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of record as the park staff for trend analysis. Rather, Dr. Michaels used a period of record that 

 tended to minimize the modern day deterioration in visibility conditions at Shenandoah 



To illustrate the misleading character of Dr. Michaels' analysis, I introduced in my testimony the 

 complete historic record of summer visibility data for the southeastern United States, as reported 

 by Husar in the above-referenced State of Science and Technology Report No. 24. Because trend 

 analyses should consider as long a time period as possible to distinguish trends caused by human 

 activity from natural meteorological variability, I believe that this period of record provides a 

 better perspective than the period used by Dr. Michaels. As my testimony correctly stated, 

 visibility in the Shenandoah area generally deteriorated from 1940 to 1970, improved from 1970 

 to 1980, and deteriorated again from 1980 to the present so that current conditions are 

 comparable to 1970 conditions. 



I appreciate this opportunity to clarify my testimony, and I apologize for any problems my earlier 

 statement may have caused. Please contact me at (303) 491-8292 if you have questions regarding 

 this matter. 



William C. Malm, Ph.D. 

 Research Physicist 



