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| Year | 2012 | 2011 | 2010 | 2009 | 2008 | 2007 | 2006 |
| January | 30.59 29.65 |
30.39 29.41 |
30.31 29.50 |
30.56 29.41 |
30.71 29.77 |
30.56 29.50 |
30.59 29.32 |
| February | 30.50 29.65 |
30.45 29.74 |
30.36 29.29 |
30.48 29.50 |
30.48 29.47 |
30.30 29.35 |
30.38 29.46 |
| March | 30.59 29.53 |
30.30 29.32 |
30.48 29.56 |
30.44 29.50 |
30.50 29.44 |
30.34 29.55 |
|
| April | 30.34 29.51 |
30.47 29.32 |
30.48 29.35 |
30.39 29.71 |
30.27 29.06 |
30.39 29.50 |
|
| May | 30.31 29.63 |
30.39 29.77 |
30.36 29.77 |
30.21 29.47 |
30.36 29.83 |
30.14 29.56 |
|
| June | 30.25 29.84 |
30.27 29.80 |
30.15 29.59 |
30.27 29.83 |
30.30 29.32 |
30.20 29.58 |
|
| July | 30.22 29.77 |
30.24 29.83 |
30.27 29.68 |
30.27 29.88 |
30.21 29.77 |
30.19 29.80 |
|
| August | 30.21 28.15 |
30.28 29.78 |
30.27 29.74 |
30.21 29.71 |
30.18 29.77 |
30.15 29.53 |
|
| September | 30.27 29.68 |
30.22 29.46 |
30.27 29.70 |
30.24 29.56 |
30.39 29.80 |
30.18 29.49 |
|
| October | 30.47 29.44 |
30.28 29.55 |
30.30 29.71 |
30.56 29.65 |
30.42 29.62 |
30.25 29.41 |
|
| November | 30.50 29.71 |
30.40 29.49 |
30.42 29.47 |
30.53 29.44 |
30.36 29.59 |
30.53 29.47 |
|
| December | 30.59 29.71 |
30.31 29.34 |
30.59 29.24 |
30.68 29.53 |
30.47 29.50 |
30.61 29.59 |
|
| Yearly Extremes | 30.59 Dec 3 28.15 Aug 27 |
30.47 Apr 14 29.29 Feb 6 |
30.59 Dec 12 29.24 Dec 19 |
30.71 Jan 21 29.44 Nov 30 |
30.56 Jan 11 29.06 Apr 15 |
30.61 Dec 9 29.32 Jan 14 |