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| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1879 | 75 | 72 | 75 | 70 | 78 | 81 | 79 | 84 | 82 | 81 | 88 | 80 | 79 |
| 1880 | 71 | 75 | 74 | 69 | 78 | 81 | 81 | 77 | 82 | 79 | 83 | 81 | 78 |
| 1881 | 78 | 75 | 77 | 81 | 68 | 79 | 82 | 82 | 78 | 83 | 83 | 78 | 79 |
| 1882 | × | × | × | × | × | × | × | × | × | × | × | × | × |
| 1883 | 80 | 86 | 89 | 78 | 78 | 81 | 84 | 78 | 82 | 85 | 83 | 82 | 82 |
| 1884 | 82 | 79 | 78 | 76 | 75 | 83 | 83 | 83 | 86 | 83 | 80 | 80 | 81 |
| 1885 | 79 | 79 | 76 | 79 | 76 | 76 | 86 | 87 | 85 | 83 | 80 | 77 | 80 |
| 1886 | 75 | 76 | 74 | 68 | 77 | 80 | 83 | 81 | 86 | 80 | 80 | 83 | 79 |
| 1887 | 80 | 82 | 83 | 75 | 76 | 84 | 85 | 85 | 84 | 77 | 82 | 84 | 81 |
| 1888 | 85 | 86 | 81 | 63 | 69 | 83 | 82 | 85 | 81 | 77 | 81 | 84 | 80 |
| 1889 | 83 | 85 | 84 | 76 | 72 | 79 | 83 | 83 | 83 | 80 | 82 | 85 | 81 |
| 1890 | 81 | 87 | 78 | 68 | 65 | 79 | 80 | 80 | 86 | 82 | 81 | 82 | 79 |
| 1891 | 79 | 79 | 73 | 68 | 69 | 75 | 84 | 85 | 82 | 77 | 74 | 75 | 77 |
| 1892 | 79 | 80 | 80 | 71 | 66 | 78 | 79 | 84 | 85 | 80 | 75 | 81 | 78 |
| 1893 | 80 | 82 | 77 | 80 | 75 | 84 | 84 | 84 | 81 | 75 | 79 | 82 | 80 |
| 1894 | 82 | 80 | 79 | 75 | 75 | 78 | 84 | 84 | 84 | 84 | 82 | 83 | 81 |
| 1895 | 87 | 87 | 82 | 74 | 75 | 82 | 82 | 83 | 86 | 81 | 75 | 80 | 81 |
| 1896 | 82 | 85 | 80 | 70 | 65 | 82 | 87 | 87 | 91 | 85 | 85 | 85 | 82 |
| 1897 | 86 | 79 | 75 | 74 | 76 | 84 | 84 | 81 | 84 | 82 | 78 | 81 | 80 |
| 1898 | 84 | 87 | 85 | 69 | 76 | 79 | 89 | 86 | 84 | 74 | 75 | 82 | 81 |
| 1899 | 86 | 84 | 75 | 75 | 72 | 83 | 86 | 87 | 81 | 82 | 81 | 76 | 81 |
| 1900 | 81 | 85 | 85 | 70 | 77 | 78 | 83 | 87 | 83 | 80 | 77 | 78 | 80 |
| 1901 | 83 | 84 | 76 | 67 | 71 | 83 | 85 | 89 | 87 | 79 | 75 | 74 | 79 |
| 1902 | 75 | 77 | 75 | 64 | 76 | 81 | 86 | 82 | 80 | 80 | 77 | 82 | 78 |
| 1903 | 80 | 77 | 72 | 73 | 79 | 81 | 86 | 83 | 83 | 77 | 79 | 76 | 79 |
| 1904 | 79 | 74 | 72 | 71 | 72 | 83 | 87 | 85 | 83 | 78 | 75 | 78 | 78 |
| 1905 | 78 | 81 | 70 | 75 | 71 | 84 | 86 | 82 | 81 | 79 | 73 | 77 | 78 |
| 1906 | 79 | 78 | 74 | 75 | 70 | 81 | 80 | 82 | 77 | 77 | 75 | 79 | 77 |
| 1907 | 76 | 75 | 72 | 76 | 80 | 84 | 84 | 87 | 85 | 79 | 78 | 76 | 79 |
| 1908 | 81 | 76 | 80 | 74 | 84 | 83 | 81 | 84 | 81 | 82 | 75 | 75 | 80 |
| 1909 | 79 | 79 | 73 | 71 | 81 | 81 | 82 | 81 | 81 | 81 | 73 | 79 | 78 |
| 1910 | 74 | 74 | 69 | 74 | 74 | 80 | 81 | 81 | 81 | 79 | 74 | 73 | 76 |
| 1911 | 77 | 75 | 72 | 74 | 66 | 81 | 87 | 87 | 83 | 83 | 80 | 73 | 78 |
| 1912 | 81 | 79 | 75 | 69 | 78 | 82 | 86 | 84 | 84 | 80 | 75 | 79 | 79 |
| 1913 | 81 | 76 | 75 | 69 | 75 | 82 | 88 | 82 | 82 | 77 | 73 | 76 | 78 |
| 1914 | 78 | 78 | 73 | 71 | 65 | 83 | 83 | 85 | 83 | 82 | 78 | 79 | 78 |
| 1915 | 78 | 81 | 76 | 72 | 76 | 80 | 86 | 84 | 81 | 82 | 74 | 75 | 79 |
| 1916 | 76 | 80 | 75 | 70 | 75 | 83 | 81 | 78 | 80 | 77 | 80 | 77 | 78 |
| 1917 | 86 | 81 | 73 | 73 | 69 | 79 | 87 | 80 | 81 | 76 | 80 | 78 | 79 |
| 1918 | 81 | 76 | 72 | 70 | 73 | 81 | 86 | 84 | 82 | 77 | 76 | 73 | 78 |
| 1919 | 79 | 78 | 74 | 75 | 72 | 81 | 81 | 83 | 84 | 79 | 75 | 76 | 78 |
| 1920 | 79 | 79 | 75 | 73 | 83 | 81 | 83 | 85 | 82 | 78 | 77 | 77 | 79 |
| 1921 | 80 | 80 | 73 | 74 | 75 | 78 | 83 | 83 | 81 | 81 | 77 | 76 | 78 |
| 1922 | 80 | 77 | 76 | 73 | 78 | 79 | 86 | 87 | 83 | 82 | 77 | 82 | 80 |
| 1923 | 77 | 79 | 75 | 69 | 77 | 81 | 84 | 84 | 85 | 81 | 75 | 75 | 79 |
| 1924 | 78 | 78 | 79 | 74 | 78 | 84 | 82 | 80 | 81 | 77 | 79 | 79 | 79 |
| 1925 | 84 | 76 | 79 | 72 | 75 | 75 | 81 | 82 | 82 | 80 | 80 | 79 | 79 |
| 1926 | 83 | 81 | 77 | 72 | 76 | 77 | 83 | 81 | 83 | 80 | 79 | 77 | 79 |
| 1927 | 77 | 81 | 72 | 72 | 73 | 74 | 85 | 84 | 81 | 79 | 77 | 78 | 78 |
| 1928 | 82 | 79 | 75 | 68 | 71 | 79 | 80 | 76 | 82 | 82 | 72 | 77 | 77 |
| 1929 | 83 | 81 | 79 | 73 | 78 | 81 | 83 | 84 | 79 | 81 | 79 | 79 | 80 |
| 1930 | 78 | 81 | 79 | 69 | 75 | 82 | 82 | 83 | 83 | 83 | 76 | 76 | 79 |
| 1931 | 81 | 79 | 74 | 72 | 77 | 84 | 85 | 85 | 83 | 80 | 77 | 78 | 80 |
| 1932 | 72 | 74 | 73 | 69 | 74 | 77 | 86 | 87 | 86 | 77 | 77 | 75 | 77 |
| 1933 | 79 | 78 | 74 | 67 | 72 | 78 | 81 | 84 | 83 | 82 | 74 | 75 | 77 |
| 1934 | 77 | 79 | 71 | 64 | 79 | 83 | 81 | 82 | 82 | 76 | 74 | 73 | 77 |
| 1935 | 76 | 73 | 76 | 72 | 73 | 83 | 84 | 84 | 83 | 84 | 75 | 74 | 78 |
| 1936 | 72 | 79 | 72 | 73 | 74 | 79 | 86 | 84 | 83 | 79 | 72 | 74 | 77 |
| 1937 | 70 | 77 | 74 | 69 | 76 | 78 | 85 | 82 | 84 | 82 | 76 | 77 | 78 |
| 1938 | 76 | 73 | 72 | 68 | 76 | 78 | 82 | 80 | 86 | 79 | 81 | 76 | 77 |
| 1939 | 79 | 76 | 76 | 73 | 78 | 78 | 84 | 83 | 85 | 79 | 75 | 73 | 78 |
| 1940 | 76 | 76 | 72 | 71 | 69 | 84 | 83 | 82 | 80 | 76 | 73 | 72 | 76 |
| 1941 | 79 | 76 | 72 | 69 | 70 | 79 | 84 | 85 | 81 | 78 | 73 | 72 | 77 |
| 1942 | 73 | 69 | 73 | 67 | 71 | 81 | 84 | 85 | 82 | 81 | 74 | 79 | 77 |
| 1943 | 74 | 73 | 74 | 71 | 77 | 74 | 82 | 84 | 84 | 80 | 76 | 76 | 77 |
| 1944 | 75 | 78 | 73 | 73 | 68 | 82 | 82 | 80 | 81 | 76 | 76 | 76 | 77 |
| 1945 | 81 | 80 | 80 | 71 | 80 | 79 | 81 | 81 | 81 | 80 | 77 | 78 | 79 |
| 1946 | 79 | 75 | 79 | 68 | 73 | 77 | 80 | 81 | 76 | 80 | 75 | 76 | 77 |
| 1947 | 75 | 75 | 73 | 73 | 72 | 75 | 82 | 84 | 82 | 81 | 74 | 76 | 77 |
| 1948 | 76 | 75 | 77 | 70 | 66 | 77 | 81 | 77 | 79 | 74 | 74 | 76 | 75 |
| 1949 | 73 | 76 | 76 | 75 | 70 | 74 | 78 | 81 | 79 | 77 | 72 | 77 | 76 |
| 1950 | 75 | 71 | 73 | 68 | 70 | 79 | 79 | 80 | 80 | 73 | 78 | 78 | 75 |
| 1951 | 75 | 81 | 75 | 73 | 65 | 77 | 79 | 77 | 74 | 74 | 76 | 77 | 75 |
| 1952 | 73 | 68 | 72 | 73 | 67 | 78 | 84 | 84 | 79 | 77 | 74 | 74 | 75 |
| 1953 | 76 | 70 | 72 | 70 | 73 | 76 | 82 | 77 | 74 | 75 | 76 | 73 | 75 |
| 1954 | 77 | 74 | 68 | 65 | 70 | 73 | 78 | 85 | 78 | 71 | 67 | 72 | 73 |
| 1955 | 78 | 73 | 71 | 63 | 75 | 77 | 76 | 80 | 77 | 78 | 71 | 70 | 74 |
| 1956 | 72 | 69 | 72 | 58 | 57 | 81 | 81 | 82 | 75 | 74 | 72 | 77 | 73 |
| 1957 | 76 | 73 | 72 | 64 | 68 | 78 | 81 | 82 | 78 | 74 | 68 | 75 | 74 |
| 1958 | 76 | 77 | 67 | 62 | 69 | 76 | 80 | 80 | 77 | 74 | 69 | 75 | 74 |
| 1959 | 71 | 71 | 74 | 65 | 67 | 76 | 80 | 77 | 78 | 73 | 74 | 74 | 73 |
| 1960 | 74 | 72 | 71 | 70 | 72 | 79 | 79 | 74 | 74 | 72 | 70 | 71 | 73 |
| 1961 | 70 | 73 | 70 | 64 | 69 | 73 | 79 | 81 | 78 | 68 | 73 | 73 | 73 |
| 1962 | 77 | 69 | 69 | 63 | 70 | 75 | 81 | 82 | 75 | 67 | 67 | 71 | 72 |
| 1963 | 73 | 71 | 71 | 69 | 59 | 78 | 82 | 84 | 74 | 73 | 69 | 74 | 73 |
| 1964 | 73 | 73 | 71 | 66 | 64 | 79 | 80 | 80 | 72 | 71 | 71 | 75 | 73 |
| 1965 | 74 | 73 | 70 | 63 | 65 | 73 | 79 | 76 | 75 | 70 | 69 | 76 | 72 |
| 1966 | 76 | 74 | 75 | 69 | 63 | 73 | 80 | 81 | 75 | 77 | 70 | 77 | 74 |
| 1967 | 73 | 68 | 69 | 66 | 73 | 78 | 81 | 75 | 80 | 69 | 67 | 74 | 73 |
| 1968 | 75 | 73 | 71 | 66 | 72 | 73 | 77 | 80 | 76 | 71 | 72 | 71 | 73 |
| 1969 | 70 | 69 | 66 | 67 | 67 | 79 | 77 | 82 | 72 | 70 | 67 | 70 | 71 |
| 1970 | 73 | 72 | 74 | 64 | 65 | 76 | 75 | 78 | 72 | 70 | 71 | 68 | 72 |
| 1971 | 75 | 73 | 65 | 54 | 69 | 79 | 82 | 82 | 80 | 74 | 71 | 71 | 73 |
| 1972 | 78 | 76 | 71 | 68 | 69 | 81 | 81 | 78 | 79 | 73 | 72 | 71 | 75 |
| 1973 | 78 | 68 | 68 | 61 | 67 | 74 | 78 | 83 | 79 | 74 | 69 | 70 | 72 |
| 1974 | 76 | 75 | 73 | 68 | 68 | 81 | 79 | 85 | 80 | 71 | 64 | 66 | 74 |
| 1975 | 71 | 68 | 74 | 67 | 72 | 76 | 83 | 78 | 73 | 73 | 72 | 73 | 73 |
| 1976 | 70 | 69 | 66 | 64 | 62 | 79 | 72 | 75 | 75 | 71 | 69 | 69 | 70 |
| 1977 | 67 | 67 | 67 | 63 | 71 | 71 | 76 | 76 | 73 | 68 | 68 | 70 | 70 |
| 1978 | 74 | 69 | 68 | 65 | 64 | 75 | 76 | 71 | 67 | 68 | 64 | 68 | 69 |
| 1979 | 69 | 66 | 65 | 64 | 64 | 68 | 73 | 68 | 67 | 70 | 66 | 69 | 67 |
| 1980 | 68 | 74 | 64 | 66 | 64 | 71 | 76 | 78 | 68 | 70 | 68 | 75 | 70 |
| 1981 | 76 | 72 | 74 | 67 | 72 | 79 | 80 | 79 | 72 | 67 | 64 | 71 | 73 |
| 1982 | 73 | 75 | 71 | 67 | 65 | 69 | 73 | 77 | 70 | 67 | 67 | 69 | 70 |
| 1983 | 67 | 68 | 69 | 56 | 66 | 76 | 77 | 79 | 75 | 63 | 70 | 67 | 69 |
| 1984 | 70 | 72 | 67 | 63 | 71 | 72 | 78 | 72 | 70 | 64 | 61 | 68 | 69 |
| 1985 | 71 | 73 | 65 | 66 | 61 | 69 | 80 | 74 | 71 | 68 | 69 | 69 | 70 |
| 1986 | 70 | 67 | 70 | 63 | 67 | 69 | 77 | 75 | 71 | 69 | 64 | 68 | 69 |
| 1987 | 72 | 68 | 68 | 61 | 63 | 70 | 78 | 79 | 71 | 62 | 62 | 69 | 69 |
| 1988 | 70 | 69 | 63 | 65 | 66 | 77 | 74 | 78 | 74 | 65 | 66 | 70 | 70 |
| 1989 | 69 | 63 | 69 | 65 | 65 | 72 | 74 | 71 | 72 | 68 | 67 | 71 | 69 |
| 1990 | 69 | 75 | 61 | 59 | 66 | 75 | 75 | 77 | 73 | 69 | 66 | 71 | 70 |
| 1991 | 75 | 70 | 65 | 61 | 59 | 72 | 76 | 75 | 71 | 69 | 69 | 69 | 69 |
| 1992 | 72 | 69 | 66 | 65 | 66 | 74 | 75 | 77 | 76 | 73 | 67 | 69 | 71 |
| 1993 | 71 | 72 | 61 | 62 | 68 | 77 | 70 | 76 | 72 | 67 | 67 | 68 | 69 |
| 1994 | 68 | 71 | 68 | 57 | 58 | 70 | 74 | 74 | 75 | 67 | 67 | 68 | 68 |
| 1995 | 71 | 70 | 66 | 64 | 68 | 75 | 76 | 78 | 69 | 73 | 66 | 70 | 71 |
| 1996 | 67 | 66 | 62 | 59 | 72 | 71 | 76 | 71 | 71 | 68 | 63 | 67 | 68 |
| 1997 | 68 | 70 | 62 | 59 | 66 | 76 | 73 | 77 | 70 | 68 | 69 | 67 | 69 |
| 1998 | 67 | 67 | 64 | 63 | 64 | 74 | 80 | 79 | 77 | 72 | 72 | 74 | 71 |
| 1999 | 72 | 71 | 67 | 68 | 69 | 71 | 81 | 78 | 74 | 73 | 69 | 75 | 72 |
| 2000 | 79 | 74 | 68 | 73 | 78 | 79 | 83 | 77 | 78 | 64 | 65 | 70 | 74 |
| 2001 | 68 | 63 | 65 | 59 | 66 | 71 | 79 | 74 | 70 | 69 | 66 | 66 | 68 |
| 2002 | 69 | 67 | 61 | 60 | 61 | 70 | 79 | 79 | 68 | 65 | 68 | 61 | 67 |
| 2003 | 67 | 70 | 63 | 63 | 61 | 70 | 71 | 75 | 72 | 68 | 65 | 67 | 68 |
| 2004 | 70 | 69 | 64 | 55 | 63 | 64 | 75 | 70 | 68 | 63 | 66 | 67 | 66 |
| 2005 | 71 | 70 | 66 | 65 | 65 | 69 | 75 | 72 | 69 | 64 | 66 | 65 | 68 |
| 2006 | 70 | 66 | 66 | 66 | 59 | 75 | 71 | 72 | 65 ) | 65 | 69 | 70 | 68 |
| 2007 | 71 | 71 | 67 | 63 | 68 | 71 | 69 | 71 | 72 | 67 | 63 | 65 | 68 |
| 2008 | 66 | 69 | 67 | 58 | 68 | 71 | 75 | 71 | 65 | 66 | 68 | 71 | 68 |
| 2009 | 70 | 70 | 65 | 57 | 61 | 72 | 79 | 73 | 67 | 65 | 69 | 68 | 68 |
| 2010 | 68 | 66 | 65 | 63 | 65 | 72 | 79 | 73 | 65 | 68 | 69 | 70 | 69 |
| 2011 | 66 | 67 | 60 ) | 62 | 71 | 73 | 78 | 72 | 75 | 69 | 68 | 68 | 69 |
| 2012 | 67 | 64 | 66 | 66 | 71 | 72 | 74 | 76 | 74 | 69 | 73 | 69 | 70 |
| 2013 | 67 | 68 | 67 | 64 | 72 | 76 | 75 | 79 | 73 | 71 | 68 | 70 | 71 |
| 2014 | 69 | 69 | 68 | 53 | 65 | 73 | 70 | 73 | 68 | 64 | 66 | 67 | 67 |
| 2015 | 70 | 68 | 67 | 59 | 56 | 73 | 73 | 73 | 71 | 61 | 67 | 68 | 67 |
| 2016 | 65 | 66 | 63 | 58 | 58 | 73 | 75 | 73 | 72 | 58 | 65 | 69 | 66 |
| 2017 | 70 | 66 | 64 | 59 | 62 | 72 | 73 | 70 | 67 | 63 | 66 | 67 | 67 |
| 2018 | 66 | 65 | 62 | 57 | 63 | 69 | 77 ) | 79 | 69 | 72 | 71 | 73 | 69 |
| 2019 | 70 | 65 | 65 | 59 | 61 | 73 | 76 | 78 | 70 | 70 | 66 | 71 | 69 |
| 2020 | 71 | 73 | 67 | 65 | 66 | 73 | 77 | 76 | 76 | 69 | 69 | 65 | 71 |
| 2021 | 70 | 69 | 67 | 60 | 71 | 71 | 74 | 73 | 73 | 73 | 72 | 71 | 70 |
| 2022 | 76 | 71 | 69 | 56 | 63 | 77 | 78 | 75 | 71 | 70 | 69 | 75 | 71 |
| 2023 | 68 | 75 | 65 | 62 | 66 | 73 | 77 | 78 | 76 | 71 | 71 | 67 | 71 |
| 2024 | 77 | 71 | 67 | 65 | 65 | 74 | 77 | 81 | 69 | 72 | 72 | 69 | 72 |
| 2025 | 72 | 72 | 70 | 72 | 65 | 72 | 74 | 77 | 72 | 71 | 71 | 70 ] | 72 ] |
Remarks
The red lines, if any, indicate data inhomogeneity caused by changes in instrumentation, observation methods and/or site location.