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| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1963 | 89 | 81 | 82 | 73 | 69 | 79 ] | |||||||
| 1964 | 66 | 68 | 69 | 77 | 77 | 81 | 83 | 86 | 82 | 77 | 72 | 71 | 76 |
| 1965 | 66 | 63 | 67 | 61 | 72 | 79 | 87 | 84 | 79 | 74 | 68 | 68 | 72 |
| 1966 | 63 | 65 | 64 | 70 | 71 | 82 | 86 | 85 | 82 | 80 | 66 | 58 | 73 |
| 1967 | 63 | 60 | 64 | 70 | 76 | 78 | 86 | 81 | 84 | 75 | 67 | 72 | 73 |
| 1968 | 61 | 59 | 66 | 70 | 80 | 85 | 85 | 85 | 81 | 73 | 69 | 70 | 74 |
| 1969 | 62 | 63 | 61 | 68 | 65 | 78 | 84 | 83 | 79 | 71 ) | 71 | 59 | 70 |
| 1970 | 60 | 59 | 56 | 63 | 70 | 77 | 80 | 83 | 81 | 76 | 69 | 62 | 70 |
| 1971 | 63 | 65 | 57 | 62 | 68 | 81 | 85 | 83 | 83 | 74 | 66 | 60 | 71 |
| 1972 | 65 | 61 | 66 | 71 | 74 | 80 | 83 | 81 | 80 | 72 | 69 | 66 | 72 |
| 1973 | 64 | 60 | 61 | 62 | 67 | 82 | 79 | 77 | 79 | 76 | 69 | 67 | 70 |
| 1974 | 61 | 65 | 70 | 67 | 69 | 86 | 87 | 83 | 82 | 74 | 63 | 59 | 72 |
| 1975 | 64 | 65 | 63 | 69 | 71 | 85 | 83 | 75 | 74 | 72 | 72 | 62 | 71 |
| 1976 | 60 | 65 | 61 | 64 | 67 | 81 | 80 | 80 | 81 | 75 | 64 | 64 | 70 |
| 1977 | 59 | 63 | 63 | 65 | 76 | 83 | 83 | 84 | 76 | 73 | 70 | 64 | 72 |
| 1978 | 63 | 63 | 62 | 66 | 74 | 78 | 82 | 79 | 81 | 73 | 68 | 60 | 71 |
| 1979 | 61 | 66 | 61 | 67 | 72 | 80 | 83 | 81 | 78 | 76 | 73 | 65 | 72 |
| 1980 | 60 | 59 | 58 | 68 | 72 | 78 | 87 | 86 | 80 | 74 | 68 | 70 | 72 |
| 1981 | 60 | 69 | 61 | 63 | 71 | 86 | 85 | 86 | 78 | 75 | 62 | 66 | 72 |
| 1982 | 62 | 60 | 61 | 70 | 68 | 76 | 84 | 86 | 79 | 75 | 70 | 66 | 71 |
| 1983 | 61 | 63 | 69 | 67 | 71 | 82 | 86 | 86 | 83 | 68 | 72 | 62 | 73 |
| 1984 | 64 | 64 | 65 | 72 | 81 | 86 | 90 | 78 | 81 | 70 | 64 | 65 | 73 |
| 1985 | 59 | 63 | 66 | 73 | 69 | 77 | 80 | 80 | 79 | 73 | 72 | 60 | 71 |
| 1986 | 62 | 62 | 64 | 66 | 69 | 79 | 83 | 83 | 76 | 73 | 65 | 59 | 70 |
| 1987 | 58 | 59 | 65 | 62 | 69 | 74 | 79 | 83 | 80 | 71 | 66 | 62 | 69 |
| 1988 | 59 | 53 | 60 | 66 | 71 | 81 | 84 | 87 | 82 | 70 | 67 | 62 | 70 |
| 1989 | 64 | 61 | 63 | 67 | 74 | 79 | 86 | 82 | 82 | 74 | 75 | 65 | 73 |
| 1990 | 61 | 70 | 63 | 72 | 69 | 78 | 81 | 79 | 80 | 75 | 71 | 65 | 72 |
| 1991 | 60 | 62 | 61 | 64 | 69 | 78 | 84 | 82 | 81 | 76 | 68 | 64 | 71 |
| 1992 | 63 | 59 | 66 | 71 | 74 | 80 | 79 | 83 | 77 | 78 | 70 | 66 | 72 |
| 1993 | 63 | 63 | 62 | 59 | 73 | 80 | 86 | 80 | 79 | 72 | 70 | 63 | 71 |
| 1994 | 61 | 62 | 57 | 56 | 71 | 80 | 81 | 75 | 81 | 75 | 63 | 62 | 69 |
| 1995 | 59 | 57 | 68 | 67 | 75 | 83 | 79 | 82 | 75 | 75 | 64 | 60 | 70 |
| 1996 | 58 | 55 | 60 | 62 | 71 | 81 | 81 | 80 | 80 | 73 | 67 | 62 | 69 |
| 1997 | 59 | 59 | 57 | 65 | 71 | 80 | 81 | 78 | 82 | 73 | 70 | 65 | 70 |
| 1998 | 63 | 59 | 57 | 71 | 72 | 81 | 87 | 89 | 88 | 80 | 69 | 63 | 73 |
| 1999 | 61 | 61 | 68 | 70 | 64 | 79 | 83 | 84 | 82 | 75 | 71 | 62 | 72 |
| 2000 | 64 | 59 | 60 | 64 | 79 | 78 | 79 | 78 | 79 | 72 | 67 | 61 | 70 |
| 2001 | 59 | 59 | 59 | 58 | 73 | 74 | 77 | 83 | 77 | 72 | 63 | 57 | 68 |
| 2002 | 62 | 60 | 58 | 62 | 71 | 77 | 84 | 82 | 81 | 76 | 69 | 63 | 70 |
| 2003 | 64 | 68 | 62 | 69 | 77 | 79 | 86 | 84 | 81 | 74 | 74 | 67 | 74 |
| 2004 | 66 | 64 | 62 | 64 | 75 | 75 | 81 | 79 | 80 | 79 | 73 | 66 | 72 |
| 2005 | 67 | 65 | 65 | 63 | 74 | 81 | 85 | 83 | 81 | 80 | 69 | 68 | 73 |
| 2006 | 60 | 71 | 62 | 71 | 76 | 81 | 88 | 86 | 82 ) | 78 | 76 | 71 | 75 |
| 2007 | 66 | 64 | 66 | 72 | 71 | 81 | 84 | 81 | 81 | 75 | 68 | 73 | 74 |
| 2008 | 64 | 62 | 69 | 64 | 72 | 76 | 85 | 81 | 78 | 76 | 67 | 68 | 72 |
| 2009 | 64 | 60 | 61 | 61 | 66 | 77 | 79 | 81 | 76 | 72 | 72 | 64 | 69 |
| 2010 | 63 | 62 | 65 | 67 | 71 | 77 | 79 | 77 | 77 | 77 | 66 | 66 | 71 |
| 2011 | 57 | 61 | 60 ] | 61 ) | 67 | 74 | 76 | 78 | 80 | 71 | 71 | 59 | 69 ] |
| 2012 | 56 | 61 | 65 | 67 | 72 | 75 | 79 | 78 | 79 | 70 | 67 | 63 | 69 |
| 2013 | 60 | 59 | 56 | 60 | 72 | 79 | 83 | 81 | 82 | 80 ) | 69 | 70 | 71 |
| 2014 | 64 | 61 | 64 | 57 | 69 | 83 | 81 | 84 | 78 | 75 | 73 | 68 | 71 |
| 2015 | 64 | 65 | 65 | 68 | 64 | 79 | 81 | 81 | 80 | 65 | 76 | 67 | 71 |
| 2016 | 61 | 62 | 62 | 67 | 73 | 80 | 82 | 82 | 84 | 70 | 69 | 66 | 72 |
| 2017 | 64 | 62 | 64 | 66 | 71 | 79 | 82 | 89 | 79 | 80 | 68 | 66 | 73 |
| 2018 | 61 | 60 | 60 | 66 | 71 | 81 | 88 ) | 83 | 84 | 77 | 71 | 66 | 72 |
| 2019 | 61 | 61 | 63 | 66 | 67 | 83 | 83 | 83 | 78 | 80 | 66 | 68 | 72 |
| 2020 | 67 | 68 | 66 | 66 | 73 | 79 | 91 | 83 | 84 | 78 | 70 | 68 | 74 |
| 2021 | 67 | 64 | 64 | 64 | 72 | 82 | 85 | 85 | 81 | 81 | 76 | 70 | 74 |
| 2022 | 62 | 65 | 68 | 69 | 72 | 80 | 86 ) | 85 | 86 | 77 | 73 | 71 | 75 |
| 2023 | 65 | 66 | 68 | 60 | 72 | 82 | 79 | 86 | 87 | 76 | 74 | 68 | 74 |
| 2024 | 64 | 62 | 64 | 70 | 70 | 80 | 81 | 87 | 84 | 81 | 73 | 64 | 73 |
| 2025 | 68 | 57 | 64 | 75 | 77 | 79 | 78 | 78 | 83 | 81 | 71 ) | 74 ] |
Remarks
The red lines, if any, indicate data inhomogeneity caused by changes in instrumentation, observation methods and/or site location.