# year value 1901 27.51294 1902 27.52811 1903 27.39046 1904 27.28374 1905 27.46337 1906 27.37278 1907 27.24288 1908 27.33466 1909 27.20634 1910 27.34788 1911 27.2944 1912 27.30255 1913 27.25193 1914 27.3095 1915 27.5627 1916 27.27582 1917 27.25915 1918 27.28667 1919 27.33166 1920 27.26128 1921 27.38242 1922 27.25405 1923 27.28343 1924 27.44282 1925 27.38144 1926 27.63548 1927 27.36252 1928 27.45339 1929 27.08248 1930 27.24784 1931 27.59021 1932 27.46404 1933 27.32669 1934 27.42421 1935 27.40851 1936 27.25812 1937 27.5421 1938 27.40332 1939 27.50232 1940 27.58052 1941 27.67431 1942 27.44391 1943 27.19202 1944 27.55023 1945 27.51013 1946 27.30564 1947 27.63761 1948 27.37952 1949 27.3188 1950 27.0878 1951 27.53325 1952 27.34034 1953 27.37078 1954 27.07631 1955 27.13806 1956 26.97724 1957 27.38896 1958 27.39048 1959 27.27904 1960 27.43374 1961 27.21746 1962 27.29844 1963 27.48715 1964 27.10387 1965 27.2619 1966 27.51405 1967 27.16678 1968 27.25075 1969 27.52525 1970 27.41383 1971 27.00643 1972 27.52286 1973 27.51031 1974 27.19656 1975 27.13599 1976 27.1641 1977 27.50845 1978 27.28185 1979 27.64229 1980 27.58069 1981 27.58785 1982 27.66186 1983 27.95902 1984 27.51596 1985 27.46706 1986 27.62364 1987 28.15749 1988 27.68229 1989 27.62982 1990 27.78691 1991 27.64796 1992 27.60998 1993 27.82511 1994 27.57218 1995 28.01549 1996 27.72899 1997 27.96346 1998 28.26879 1999 27.69479 2000 27.79798 2001 27.83911 2002 28.15062 2003 28.00956 2004 27.92834 2005 28.09792 2006 28.14379 2007 27.94387 2008 27.90307 2009 28.39997 2010 28.22369 2011 28.22145 2012 28.05456 2013 27.94793 2014 28.23961 2015 28.36471 2016 28.26682 2017 28.23534 2018 28.09934