Year mm per year 1961 1500 1962 1500 1963 1500 1964 1500 1965 1500 1966 1500 1967 1500 1968 1500 1969 1500 1970 1500 1971 1500 1972 1500 1973 1500 1974 1500 1975 1500 1976 1500 1977 1500 1978 1500 1979 1500 1980 1500 1981 1500 1982 1500 1983 1500 1984 1500 1985 1500 1986 1500 1987 1500 1988 1500 1989 1500 1990 1500 1991 1500 1992 1500 1993 1500 1994 1500 1995 1500 1996 1500 1997 1500 1998 1500 1999 1500 2000 1500 2001 1500 2002 1500 2003 1500 2004 1500 2005 1500 2006 1500 2007 1500 2008 1500 2009 1500 2010 1500 2011 1500 2012 1500 2013 1500 2014 1500 2015 1500 2016 1500 2017 1500 2018 1500 2019 1500 2020 1500