15.4 | 6.3 | 10.9 |
| May | 15.9 | 4.4 | 10.2 |
| June | 17.6 | 10.2 | 13.9 |
| July | 20.7 | 12.7 | 16.7 |
| August | 20.6 | 13.3 | 17.0 |
| September | 18.3 | 8.9 | 13.6 |
| October | 14.2 | 6.9 | 10.6 |
| November | 9.0 | 5.1 | 7.1 |
| December | 9.1 | 3.1 | 6.1 |
| 1988 |   |   |   |
| January | 8.2 | 3.2 | 5.7 |
| February | 7.4 | 3.1 | 5.3 |
| March | 11.1 | 5.0 | 8.1 |
| April | 10.9 | 5.9 | 8.4 |
| May | 17.3 | 7.7 | 12.5 |
| June |   |   |   |
| July | 18.6 | 11.9 | 15.3 |
| August | 21.5 | 11.7 | 16.6 |
| September | 17.7 | 9.4 | 13.6 |
| October | 14.6 | 8.8 | 11.7 |
| November | 10.2 | 2.7 | 6.5 |
| December | 9.8 | 4.9 | 7.4 |
| 1989 |   |   |   |
| January | 9.7 | 4.2 | 7.0 |
| February | 9.2 | 4.6 | 6.9 |
| March | 12.2 | 3.2 | 7.7 |
| April | 8.8 | 4.1 | 6.5 |
| May | 20.3 | 9.1 | 14.7 |
| June | 21.9 | 12.1 | 17.0 |
| July | 24.9 | 14.2 | 19.6 |
| August | 20.3 | 10.5 | 15.4 |
| September | 20.9 | 10.7 | 15.8 |
| October | 16.6 | 8.3 | 12.5 |
| November | 5.3 | -1.9 | 1.7 |
| December | 8.9 | 8.9 | 8.9 |
| 1990 |   |   |   |
| January | 9.7 | 4.9 | 7.3 |
| February | 8.6 | 7.8 | 8.2 |
| March | 13.1 | 4.6 | 8.9 |
| April | 14.4 | 3.3 | 8.9 |
| May | 20.7 | 8.3 | 14.5 |
| June | 19.7 | 10.0 | 14.9 |
| July | 23.7 | 12.2 | 18.0 |
| August | 25.3 | 14.1 | 19.7 |
| September | 18.9 | 8.3 | 13.6 |
| October | 15.7 | 7.2 | 11.5 |
| November | 9.9 | 4.6 | 7.3 |
| December | 6.3 | 0.7 | 3.5 |
| 1991 |   |   |   |
| January | 7.3 | 0.4 | 3.9 |
| February | 4.3 | -1.9 | 1.2 |
| March | 11.3 | 4.1 | 7.7 |
| April | 12.0 | 3.4 | 7.7 |
| May | 14.6 | 6.5 | 10.6 |
| June | 16.3 | 7.8 | 12.1 |
| July | 22.1 | 12.6 | 17.4 |
| August | 22.6 | 12.1 | 17.4 |
| September | 19.9 | 9.7 | 14.8 |
| October | 13.0 | 6.3 | 9.7 |
| November | 9.7 | 3.4 | 6.6 |
| December | 6.9 | 0.9 | 3.9 |
| 1992 |   |   |   |
| January |   |   |   |
| February | 7.5 | 2.5 | 5.0 |
| March | 10.4 | 7.1 | 8.8 |
| April | 12.2 | 3.0 | 7.6 |
| May | 19.3 | 8.2 | 13.8 |
| June | 21.4 | 10.5 | 16.0 |
| July | 21.0 | 10.9 | 16.0 |
| August | 21.2 | 9.9 | 15.6 |
| September | 17.9 | 8.3 | 13.1 |
| October | 11.3 | 4.8 | 8.1 |
| November | 9.5 | 3.6 | 6.6 |
| December | 8.2 | 2.5 | 5.4 |
| 1993 |   |   |   |
| January | 9.2 | 2.1 | 5.7 |
| February | 6.9 | 6.9 | 6.9 |
| March | 11.3 | 10.0 | 10.7 |
| April | 14.5 | 9.3 | 11.9 |
| May | 17.3 | 11.4 | 14.4 |
| June | 21.3 | 9.3 | 15.3 |
| July | 20.3 | 7.7 | 14.0 |
| August | 20.7 | 3.7 | 12.2 |
| September | 17.1 | 0.8 | 9.0 |
| October | 12.1 | 1.8 | 7.0 |
| November | 12.9 | 2.6 | 7.8 |
| December | 8.4 | 2.3 | 5.4 |
| 1994 |   |   |   |
| January | 8.6 | 3.0 | 5.8 |
| February | 6.6 | 6.8 | 6.7 |
| March | 11.9 | 7.2 | 9.6 |
| April | 12.4 | 11.7 | 12.1 |
| May | 15.7 | 13.6 | 14.7 |
| June | 21.4 | 10.0 | 15.7 |
| July | 26.7 | 7.7 | 17.2 |
| August | 22.1 | 7.0 | 14.6 |
| September | 17.0 | 5.8 | 11.4 |
| October | 14.4 | 5.9 | 10.2 |
| November | 22.1 | 7.0 | 14.6 |
| December | 9.6 | 1.4 | 5.5 |
|   |   |   |   |
| 1995 |   |   |   |
| January | 7.7 | 3.7 | 5.7 |
| February | 9.6 | 4.8 | 7.2 |
| March | 10.4 | 7.6 | 9.0 |
| April | 15.1 | 11.3 | 13.2 |
| May | 18.4 | 13.5 | 16.0 |
| June | 19.6 | 11.9 | 15.8 |
| July | 26.7 | 9.7 | 18.2 |
| August | 26.9 | 5.4 | 16.2 |
| September | 18.3 | 2.4 | 10.4 |
| October | 17.3 | 1.6 | 9.5 |
| November | 10.6 | 1.6 | 6.1 |
| December | 4.3 | -0.5 | 1.9 |
|   |   |   |   |
| 1996 |   |   |   |
| January | 5.6 | 1.8 | 3.7 |
| February | 5.5 | 4.4 | 5.0 |
| March | 7.8 | 7.9 | 7.9 |
| April | 14.1 | 10.1 | 12.1 |
| May | 14.5 | 11.4 | 13.0 |
| June | 25.9 | 11.4 | 18.7 |
| July | 23.5 | 7.5 | 15.5 |
| August | 23.2 | 6.4 | 14.8 |
| September | 18.5 | 0.1 | 9.3 |
| October | 15.9 | 7.1 | 11.5 |
| November | 9.3 | 2.1 | 5.7 |
| December | 4.8 | 4.3 | 4.6 |
|   |   |   |   |
| 1997 |   |   |   |
| January | 4.9 | 3.6 | 4.3 |
| February | 10.3 | 5.5 | 7.9 |
| March | 13.5 | 8.4 | 11.0 |
| April | 14.7 | 10.8 | 12.8 |
| May | 18.7 | 13.2 | 16.0 |
| June | 20.3 | 12.3 | 16.3 |
| July | 23.1 | 9.3 | 16.2 |
| August | 26.4 | 3.0 | 14.7 |
| September | 19.8 | 4.5 | 12.2 |
| October | 15.0 | 3.5 | 9.3 |
| November | 11.4 | 3.5 | 7.5 |
| December | 8.0 | 3.7 | 5.9 |
The record is incomplete between 1958 and 1961, 1975 and 1977