From an early Bruce Jones Encyclopedia of Formula 1, Races where it rained immediately before or during the race:
1950 Indianapolis
1951 Switzerland
1952 Belgium
1952 France
1952 Netherlands
1953 Britain
1954 Argentina
1954 France
1954 Britain
1954 Switzerland
1955 Netherlands
1956 Belgium
1956 Italy
1957 -
1958 Portugal
1959 -
14 races out of 86 - 16%
1960 Monaco
1961 Britain
1961 Germany
1962 Germany
1962 Italy
1963 Belgium
1963 France
1964 -
1965 Belgium
1965 USA
1966 Belgium
1966 Britain
1966 Germany
1967 Canada
1968 Netherlands
1968 France
1968 Germany
1969 -
16 races out of 89 - 18%
1970 -
1971 Netherlands
1971 Canada
1972 Spain
1972 Monaco
1972 USA
1973 Canada
1974 Brazil
1974 Spain
1974 Germany
1975 Monaco
1975 Netherlands
1975 Britain
1975 Austria
1976 Germany
1976 Japan
1977 Belgium
1977 Austria
1977 US East
1978 Austria
1979 South Africa
1979 US East
21 races from 144 - 15%
So the conclusion is that when you're young you siomply don't notice the rain
