| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1991 | 35.1 | 39.2 | 46.7 | 57.4 | 65.9 | 71.4 | 73.6 | 72.1 | 64.5 | 54.7 | 38.7 | 42.4 | 55.3 |
| 1992 | 35.4 | 41.3 | 45.9 | 52.6 | 60.2 | 68.1 | 72.6 | 67.1 | 65.3 | 52.2 | 40.1 | 40.1 | 53.4 |
| 1993 | 37.5 | 39.4 | 42.2 | 48.9 | 60.4 | 71.3 | 73.5 | 73.6 | 65.4 | 65.0 | 38.5 | 37.4 | 53.6 |
| 1994 | 35.9 | 37.0 | 44.6 | 52.2 | 60.5 | 70.8 | 71.5 | 69.2 | 62.1 | 56.0 | 48.1 | 40.4 | 54.0 |
| 1995 | 37.3 | 37.1 | 47.8 | 50.9 | 63.9 | 67.2 | 71.5 | 73.0 | 63.6 | 50.5 | 42.1 | 45.8 | 54.2 |
| 1996 | 34.2 | 42.2 | 39.9 | 50.8 | 70.4 | 71.2 | 76.2 | 75.1 | 67.0 | 55.7 | 49.0 | 43.5 | 56.3 |
| 1997 | 40.0 | 45.0 | 55.7 | 55.2 | 65.1 | 68.6 | 73.0 | 73.4 | 67.1 | 56.6 | 43.8 | 36.2 | 56.6 |
| 1998 | 42.6 | 39.5 | 45.1 | 51.7 | 68.0 | 77.9 | 80.1 | 77.5 | 75.3 | 61.3 | 52.8 | 43.5 | 59.6 |
| 1999 | 42.7 | 46.8 | 49.9 | 62.0 | 66.8 | 75.3 | 77.2 | 74.2 | 63.2 | 53.6 | 46.9 | 39.4 | 58.1 |
| 2000 | 43.5 | 49.0 | 54.3 | 55.8 | 68.8 | 71.6 | 73.1 | 73.1 | 66.4 | 59.0 | 45.2 | 34.9 | 57.8 |
| 2001 | 35.2 | 44.0 | 42.7 | 57.9 | 63.6 | 68.2 | 74.2 | 73.2 | 65.1 | 51.8 | 50.4 | 41.9 | 55.7 |
| 2002 | 41.6 | 36.3 | 49.4 | 62.3 | 65.3 | 70.4 | 72.7 | 73.6 | 68.2 | 60.8 | 43.4 | 41.2 | 57.1 |
| 2003 | 37.1 | 43.0 | 48.3 | 57.5 | 68.3 | 71.1 | 72.8 | 73.5 | 66.3 | 57.9 | 53.6 | 39.5 | 57.4 |
| 2004 | 40.0 | 41.5 | 52.5 | 55.7 | 66.0 | 71.3 | 72.8 | 68.9 | 65.6 | 66.2 | 50.3 | 34.2 | 57.1 |
| 2005 | 42.0 | 44.4 | 44.7 | 50.9 | 60.1 | 70.6 | 73.4 | 73.1 | 69.7 | 54.1 | 47.5 | 32.0 | 55.2 |
| 2006 | 37.1 | 33.9 | 48.6 | 58.4 | 63.5 | 68.6 | 73.5 | 74.7 | 59.8 | 55.4 | 43.4 | 38.8 | 54.6 |
| 2007 | 35.8 | 37.0 | 49.6 | 50.7 | 63.9 | 71.1 | 71.7 | 75.0 | 68.1 | 58.4 | 47 | 37.9 | 55.5 |
| 2008 | 33.3 | 39.2 | 28.0 | ||||||||||