Year	mm per year
1961	537
1962	537
1963	537
1964	537
1965	537
1966	537
1967	537
1968	537
1969	537
1970	537
1971	537
1972	537
1973	537
1974	537
1975	537
1976	537
1977	537
1978	537
1979	537
1980	537
1981	537
1982	537
1983	537
1984	537
1985	537
1986	537
1987	537
1988	537
1989	537
1990	537
1991	537
1992	537
1993	537
1994	537
1995	537
1996	537
1997	537
1998	537
1999	537
2000	537
2001	537
2002	537
2003	537
2004	537
2005	537
2006	537
2007	537
2008	537
2009	537
2010	537
2011	537
2012	537
2013	537
2014	537
2015	537
2016	537
2017	537
2018	537
2019	537
2020	537