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| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1938 | 3.1 | 3.2 | 4.4 | 6.7 | 9.3 | 11.2 | 16.9 | 21.5 | 14.8 | 8.8 | 5.5 | 3.2 | 9.1 |
| 1939 | 2.8 | 2.9 | 4.3 | 6.3 | 8.5 | 12.7 | 17.5 | 19.5 | 16.0 | 8.1 | 5.3 | 3.5 | 9.0 |
| 1940 | 3.1 | 3.2 | 4.5 | 6.1 | 8.4 | 11.5 | 16.7 | 19.1 | 13.5 | 8.8 | 5.6 | 4.1 | 8.7 |
| 1941 | 2.9 | 3.1 | 3.5 | 5.6 | 8.3 | 11.7 | 12.9 | 15.9 | 14.5 | 8.8 | 5.6 | 3.7 | 8.0 |
| 1942 | 2.7 | 2.8 | 4.1 | 5.9 | 8.1 | 11.9 | 17.2 | 17.5 | 13.7 | 8.9 | 5.2 | 3.5 | 8.5 |
| 1943 | 3.6 | 2.9 | 4.1 | 5.9 | 8.5 | 12.0 | 19.2 | 21.1 | 15.1 | 9.7 | 4.4 | 3.5 | 9.2 |
| 1944 | 2.7 | 2.7 | 4.1 | 5.5 | 8.4 | 13.6 | 18.4 | 20.4 | 14.8 | 8.7 | 4.8 | 2.8 | 8.9 |
| 1945 | 2.7 | 2.9 | 4.1 | 6.1 | 7.7 | 11.1 | 13.2 | 18.1 | 13.9 | 9.2 | 5.6 | 3.1 | 8.1 |
| 1946 | 2.8 | 2.9 | 3.7 | 6.4 | 8.1 | 14.0 | 17.6 | 21.9 | 13.6 | 10.1 | 5.7 | 3.7 | 9.2 |
| 1947 | 3.3 | 3.2 | 3.6 | 6.4 | 8.4 | 11.1 | 16.8 | 18.5 | 13.6 | 9.3 | 5.5 | 3.5 | 8.6 |
| 1948 | 3.9 | 3.3 | 4.5 | 7.1 | 10.0 | 12.7 | 17.1 | 21.1 | 14.1 | 8.5 | 5.6 | 4.1 | 9.3 |
| 1949 | 3.7 | 3.7 | 3.5 | 5.7 | 9.5 | 11.1 | 16.3 | 21.1 | 14.8 | 9.1 | 5.1 | 3.3 | 8.9 |
| 1950 | 3.0 | 3.1 | 4.2 | 6.3 | 9.3 | 13.3 | 19.4 | 22.6 | 14.8 | 7.6 | 5.5 | 3.7 | 9.4 |
| 1951 | 2.8 | 2.9 | 3.9 | 6.2 | 8.9 | 11.9 | 16.8 | 22.7 | 13.3 | 8.4 | 5.6 | 4.1 | 9.0 |
| 1952 | 3.1 | 2.7 | 3.7 | 6.3 | 8.5 | 12.8 | 16.7 | 19.6 | 14.6 | 9.7 | 4.6 | 2.7 | 8.8 |
| 1953 | 2.8 | 2.7 | 4.4 | 6.2 | 8.5 | 11.5 | 17.2 | 17.9 | 14.6 | 8.9 | 4.1 | 3.5 | 8.5 |
| 1954 | 2.8 | 3.5 | 3.8 | 6.5 | 8.3 | 10.5 | 13.4 | 17.5 | 15.4 | 8.7 | 5.2 | 4.2 | 8.3 |
| 1955 | 2.8 | 3.2 | 4.3 | 6.3 | 8.4 | 12.5 | 20.5 | 19.3 | 13.7 | 9.5 | 5.4 | 4.3 | 9.2 |
| 1956 | 2.9 | 3.3 | 4.1 | 5.9 | 9.3 | 11.2 | 14.1 | 16.1 | 15.5 | 9.8 | 5.3 | 3.9 | 8.5 |
| 1957 | 3.5 | 3.1 | 3.9 | 6.2 | 8.7 | 11.2 | 16.7 | 18.1 | 13.4 | 9.4 | 6.2 | 4.3 | 8.7 |
| 1958 | 3.4 | 3.4 | 3.8 | 5.9 | 8.5 | 12.1 | 17.6 | 18.1 | 14.9 | 8.9 | 5.3 | 4.3 | 8.9 |
| 1959 | 2.8 | 3.5 | 5.1 | 7.3 | 9.0 | 12.6 | 17.0 | 17.9 | 14.9 | 9.7 | 5.4 | 3.3 | 9.0 |
| 1960 | 2.6 | 3.2 | 4.0 | 5.6 | 8.4 | 11.4 | 15.5 | 20.2 | 15.6 | 8.4 | 5.5 | 3.2 | 8.6 |
| 1961 | 2.6 | 3.1 | 4.1 | 6.7 | 8.8 | 13.6 | 19.3 | 19.8 | 17.0 | 8.6 | 5.8 | 4.3 | 9.5 |
| 1962 | 3.5 | 3.0 | 3.9 | 7.4 | 9.9 | 12.4 | 16.9 | 17.4 | 14.8 | 7.7 | 5.4 | 4.0 | 8.9 |
| 1963 | 3.7 | 3.6 | 4.6 | 6.5 | 8.7 | 12.1 | 16.8 | 17.5 | 13.8 | 9.1 | 6.1 | 4.0 | 8.9 |
| 1964 | 3.2 | 2.5 | 4.4 | 5.7 | 8.7 | 12.0 | 15.1 | 17.7 | 12.6 | 9.1 | 5.8 | 4.1 | 8.4 |
| 1965 | 3.7 | 3.2 | 4.1 | 5.7 | 8.9 | 13.0 | 15.2 | 18.6 | 14.8 | 9.4 | 5.2 | 4.1 | 8.8 |
| 1966 | 3.5 | 3.3 | 4.5 | 6.1 | 8.7 | 12.5 | 15.5 | 18.5 | 12.5 | 11.1 | 5.9 | 3.4 | 8.8 |
| 1967 | 2.8 | 3.0 | 4.4 | 6.5 | 10.5 | 11.8 | 17.1 | 19.4 | 14.0 | 9.6 | 5.1 | 4.4 | 9.1 |
| 1968 | 3.7 | 3.5 | 5.3 | 7.1 | 9.2 | 13.4 | 17.3 | 17.7 | 13.7 | 8.0 | 7.2 | 4.3 | 9.2 |
| 1969 | 2.7 | 2.7 | 3.3 | 5.2 | 8.0 | 11.7 | 17.2 | 16.0 | 13.6 | 8.5 | 5.3 | 3.1 | 8.1 |
| 1970 | 3.0 | 3.0 | 3.8 | 7.1 | 10.2 | 12.9 | 17.6 | 18.9 | 13.7 | 9.1 | 6.0 | 3.3 | 9.1 |
| 1971 | 3.1 | 3.1 | 3.8 | 5.6 | 8.4 | 11.4 | 15.9 | 16.8 | 13.7 | 9.8 | 5.1 | 3.7 | 8.4 |
| 1972 | 3.0 | 3.3 | 4.5 | 6.9 | 9.1 | 12.3 | 17.3 | 19.6 | 13.9 | 9.5 | 5.4 | 4.0 | 9.1 |
| 1973 | 3.3 | 3.0 | 3.5 | 5.6 | 8.2 | 12.2 | 17.2 | 19.9 | 14.5 | 7.8 | 5.2 | 3.7 | 8.7 |
| 1974 | 3.7 | 3.2 | 3.6 | 6.3 | 8.3 | 12.1 | 16.1 | 17.7 | 14.0 | 8.3 | 4.8 | 2.8 | 8.4 |
| 1975 | 3.5 | 2.8 | 4.2 | 6.8 | 9.1 | 13.5 | 16.5 | 19.8 | 14.0 | 8.7 | 5.8 | 3.4 | 9.0 |
| 1976 | 3.2 | 3.3 | 3.6 | 6.2 | 8.4 | 11.2 | 15.7 | 15.6 | 13.0 | 8.3 | 4.5 | 3.8 | 8.1 |
| 1977 | 2.3 | 2.5 | 3.9 | 5.5 | 8.1 | 11.2 | 16.8 | 16.6 | 13.7 | 8.6 | 5.4 | 3.7 | 8.2 |
| 1978 | 2.7 | 2.2 | 3.8 | 6.1 | 8.2 | 13.0 | 19.8 | 19.7 | 13.3 | 7.6 | 5.1 | 3.5 | 8.8 |
| 1979 | 2.5 | 3.3 | 3.6 | 5.4 | 8.0 | 12.2 | 14.9 | 17.8 | 13.2 | 9.4 | 5.2 | 4.3 | 8.3 |
| 1980 | 3.1 | 2.6 | 3.6 | 5.0 | 8.2 | 11.5 | 15.3 | 14.9 | 13.2 | 8.1 | 5.5 | 4.4 | 8.0 |
| 1981 | 2.9 | 2.8 | 3.6 | 6.5 | 8.2 | 11.5 | 16.8 | 17.8 | 13.5 | 8.5 | 4.2 | 4.0 | 8.4 |
| 1982 | 3.0 | 3.1 | 4.3 | 6.1 | 9.0 | 11.4 | 16.3 | 20.8 | 14.4 | 9.3 | 5.6 | 4.0 | 8.9 |
| 1983 | 3.2 | 2.8 | 4.1 | 6.9 | 8.8 | 9.8 | 14.5 | 18.6 | 14.6 | 7.3 | 6.0 | 3.8 | 8.4 |
| 1984 | 3.4 | 2.8 | 3.5 | 5.7 | 9.0 | 14.7 | 19.5 | 20.4 | 14.6 | 7.9 | 5.0 | 3.6 | 9.2 |
| 1985 | 2.6 | 3.6 | 4.0 | 6.8 | 8.4 | 11.4 | 16.8 | 22.0 | 13.6 | 8.8 | 5.3 | 3.2 | 8.9 |
| 1986 | 2.8 | 2.9 | 4.0 | 6.5 | 8.5 | 10.8 | 14.7 | 19.6 | 14.4 | 7.4 | 4.8 | 3.6 | 8.3 |
| 1987 | 2.8 | 3.2 | 4.0 | 6.0 | 8.1 | 12.0 | 17.7 | 18.3 | 14.4 | 9.0 | 5.1 | 3.5 | 8.7 |
| 1988 | 3.0 | 2.9 | 3.9 | 6.5 | 8.4 | 12.7 | 13.8 | 20.7 | 14.3 | 8.7 | 5.1 | 4.0 | 8.7 |
| 1989 | 3.4 | 3.3 | 4.8 | 6.2 | 8.4 | 11.0 | 18.2 | 20.2 | 15.4 | 9.6 | 6.1 | 4.0 | 9.2 |
| 1990 | 2.9 | 3.9 | 4.8 | 6.5 | 9.4 | 13.6 | 17.6 | 20.2 | 15.2 | 10.7 | 6.5 | 4.7 | 9.7 |
| 1991 | 4.3 | 3.4 | 3.9 | 7.0 | 9.4 | 14.3 | 16.4 | 18.5 | 14.1 | 10.0 | 5.4 | 3.7 | 9.2 |
| 1992 | 3.3 | 3.5 | 4.2 | 6.3 | 8.7 | 12.1 | 17.6 | 17.7 | 13.6 | 9.7 | 5.3 | 4.3 | 8.9 |
| 1993 | 3.8 | 3.4 | 4.1 | 5.9 | 8.6 | 11.5 | 14.6 | 17.2 | 14.6 | 9.0 | 5.8 | 4.0 | 8.5 |
| 1994 | 2.8 | 3.8 | 4.0 | 5.9 | 9.5 | 12.2 | 18.0 | 22.0 | 16.6 | 9.1 | 5.8 | 3.6 | 9.4 |
| 1995 | 3.4 | 3.4 | 4.4 | 6.4 | 10.4 | 12.6 | 18.8 | 19.4 | 14.3 | 11.0 | 5.8 | 4.1 | 9.5 |
| 1996 | 3.1 | 3.0 | 3.8 | 5.7 | 8.2 | 11.7 | 15.8 | 17.6 | 14.5 | 8.7 | 4.8 | 3.7 | 8.4 |
| 1997 | 3.6 | 3.4 | 3.7 | 6.1 | 7.9 | 12.4 | 16.8 | 16.6 | 13.1 | 9.0 | 6.5 | 3.8 | 8.6 |
| 1998 | 2.6 | 3.0 | 4.4 | 7.1 | 9.2 | 12.1 | 15.5 | 18.2 | 15.6 | 10.4 | 5.1 | 3.6 | 8.9 |
| 1999 | 3.3 | 3.1 | 3.7 | 6.5 | 8.9 | 13.5 | 17.3 | 22.0 | 15.7 | 9.7 | 5.8 | 4.0 | 9.5 |
| 2000 | 3.5 | 3.0 | 4.0 | 6.4 | 10.5 | 13.3 | 20.2 | 22.8 | 17.0 | 9.0 | 5.1 | 3.5 | 9.9 |
| 2001 | 3.0 | 2.7 | 4.1 | 6.9 | 9.6 | 13.1 ) | 17.2 | 18.0 | 13.1 | 9.4 | 5.5 | 3.1 | 8.8 |
| 2002 | 3.5 | 4.1 | 4.4 | 7.3 | 9.4 | 11.6 | 15.8 | 16.7 | 13.2 | 9.2 | 5.2 | 2.9 | 8.6 |
| 2003 | 3.1 | 2.8 | 4.0 | 6.8 | 9.1 | 12.7 | 13.5 | 17.7 | 14.0 | 9.0 | 5.9 | 3.7 | 8.5 |
| 2004 | 3.4 | 3.6 | 4.0 | 5.7 | 9.6 | 13.1 | 16.3 | 17.6 | 14.9 | 9.3 | 6.2 | 3.7 | 9.0 |
| 2005 | 3.2 | 2.8 | 3.8 | 6.1 | 8.0 | 13.4 | 15.8 | 19.8 | 14.7 | 9.5 | 6.3 | 3.8 | 8.9 |
| 2006 | 3.0 | 3.2 | 4.3 | 5.8 | 9.2 | 12.5 | 18.1 | 22.8 | 15.5 | 9.4 | 6.2 | 3.8 | 9.5 |
| 2007 | 3.4 | 3.5 | 4.3 | 6.0 | 8.9 | 14.1 | 15.9 | 20.4 | 15.5 | 9.4 | 4.9 | 3.9 | 9.2 |
| 2008 | 2.8 | 3.2 | 5.2 | 7.1 | 9.3 | 12.0 | 18.2 | 18.3 | 15.5 | 9.8 | 5.7 | 4.4 | 9.3 |
| 2009 | 3.9 | 3.3 | 4.6 | 6.2 | 9.6 | 13.0 | 15.3 | 18.2 | 14.6 | 9.6 | 5.7 | 3.9 | 9.0 |
| 2010 | 3.7 | 3.3 | 3.8 | 6.1 | 8.8 | 15.1 | 19.0 | 23.1 | 15.9 | 10.1 | 6.4 | 4.4 | 10.0 |
| 2011 | 3.2 | 3.7 | 4.1 | 6.5 | 8.3 | 13.3 | 17.8 ) | 21.4 | 16.6 | 9.9 | 6.2 | 3.9 | 9.6 |
| 2012 | 3.0 | 2.7 | 3.8 | 6.5 | 9.5 | 12.0 | 17.4 | 20.2 | 18.6 | 9.6 | 6.4 | 3.7 | 9.5 |
| 2013 | 3.0 | 3.0 | 3.8 | 6.2 | 8.8 | 13.5 | 18.6 | 20.2 | 15.0 | 9.9 | 6.4 | 4.3 | 9.4 |
| 2014 | 2.9 | 3.2 | 4.0 | 5.9 | 9.2 | 13.1 | 19.0 | 19.6 | 14.6 | 8.6 | 6.0 | 3.7 | 9.2 |
| 2015 | 3.6 | 4.0 | 5.7 | 6.7 | 9.0 | 12.2 | 17.7 | 18.1 | 15.1 | 8.1 | 5.5 | 4.1 | 9.2 |
| 2016 | 2.7 | 3.0 | 4.3 | 6.2 | 9.0 | 12.0 | 17.5 | 21.1 | 14.9 | 7.3 | 4.1 | 3.3 | 8.8 |
| 2017 | 3.0 | 3.4 | 4.2 | 6.2 | 9.2 | 11.7 | 17.8 | 17.2 | 14.3 | 8.2 | 5.6 | 3.6 | 8.7 |
| 2018 | 3.6 | 3.0 | 4.6 | 6.6 | 9.7 | 12.5 | 17.4 | 18.0 | 14.7 | 10.0 | 6.0 | 3.8 | 9.2 |
| 2019 | 3.2 | 3.1 | 4.7 | 6.4 | 10.7 | 13.1 | 18.7 | 18.7 | 15.6 | 10.4 | 5.0 | 3.8 | 9.5 |
| 2020 | 3.3 | 3.5 | 4.9 | 6.4 | 10.1 | 13.7 | 17.8 | 19.9 | 17.5 | 9.8 | 6.0 | 3.2 | 9.7 |
| 2021 | 2.9 | 3.8 | 5.1 | 6.7 | 9.7 | 13.6 | 21.8 | 19.5 | 15.3 | 9.8 | 7.0 | 4.5 | 10.0 |
| 2022 | 3.5 | 3.4 | 5.0 | 6.9 | 10.4 | 13.0 | 19.1 | 20.6 | 16.4 | 10.2 | 6.1 | 3.7 | 9.9 |
| 2023 | 3.0 | 3.2 | 5.8 | 7.3 | 10.0 | 15.6 | 20.8 | 23.0 | 18.3 | 10.9 | 6.9 | 3.9 | 10.7 |
| 2024 | 3.9 | 3.5 | 4.3 | 8.0 | 9.4 | 14.3 | 20.0 | 20.8 ) | 14.9 | 10.4 | 6.2 ) | 10.5 ] |
Remarks
The red lines, if any, indicate data inhomogeneity caused by changes in instrumentation, observation methods and/or site location.