Year | Mean | Minimum | Maximum | Year | Mean | Minimum | Maximum | Year | Mean | Minimum | Maximum |
1956 | 245.06 | 136.47 | 351.19 | 1976 | 243.54 | 119.79 | 337.48 | 1996 | 278.82 | 138.24 | 398.96 |
1957 | 256.42 | 158.08 | 413.65 | 1977 | 271.64 | 213.89 | 332.08 | 1997 | 252.24 | 172.36 | 368.83 |
1958 | 243.08 | 102.72 | 334.09 | 1978 | 253.10 | 158.26 | 327.88 | 1998 | 268.57 | 186.87 | 377.88 |
1959 | 234.32 | 165.23 | 374.91 | 1979 | 221.80 | 111.09 | 395.10 | 1999 | 274.88 | 182.86 | 418.80 |
1960 | 271.03 | 170.02 | 393.48 | 1980 | 240.94 | 140.16 | 340.74 | 2000 | 250.17 | 141.92 | 325.56 |
1961 | 272.20 | 216.85 | 370.95 | 1981 | 268.23 | 184.37 | 342.18 | 2001 | 258.87 | 170.82 | 352.81 |
1962 | 268.49 | 174.43 | 371.72 | 1982 | 201.89 | 82.75 | 343.39 | 2002 | 263.42 | 203.82 | 333.61 |
1963 | 235.06 | 145.33 | 317.53 | 1983 | 273.89 | 167.80 | 334.67 | 2003 | 251.50 | 203.82 | 333.61 |
1964 | 273.34 | 157.35 | 382.23 | 1984 | 253.62 | 175.54 | 379.11 | 2004 | 256.18 | 203.82 | 333.61 |
1965 | 219.27 | 79.42 | 388.01 | 1985 | 278.95 | 213.49 | 329.46 | 2005 | 255.32 | 142.15 | 341.30 |
1966 | 268.52 | 138.21 | 453.26 | 1986 | 248.17 | 165.43 | 398.32 | 2006 | 251.98 | 165.74 | 333.61 |
1967 | 264.49 | 172.68 | 363.09 | 1987 | 223.63 | 112.91 | 374.83 | 2007 | 265.49 | 155.02 | 375.12 |
1968 | 232.71 | 118.47 | 350.52 | 1988 | 266.14 | 202.05 | 362.54 | 2008 | 265.05 | 174.25 | 351.64 |
1969 | 262.85 | 191.72 | 345.35 | 1989 | 265.76 | 194.16 | 365.42 | 2009 | 275.56 | 203.82 | 418.85 |
1970 | 293.40 | 188.40 | 419.12 | 1990 | 266.07 | 191.38 | 359.25 | 2010 | 297.76 | 181.87 | 461.59 |
1971 | 280.67 | 175.70 | 334.14 | 1991 | 240.23 | 108.17 | 336.79 | 2011 | 263.47 | 170.02 | 354.29 |
1972 | 212.78 | 65.16 | 374.90 | 1992 | 227.83 | 109.13 | 324.48 | 2012 | 274.63 | 189.96 | 373.84 |
1973 | 297.41 | 177.06 | 435.92 | 1993 | 203.56 | 74.11 | 349.63 | 2013 | 259.63 | 203.82 | 333.61 |
1974 | 243.77 | 120.97 | 369.68 | 1994 | 202.31 | 97.45 | 316.28 | 2014 | 267.24 | 152.87 | 484.31 |
1975 | 284.51 | 195.82 | 383.66 | 1995 | 255.87 | 144.73 | 361.30 | 2015 | 245.64 | 137.87 | 333.61 |
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| 2016 | 260.45 | 203.82 | 333.61 |