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| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1958 | 27.7 | 28.0 | 27.6 | 24.4 | 21.1 | 19.2 | 24.7 ] | ||||||
| 1959 | 15.3 | 19.3 | 19.3 | 20.9 | 23.8 | 25.7 | 28.3 | 28.1 | 27.4 | 24.1 | 22.1 | 19.6 | 22.8 |
| 1960 | 16.8 | 17.1 | 20.3 | 20.5 | 23.1 | 26.1 | 28.5 | 27.4 | 27.4 | 24.8 | 22.1 | 17.5 | 22.6 |
| 1961 | 15.5 | 15.7 | 18.6 | 21.1 | 24.2 | 27.6 | 28.4 | 28.4 | 27.8 | 25.1 | 22.0 | 18.5 | 22.7 |
| 1962 | 14.0 | 16.3 | 16.8 | 19.3 | 24.0 | 26.6 | 28.2 | 27.9 | 27.5 | 24.9 | 21.4 | 17.5 | 22.0 |
| 1963 | 11.3 | 12.9 | 17.2 | 20.8 | 25.5 | 26.3 | 28.3 | 28.5 | 27.7 | 23.7 | 21.3 | 18.0 | 21.8 |
| 1964 | 17.2 | 16.2 | 18.1 | 24.1 | 23.9 | 25.5 | 27.9 | 27.7 | 27.9 | 25.5 | 21.4 | 18.5 | 22.8 |
| 1965 | 15.8 | 16.5 | 16.6 | 20.4 | 22.6 | 25.5 | 28.4 | 27.7 | 26.5 | 23.3 | 22.3 | 18.0 | 22.0 |
| 1966 | 16.4 | 17.9 | 19.9 | 21.9 | 23.3 | 25.5 | 28.1 | 28.5 | 26.4 | 23.6 | 21.9 | 18.9 | 22.7 |
| 1967 | 15.2 | 15.5 | 18.2 | 22.6 | 25.0 | 26.5 | 28.8 | 28.5 | 26.8 | 24.0 | 22.9 | 15.6 | 22.5 |
| 1968 | 15.4 | 13.6 | 17.4 | 20.1 | 23.4 | 25.2 | 28.6 | 28.4 | 27.0 | 23.8 | 21.4 | 19.5 | 22.0 |
| 1969 | 18.3 | 17.6 | 18.3 | 22.1 | 24.6 | 24.5 | 28.9 | 28.1 | 28.0 | 23.8 | 20.6 | 17.1 | 22.7 |
| 1970 | 14.9 | 16.7 | 16.6 | 19.5 | 23.6 | 25.9 | 27.7 | 28.3 | 27.5 | 25.1 | 21.2 | 18.6 | 22.1 |
| 1971 | 14.4 | 15.6 | 17.5 | 20.9 | 23.1 | 27.6 | 29.0 | 27.6 | 26.9 | 24.2 | 21.2 | 18.4 | 22.2 |
| 1972 | 18.1 | 16.8 | 17.2 | 20.4 | 23.6 | 26.3 | 27.7 | 27.1 | 25.7 | 23.9 | 21.6 | 18.4 | 22.2 |
| 1973 | 16.9 | 18.6 | 18.8 | 22.6 | 23.6 | 26.0 | 27.6 | 27.8 | 26.7 | 24.9 | 20.6 | 16.1 | 22.5 |
| 1974 | 16.0 | 16.0 | 17.9 | 20.8 | 24.2 | 25.5 | 27.8 | 28.2 | 26.4 | 25.4 | 22.1 | 19.8 | 22.5 |
| 1975 | 16.6 | 16.9 | 17.8 | 22.0 | 23.3 | 26.1 | 28.0 | 27.7 | 27.0 | 25.9 | 21.6 | 17.1 | 22.5 |
| 1976 | 15.0 | 18.1 | 17.9 | 21.8 | 24.3 | 26.6 | 26.8 | 27.9 | 26.2 | 23.7 | 20.3 | 17.8 | 22.2 |
| 1977 | 16.2 | 15.1 | 19.0 | 21.8 | 24.5 | 26.5 | 28.7 | 27.7 | 27.3 | 24.8 | 21.3 | 19.3 | 22.7 |
| 1978 | 15.5 | 15.3 | 17.5 | 20.8 | 23.4 | 26.2 | 27.8 | 27.4 | 26.9 | 24.8 | 21.8 | 18.8 | 22.2 |
| 1979 | 17.2 | 17.2 | 18.7 | 21.5 | 22.3 | 26.1 | 29.2 | 27.7 | 27.2 | 23.6 | 20.9 | 19.4 | 22.6 |
| 1980 | 16.1 | 15.5 | 19.2 | 20.9 | 24.2 | 27.9 | 29.1 | 28.3 | 27.2 | 24.4 | 22.5 | 16.6 | 22.7 |
| 1981 | 15.2 | 16.4 | 19.3 | 21.8 | 22.6 | 25.6 | 27.8 | 28.4 | 26.3 | 24.4 | 20.8 | 17.0 | 22.1 |
| 1982 | 14.9 | 16.9 | 19.7 | 19.8 | 24.3 | 25.3 | 28.4 | 27.8 | 26.3 | 23.8 | 22.3 | 18.1 | 22.3 |
| 1983 | 17.1 | 16.1 | 18.1 | 22.8 | 24.5 | 26.5 | 28.8 | 28.4 | 27.8 | 25.9 | 21.0 | 16.5 | 22.8 |
| 1984 | 15.1 | 15.3 | 17.5 | 21.2 | 23.4 | 27.0 | 28.4 | 28.4 | 27.4 | 24.4 | 22.5 | 18.2 | 22.4 |
| 1985 | 16.3 | 17.3 | 19.5 | 20.4 | 24.7 | 26.1 | 28.0 | 27.6 | 27.2 | 25.2 | 20.4 | 17.9 | 22.6 |
| 1986 | 14.6 | 14.4 | 16.9 | 21.3 | 23.4 | 26.6 | 28.9 | 28.2 | 27.0 | 23.9 | 21.8 | 18.1 | 22.1 |
| 1987 | 15.8 | 16.2 | 19.1 | 21.6 | 24.2 | 25.6 | 28.5 | 28.8 | 26.7 | 25.8 | 23.1 | 18.7 | 22.8 |
| 1988 | 18.5 | 17.6 | 19.2 | 20.7 | 23.6 | 27.3 | 29.5 | 28.1 | 27.2 | 25.0 | 19.9 | 17.8 | 22.9 |
| 1989 | 17.8 | 17.5 | 17.9 | 21.5 | 23.5 | 26.6 | 28.2 | 27.9 | 27.4 | 24.4 | 21.4 | 17.9 | 22.7 |
| 1990 | 16.6 | 18.6 | 18.8 | 20.2 | 23.7 | 27.2 | 29.0 | 29.0 | 27.1 | 24.1 | 22.6 | 18.8 | 23.0 |
| 1991 | 17.4 | 16.2 | 20.3 | 21.9 | 24.5 | 28.6 | 29.5 | 28.8 | 27.7 | 24.2 | 21.4 | 19.2 | 23.3 |
| 1992 | 16.5 | 16.1 | 20.4 | 21.7 | 23.3 | 26.1 | 28.1 | 28.4 | 27.3 | 24.6 | 21.3 | 19.2 | 22.8 |
| 1993 | 17.4 | 16.6 | 18.0 | 20.7 | 24.7 | 27.0 | 29.1 | 28.9 | 27.4 | 25.0 | 23.0 | 18.7 | 23.0 |
| 1994 | 17.3 | 17.1 | 17.2 | 22.8 | 23.3 | 26.9 | 29.3 | 28.7 | 26.5 | 24.1 | 22.5 | 20.2 | 23.0 |
| 1995 | 16.0 | 15.4 | 17.4 | 22.1 | 23.4 | 26.5 | 28.2 | 28.7 | 27.6 | 26.0 | 20.7 | 17.9 | 22.5 |
| 1996 | 16.9 | 16.1 | 18.6 | 19.5 | 22.7 | 27.6 | 29.3 | 27.9 | 27.8 | 24.5 | 23.1 | 18.1 | 22.7 |
| 1997 | 16.1 | 17.0 | 19.7 | 21.9 | 24.5 | 26.0 | 28.1 | 27.7 | 26.5 | 24.1 | 22.6 | 19.9 | 22.8 |
| 1998 | 18.3 | 18.2 | 20.1 | 23.5 | 26.0 | 27.5 | 29.6 | 29.7 | 27.9 | 26.3 | 23.2 | 20.2 | 24.2 |
| 1999 | 17.9 | 17.0 | 20.7 | 21.6 | 23.4 | 27.4 | 28.1 | 28.2 | 27.8 | 25.8 | 22.2 | 18.4 | 23.2 |
| 2000 | 17.9 | 16.4 | 18.8 | 20.8 | 23.5 | 27.3 | 28.4 | 28.1 | 26.1 | 25.9 | 23.3 | 19.9 | 23.0 |
| 2001 | 17.6 | 18.4 | 18.0 | 21.3 | 23.5 | 27.5 | 29.6 | 29.5 | 27.4 | 25.4 | 21.8 | 18.9 | 23.2 |
| 2002 | 16.4 | 17.0 | 19.4 | 22.8 | 24.9 | 27.0 | 27.8 ) | 28.5 | 27.1 | 24.6 | 20.9 | 19.4 | 23.0 |
| 2003 | 15.2 | 17.8 | 18.2 | 22.5 | 24.1 | 26.2 | 29.7 | 29.6 | 28.5 | 24.6 | 23.6 | 18.4 | 23.2 |
| 2004 | 16.3 | 17.0 | 19.1 | 21.8 | 25.2 | 26.7 | 28.7 | 28.8 | 27.2 | 24.7 | 22.7 | 20.0 | 23.2 |
| 2005 | 16.2 | 17.7 | 16.8 | 21.2 | 24.1 | 26.9 | 29.5 | 28.9 | 28.1 | 26.1 | 22.8 | 16.8 | 22.9 |
| 2006 | 17.9 | 17.8 | 18.1 | 21.6 | 24.8 | 26.5 | 28.8 | 29.3 | 27.1 | 25.9 | 22.6 | 19.6 | 23.3 |
| 2007 | 17.5 | 18.1 | 19.6 | 20.8 | 23.8 | 26.6 | 29.7 | 28.6 | 28.0 | 26.3 | 22.2 | 19.6 | 23.4 |
| 2008 | 18.2 | 15.8 ) | 18.5 | 21.3 | 24.3 | 27.5 | 29.5 | 28.7 | 27.8 | 25.8 | 22.3 | 18.8 | 23.2 |
| 2009 | 16.2 | 19.8 | 19.5 | 20.6 | 23.5 | 26.5 | 29.2 | 29.3 | 28.7 | 25.1 | 22.5 | 18.0 | 23.2 |
| 2010 | 16.7 | 18.2 | 19.7 | 21.0 | 23.9 | 26.6 | 28.8 | 29.1 | 28.1 | 25.3 | 21.5 | 17.7 | 23.1 |
| 2011 | 14.6 | 17.5 | 17.1 | 20.2 | 23.5 | 27.8 | 28.9 | 28.5 | 27.7 | 24.8 | 23.6 | 18.6 | 22.7 |
| 2012 | 16.9 | 17.3 | 19.5 | 21.6 | 24.3 | 27.3 | 29.4 | 28.6 | 27.0 | 24.2 | 20.8 | 18.3 | 22.9 |
| 2013 | 16.7 | 18.7 | 20.2 | 20.7 | 23.9 | 27.9 | 29.4 | 29.7 | 28.4 | 25.5 | 20.9 | 17.1 | 23.3 |
| 2014 | 16.8 | 17.7 | 18.3 | 20.9 | 23.6 | 27.1 | 29.6 | 28.8 | 28.6 | 25.3 ) | 22.7 | 17.6 | 23.1 |
| 2015 | 16.4 | 16.5 | 19.1 | 22.2 | 24.6 | 29.0 | 28.9 | 28.8 | 27.5 | 25.4 | 23.7 | 20.1 | 23.5 |
| 2016 | 17.6 | 16.6 | 18.7 | 23.0 | 25.7 | 28.4 | 29.8 | 29.5 | 28.5 | 27.4 | 23.0 | 20.7 | 24.1 |
| 2017 | 18.4 | 17.2 | 18.3 | 21.5 | 24.3 | 26.8 | 29.6 | 30.4 | 28.7 | 27.0 | 22.9 | 18.2 | 23.6 |
| 2018 | 17.2 | 16.5 | 19.7 | 21.5 | 25.5 | 27.5 | 28.3 | 28.6 | 28.2 | 23.9 | 23.0 | 20.3 | 23.4 |
| 2019 | 18.0 | 19.7 | 19.8 | 22.4 | 24.3 | 26.7 | 28.9 | 29.1 | 28.0 | 25.8 | 22.9 | 19.9 | 23.8 |
| 2020 | 18.3 | 18.5 | 20.0 | 19.8 | 24.9 | 28.5 | 29.4 | 29.3 | 27.4 | 25.7 | 23.6 | 19.2 | 23.7 |
| 2021 | 16.3 ) | 18.3 | 20.6 | 21.6 | 26.0 | 27.6 | 28.8 | 28.4 | 28.5 | 25.8 | 21.6 | 18.7 | 23.5 |
| 2022 | 17.8 | 17.2 | 20.4 | 22.5 | 23.5 | 27.3 | 29.9 | 29.9 | 28.1 | 26.0 | 23.6 | 18.5 | 23.7 |
| 2023 | 17.3 | 19.0 | 20.1 | 22.4 | 24.4 | 27.2 | 29.6 | 28.5 | 29.0 | 25.7 | 22.5 | 19.5 | 23.8 |
| 2024 | 17.5 | 19.5 | 19.5 | 23.9 | 24.6 | 27.1 | 30.6 | 30.0 | 29.0 | 27.3 | 23.7 | 18.4 | 24.3 |
| 2025 | 16.7 | 15.9 | 19.0 | 20.5 | 22.9 ] | 18.0 ] |
Remarks
The red lines, if any, indicate data inhomogeneity caused by changes in instrumentation, observation methods and/or site location.