Year Jan. Feb. Mar. Apr. May. Jun. Jul. Aug. Sep. Oct. Nov. Dec. 24-Hour Annual Avg. 
1998 4163 4721 5213 5651 6123 6221 5644 6358 6289 6244 5452   
1999              
2000     6538  7856 7579 6563 5952 5323 4887  
2001 5647 5655 6562         4389  
2002              
2003  8165 8214 8939          
2004      7701 8629 8893      
2005              
2006              
2007              
2008              
2009              
2010              
2011              
2012              
2013   7568 8233 9331 9972