YEAR Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Annual 24-hr Avg. 1990 17106 18459 18608 18818 17204 18048 17541 18096 18179 18276 17940 15384 17795 1991 16436 18045 17458 19514 17741 18667 18107 1992 16220 17650 18807 17075 18429 18723 18869 18956 19146 18164 17544 1993 17384 19444 19756 21187 19438 20247 19085 18918 21571 20943 19881 19023 19731 1994 18778 21054 21878 22033 20512 20908 19241 20472 22413 21901 21240 19541 20820 1995 20038 22193 22199 23155 21461 21867 19964 21305 23048 22915 21377 19841 21614 1996 20093 22267 21864 23320 20556 20313 19072 20715 22725 21877 21134 17953 20991 1997 18988 21562 21319 23039 21416 20822 19008 20039 22293 21741 20699 18802 20811 1998 18219 20982 20535 21653 20239 20357 18691 20031 21882 21768 20339 18874 20298 1999 20150 20962 20923 22282 21185 20809 20079 20844 22074 22155 20542 18908 20909 2000 19170 21683 21426 22502 20670 20356 19070 20416 22297 22679 20961 19504 20895 2001 19046 21306 21254 22330 19917 19520 18546 19611 20645 20894 19598 17651 20027 2002 17967 19807 19162 21427 19346 18203 17359 18538 20259 20606 19303 18136 19176 2003 18652 20039 19154 20950 19355 21371 18019 18726 21337 21107 18747 18239 19641 2004 18241 20025 20616 21515 19253 18848 17680 18546 21986 20678 19663 18124 19598 2005 18697 20446 20345 21329 19365 18779 17794 19848 20688 20384 19150 17666 19541 2006 18020 20215 19971 20677 19471 18494 21167 20860 19390 17501 2007 18372 20423 20235 20971 19505 18750 17673 19191 20718 21768 19452 17541 19550 2008 17053 19984 19314 21376 19821 17781 17436