# year value 1901 28.59284 1902 28.58982 1903 28.46804 1904 28.36268 1905 28.53006 1906 28.4288 1907 28.30969 1908 28.39253 1909 28.25696 1910 28.41434 1911 28.35524 1912 28.38736 1913 28.35936 1914 28.38863 1915 28.63875 1916 28.33889 1917 28.32046 1918 28.37291 1919 28.40359 1920 28.32877 1921 28.44751 1922 28.29095 1923 28.33412 1924 28.49826 1925 28.42534 1926 28.71444 1927 28.39769 1928 28.50125 1929 28.09048 1930 28.28131 1931 28.67184 1932 28.51114 1933 28.36656 1934 28.47556 1935 28.48309 1936 28.29553 1937 28.62235 1938 28.49407 1939 28.58879 1940 28.65071 1941 28.74257 1942 28.506 1943 28.22367 1944 28.62096 1945 28.54468 1946 28.30925 1947 28.69831 1948 28.41872 1949 28.37992 1950 28.13904 1951 28.59763 1952 28.38347 1953 28.34976 1954 28.10613 1955 28.20434 1956 28.01498 1957 28.43562 1958 28.41321 1959 28.3063 1960 28.47822 1961 28.24763 1962 28.35503 1963 28.55691 1964 28.14444 1965 28.34013 1966 28.60976 1967 28.20192 1968 28.35203 1969 28.59467 1970 28.48209 1971 28.06189 1972 28.59489 1973 28.60322 1974 28.28175 1975 28.187 1976 28.2415 1977 28.56855 1978 28.29979 1979 28.70892 1980 28.61529 1981 28.65714 1982 28.73293 1983 29.02702 1984 28.61529 1985 28.55843 1986 28.71488 1987 29.30109 1988 28.75582 1989 28.71366 1990 28.86337 1991 28.71926 1992 28.67915 1993 28.91366 1994 28.59741 1995 29.1267 1996 28.81977 1997 29.05078 1998 29.34245 1999 28.78481 2000 28.88287 2001 28.91423 2002 29.25398 2003 29.08787 2004 29.03463 2005 29.17489 2006 29.23009 2007 29.03555 2008 29.01817 2009 29.47766 2010 29.32481 2011 29.29181 2012 29.1275 2013 28.98401 2014 29.31117 2015 29.45441 2016 29.32144 2017 29.30251 2018 29.14193