| Year | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Annual |
| 1991 | 22.0 | 24.0 | 34.0 | 34.0 | 48.0 | 67.0 | 69.0 | 66.0 | 47.0 | 41.0 | 20.0 | 27.0 | 20.0 |
| 1992 | 19.0 | 30.0 | 30.0 | 36.0 | 40.0 | 58.0 | 68.0 | 57.0 | 50.0 | 42.0 | 23.0 | 26.0 | 19.0 |
| 1993 | 26.0 | 30.0 | 21.0 | 34.0 | 49.0 | 57.0 | 70.0 | 71.0 | 49.0 | 25.0 | 22.0 | 32.0 | 21.0 |
| 1994 | 21.0 | 22.0 | 28.0 | 31.0 | 46.0 | 65.0 | 58.0 | 64.0 | 45.0 | 37.0 | 35.0 | 26.0 | 21.0 |
| 1995 | 24.0 | 23.0 | 27.0 | 39.0 | 50.0 | 54.0 | 67.0 | 70.0 | 47.0 | 35.0 | 28.0 | 22.0 | 22.0 |
| 1996 | 15.0 | 18.0 | 24.0 | 32.0 | 41.0 | 59.0 | 72.0 | 70.0 | 48.0 | 38.0 | 32.0 | 24.0 | 15.0 |
| 1997 | 25.0 | 31.0 | 38.0 | 42.0 | 51.0 | 56.0 | 67.0 | 68.0 | 56.0 | 40.0 | 25.0 | 24.0 | 24.0 |
| 1998 | 28.0 | 28.0 | 26.0 | 40.0 | 52.0 | 62.0 | 76.0 | 74.0 | 70.0 | 46.0 | 35.0 | 25.0 | 25.0 |
| 1999 | 16.0 | 29.0 | 31.0 | 36.0 | 52.0 | 68.0 | 73.0 | 65.0 | 46.0 | 37.0 | 28.0 | 25.0 | 16.0 |
| 2000 | 20.0 | 26.0 | 34.0 | 34.0 | 55.0 | 57.0 | 66.0 | 66.0 | 50.0 | 46.0 | 27.0 | 21.0 | 20.0 |
| 2001 | 21.0 | 26.0 | 32.0 | 36.0 | 49.0 | 61.0 | 64.0 | 68.0 | 47.0 | 34.0 | 29.0 | 23.0 | 21.0 |
| 2002 | 14.0 | 18.0 | 16.0 | 45.0 | 47.0 | 60.0 | 68.0 | 68.0 | 57.0 | 44.0 | 28.0 | 26.0 | 14.0 |
| 2003 | 21.0 | 29.0 | 30.0 | 34.0 | 54.0 | 62.0 | 67.0 | 67.0 | 53.0 | 39.0 | 28.0 | 25.0 | 21.0 |
| 2004 | 24.0 | 28.0 | 42.0 | 37.0 | 48.0 | 64.0 | 67.0 | 57.0 | 56.0 | 48.0 | 34.0 | 19.0 | 19.0 |
| 2005 | 24.0 | 28.0 | 32.0 | 36.0 | 42.0 | 64.0 | 65.0 | 67.0 | 60.0 | 33.0 | 27.0 | 26.0 | 24.0 |
| 2006 | 22.0 | 20.0 | 28.0 | 42.0 | 48.0 | 61.0 | 66.0 | 63.0 | 50.0 | 39.0 | 26.0 | 23.0 | 20.0 |
| 2007 | 24.0 | 20.0 | 27.0 | 34.0 | 52.0 | 63.0 | 64.0 | 72.0 | 62.0 | 40.0 | 32.0 | 24.0 | 20.0 |
| 2008 | 21.0 | 26.0 | 28.0 | 35.0 | 46.0 | 66.0 | |||||||