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| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1951 | 4.5 | 5.2 | 5.9 | 5.5 | 6.1 | 4.6 | 4.4 | 4.8 | 5.1 | 4.6 | 5.5 | 5.1 | 5.1 |
| 1952 | 5.4 | 5.5 | 5.0 | 6.1 | 6.6 | 4.9 | 4.1 | 3.6 | 4.2 | 5.4 | 4.8 | 4.7 | 5.0 |
| 1953 | 6.0 | 5.6 | 5.9 | 5.1 | 4.9 | 4.4 | 3.8 | 3.6 | 4.7 | 5.0 | 5.9 | 5.2 | 5.0 |
| 1954 | 4.8 | 5.5 | 4.7 | 5.6 | 6.0 | 3.5 | 3.7 | 3.9 | 4.4 | 3.8 | 5.3 | 6.0 | 4.8 |
| 1955 | 6.3 | 6.5 | 5.9 | 5.8 | 5.3 | 5.0 | 5.2 | 4.0 | 4.5 | 4.9 | 5.6 | 6.4 | 5.5 |
| 1956 | 5.6 | 6.1 | 5.6 | 5.9 | 6.5 | 5.0 | 3.4 | 4.6 | 4.3 | 4.5 | 5.3 | 6.1 | 5.2 |
| 1957 | 4.8 | 5.2 | 5.7 | 5.1 | 5.3 | 4.5 | 4.2 | 4.6 | 4.3 | 5.0 | 5.7 | 5.2 | 5.0 |
| 1958 | 6.6 | 5.5 | 5.6 | 4.9 | 5.3 | 4.8 | 4.1 | 4.7 | 4.2 | 4.1 | 4.8 | 6.6 | 5.1 |
| 1959 | 5.9 | 5.1 | 3.8 | 6.3 | 5.2 | 3.3 | 4.3 | 4.2 | 4.2 | 5.1 | 4.9 | 5.2 | 4.8 |
| 1960 | 5.8 | 5.3 | 5.3 | 5.2 | 4.5 | 4.6 | 4.9 | 4.1 | 4.6 | 4.3 | 5.0 | 6.0 | 5.0 |
| 1961 | 4.4 | 4.9 | 5.0 | 5.3 | 5.4 | 4.5 | 4.2 | 3.6 | 4.2 | 4.3 | 4.8 | 5.1 | 4.6 |
| 1962 | 6.0 | 3.5 | 4.2 | 5.0 | 4.0 | 4.5 | 3.3 | 3.7 | 3.7 | 3.9 | 4.6 | 4.8 | 4.3 |
| 1963 | 5.3 | 4.6 | 3.9 | 4.1 | 5.7 | 4.7 | 3.9 | 2.8 | 3.6 | 3.7 | 4.8 | 3.9 | 4.3 |
| 1964 | 4.1 | 4.2 | 4.3 | 4.2 | 4.2 | 4.1 | 3.8 | 4.1 | 3.6 | 4.2 | 4.2 | 4.7 | 4.1 |
| 1965 | 5.3 | 5.1 | 4.3 | 5.0 | 4.0 | 3.8 | 3.4 | 3.8 | 3.9 | 3.6 | 4.7 | 5.2 | 4.3 |
| 1966 | 5.6 | 4.6 | 5.7 | 4.7 | 4.6 | 4.2 | 4.5 | 3.4 | 4.0 | 4.7 | 4.3 | 4.6 | 4.6 |
| 1967 | 5.1 | 3.9 | 4.3 | 4.9 | 3.8 | 4.0 | 3.7 | 2.9 | 4.3 | 4.0 | 4.5 | 3.7 | 4.1 |
| 1968 | 5.1 | 4.0 | 4.4 | 3.6 | 3.8 | 6.3 | 4.8 | 4.9 | 5.1 | 5.9 | 5.8 | 6.7 | 5.0 |
| 1969 | 5.3 | 6.8 | 5.9 | 5.6 | 6.8 | 5.7 | 4.8 | 4.2 | 5.8 | 5.1 | 5.8 | 5.8 | 5.6 |
| 1970 | 6.0 | 5.7 | 5.8 | 6.5 | 5.5 | 5.5 | 4.9 | 4.0 | 5.7 | 5.2 | 6.0 | 6.4 | 5.6 |
| 1971 | 6.0 | 5.7 | 6.5 | 6.3 | 5.8 | 5.1 | 4.8 | 4.4 | 5.1 | 5.7 | 5.8 | 6.2 | 5.6 |
| 1972 | 5.6 | 5.5 | 5.3 | 5.8 | 7.1 | 5.9 | 5.2 | 5.8 | 6.3 | 7.2 | 7.9 | 6.2 | 6.2 |
| 1973 | 5.6 | 5.5 | 5.3 | 5.5 | 7.6 | 3.9 | 5.2 | 4.6 | 3.7 | 5.5 | 5.6 | 6.0 | 5.3 |
| 1974 | 7.6 | 5.3 | 4.1 | 4.7 | 5.3 | 3.8 | 3.8 | 4.3 | 4.4 | 5.3 | 6.2 | 5.2 | 5.0 |
| 1975 | 5.0 | 5.1 | 4.4 | 4.8 | 4.5 | 3.8 | 3.7 | 4.5 | 4.7 ) | 4.4 | 4.8 | 5.0 ) | 4.6 |
| 1976 | 5.3 | 5.2 | 4.8 | 5.0 | 5.8 | 4.4 | 5.1 | 4.0 | 4.1 | 5.6 | 5.5 | 4.8 | 5.0 |
| 1977 | 5.1 | 4.4 | 5.0 | 5.7 ) | 5.5 | 4.3 | 4.5 | 3.7 | 4.5 | 5.3 | 4.6 | 4.8 ) | 4.8 |
| 1978 | 4.7 | 4.8 | 4.2 | 4.5 | 5.4 | 4.5 | 3.8 | 3.6 | 4.3 | 4.6 | 4.6 | 5.4 | 4.5 |
| 1979 | 5.4 | 5.3 | 4.7 | 4.3 | 4.5 | 4.8 | 3.7 | 4.4 | 3.6 | 4.6 | 5.0 | 4.7 | 4.6 |
| 1980 | 5.6 | 4.5 | 5.0 | 3.9 | 4.7 | 4.3 | 2.9 | 3.0 | 4.3 | 4.9 | 5.1 | 5.5 | 4.5 |
| 1981 | 5.5 | 4.4 | 5.0 | 4.9 | 5.4 | 3.1 | 4.1 | 5.0 | 4.9 | 5.4 | 5.7 | 5.4 | 4.9 |
| 1982 | 5.3 | 3.9 | 4.6 | 5.3 | 4.5 | 4.8 | 4.1 | 3.6 | 4.4 | 4.7 | 5.2 | 4.9 | 4.6 |
| 1983 | 5.0 | 5.2 | 4.0 | 5.3 | 4.7 | 3.8 | 2.8 | 3.8 | 4.3 | 5.3 | 5.0 | 5.3 | 4.5 |
| 1984 | 5.6 | 5.6 | 4.9 | 4.1 | 3.8 | 3.9 | 3.0 | 4.5 | 3.9 | 5.1 | 5.5 | 4.8 | 4.6 |
| 1985 | 5.2 | 4.8 | 3.8 | 3.6 | 4.5 | 3.9 | 3.6 | 3.9 | 3.9 | 4.8 | 4.8 | 5.0 | 4.3 |
| 1986 | 5.1 | 4.4 | 4.7 | 4.6 | 4.9 | 4.3 | 3.8 ) | 4.4 | 4.3 | 5.0 | 5.2 | 5.1 | 4.7 |
| 1987 | 5.3 | 5.1 | 4.7 | 5.0 | 5.7 | 3.6 | 3.6 | 3.7 | 4.1 | 5.2 | 4.6 | 5.1 | 4.6 |
| 1988 | 4.7 | 4.3 | 4.8 ) | 4.3 | 4.2 | 3.2 | 3.2 | 4.0 | 3.9 | 5.5 | 4.5 | 4.9 | 4.3 |
| 1989 | 4.8 | 4.3 | 4.9 | 4.9 | 4.4 | 4.4 | 4.1 | 3.7 | 3.8 | 4.0 | 4.9 | 4.9 | 4.4 |
| 1990 | 4.7 | 3.7 | 4.4 | 5.5 | 4.8 | 3.6 | 3.2 | 3.9 | 4.0 | 4.0 | 5.0 | 5.0 | 4.3 |
| 1991 | 5.6 | 4.4 | 4.3 | 4.4 | 5.4 | 3.8 ) | 3.6 ) | 4.1 ) | 4.5 | 4.8 | 4.6 | 4.6 | 4.5 |
| 1992 | 4.1 | 4.3 ) | 4.4 ) | 5.1 | 4.1 | 4.1 | 3.9 | 4.4 | 4.9 | 4.4 | 5.3 | 4.8 | 4.5 |
| 1993 | 4.8 | 4.6 | 4.8 | 5.2 | 3.8 | 4.0 | 4.0 | 3.6 | 4.1 | 5.0 | 4.6 | 5.1 | 4.5 |
| 1994 | 4.1 | 5.9 | 4.9 | 5.0 | 4.8 | 3.8 | 3.8 | 3.9 | 5.2 | 4.9 ) | 4.9 | 4.7 | 4.7 |
| 1995 | 5.1 | 4.4 | 4.0 | 4.7 | 4.3 | 3.5 | 4.1 | 3.7 | 4.1 ) | 3.9 ) | 6.0 | 5.5 | 4.4 |
| 1996 | 5.4 | 4.4 | 5.0 | 4.8 | 4.2 | 3.7 | 3.4 | 3.1 | 3.6 | 4.8 | 5.1 | 5.0 | 4.4 |
| 1997 | 5.5 | 4.4 | 4.6 | 4.4 | 4.4 | 3.9 | 4.1 | 4.1 | 4.0 | 4.6 | 5.0 | 5.0 ) | 4.5 |
| 1998 | 5.0 | 4.4 | 4.8 | 5.6 | 5.4 | 4.2 | 3.5 | 3.5 | 4.4 | 4.2 | 5.3 | 4.9 | 4.6 |
| 1999 | 5.0 | 4.9 ) | 4.8 | 4.0 | 4.4 | 4.5 | 3.8 | 4.3 | 4.4 | 5.2 | 5.1 | 4.6 | 4.6 |
| 2000 | 5.6 | 4.3 | 4.3 | 4.4 | 3.3 | 4.0 | 3.6 | 3.5 | 4.4 | 4.7 | 4.6 | 5.1 | 4.3 |
| 2001 | 5.0 | 4.5 | 5.4 | 4.9 | 5.2 | 4.8 | 3.8 | 3.6 | 4.3 | 4.8 | 4.8 | 5.0 | 4.7 |
| 2002 | 4.7 | 4.3 | 4.6 | 4.6 | 3.7 | 5.2 | 3.6 | 4.0 | 3.6 | 5.5 | 6.0 | 4.7 | 4.5 |
| 2003 | 5.4 | 3.9 | 5.1 | 4.8 | 4.6 | 4.3 | 3.7 | 3.5 | 3.7 | 4.3 | 4.6 | 5.2 | 4.4 |
| 2004 | 5.2 | 5.6 | 5.0 | 4.8 | 4.6 | 5.0 | 4.1 | 3.8 | 4.3 | 4.4 | 5.4 | 6.1 | 4.9 |
| 2005 | 5.5 | 5.3 | 5.2 | 5.1 | 4.7 | 3.7 | 4.0 | 4.1 | 3.9 | 4.7 | 4.5 ) | 5.8 | 4.7 |
| 2006 | 5.2 | 4.6 | 5.4 | 4.6 | 5.3 | 3.3 | 3.4 | 3.3 | 3.7 | 4.4 | 4.6 | 4.6 | 4.4 |
| 2007 | 3.8 | 4.5 | 4.5 | 3.7 | 5.1 | 3.9 | 3.7 | 4.2 | 4.5 | 4.2 | 5.1 | 4.4 ) | 4.3 |
| 2008 | 4.6 | 4.5 | 3.9 | 3.4 | 4.0 | 4.3 | 3.6 | 4.2 | 3.8 | 4.0 | 4.9 | 5.2 | 4.2 |
| 2009 | 5.3 | 4.8 | 4.8 | 5.0 | 4.7 | 3.9 | 3.8 | 3.6 | 3.5 | 4.2 | 4.8 | 4.5 | 4.4 |
| 2010 | 5.5 | 4.1 | 4.7 | 4.3 | 4.9 | 4.0 | 3.3 | 4.1 | 4.1 | 3.7 | 5.2 | 5.1 | 4.4 |
| 2011 | 4.8 | 4.4 | 3.9 | 5.7 | 4.7 | 4.1 | 3.3 ) | 3.6 | 4.5 | 4.7 | 4.9 | 5.0 | 4.5 |
| 2012 | 4.7 | 5.0 | 4.5 | 3.9 | 4.1 | 3.9 | 3.7 | 3.7 | 4.1 | 4.8 | 6.2 | 5.5 | 4.5 |
| 2013 | 5.6 | 5.3 | 5.7 | 4.9 | 4.7 | 4.0 | 3.9 | 4.5 | 4.1 | 4.8 | 4.7 | 5.9 | 4.8 |
| 2014 | 4.4 | 5.6 | 4.9 | 5.6 | 5.2 | 4.2 | 4.1 | 3.8 | 4.3 | 4.8 | 4.5 | 5.8 | 4.8 |
| 2015 | 5.2 | 6.0 | 5.0 | 5.4 | 5.1 | 4.5 | 4.3 | 4.2 | 3.8 | 5.9 | 5.3 | 4.2 | 4.9 |
| 2016 | 5.2 | 4.7 | 4.2 | 5.3 | 5.1 | 4.1 | 4.1 | 4.4 | 4.3 | 5.0 | 4.3 | 5.3 | 4.7 |
| 2017 | 4.9 | 4.6 | 4.0 | 5.3 | 5.6 | 5.4 | 4.9 | 4.4 | 4.4 | 4.8 | 4.9 | 4.9 | 4.8 |
| 2018 | 5.2 | 5.0 | 5.2 | 5.2 | 4.4 | 5.2 | 4.7 | 4.5 | 4.1 | 4.5 | 4.1 | 5.1 | 4.8 |
| 2019 | 5.2 | 3.9 | 4.5 | 4.7 | 5.3 | 5.0 | 3.8 | 5.1 | 4.3 | 4.8 | 5.2 | 4.9 | 4.7 |
| 2020 | 4.8 | 4.7 | 5.6 | 4.8 | 5.0 | 4.3 | 4.6 | 4.7 | 5.1 | 4.3 | 5.3 | 4.6 | 4.8 |
| 2021 | 5.3 | 5.2 | 5.0 | 5.5 | 4.7 | 5.2 | 3.7 | 4.3 | 3.7 | 4.6 | 5.1 | 4.8 | 4.8 |
| 2022 | 6.2 | 5.3 | 5.0 | 5.3 | 5.3 | 4.8 | 4.0 | 4.0 | 4.4 | 4.5 | 4.8 | 6.2 | 5.0 |
| 2023 | 4.4 | 4.3 | 5.0 | 5.4 | 5.2 | 4.3 | 4.8 | 5.1 | 3.7 | 4.6 ) | 5.0 | 5.1 | 4.7 |
| 2024 | 6.3 | 4.8 | 4.4 | 5.1 | 5.6 | 5.4 | 4.3 | 4.1 | 4.3 | 4.5 | 4.9 ) | 4.9 ] |
Remarks
The red lines, if any, indicate data inhomogeneity caused by changes in instrumentation, observation methods and/or site location.