Year	mm Hg
1980	138.8
1981	138.7
1982	138.7
1983	138.6
1984	138.5
1985	138.3
1986	138.2
1987	138
1988	137.9
1989	137.7
1990	137.5
1991	137.3
1992	136.9
1993	136.5
1994	136.1
1995	135.9
1996	135.7
1997	135.5
1998	135.5
1999	135.5
2000	135.5
2001	135.6
2002	135.7
2003	135.8
2004	136
2005	136.2
2006	136.4
2007	136.6
2008	136.8