| Years | Number of observers | Start date | End date | Number of observations |
| 1993 | 5 | May 5 | November 30 | 338 |
| 1994 | 26 | February 1 | December 27 | 2616 |
| 1995 | 28 | February 1 | December 27 | 4089 |
| 1996 | 30 | January 1 | December 20 | 3596 |
| 1998 | 9 | April 1 | December 30 | 834 |
| 1999 | 19 | April 1 | November 30 | 1786 |
| 2000 | 20 | January 10 | December 30 | 2290 |
| 2002 | 29 | March 1 | November 30 | 3223 |
| 2003 | 29 | March 1 | November 28 | 2872 |
| 2004 | 9 | April 1 | December 28 | 776 |
| 2005 | 20 | April 1 | November 30 | 1704 |
| 2010 | 2 | January 11 | November 29 | 492 |
| 2011 | 3 | January 11 | December 30 | 596 |
| 2012 | 2 | May 3 | December 4 | 224 |