There are several data completeness issues that may cause problems within the modeling

analysis. First, a site may have less than 3 years of complete data. This will be a problem for the

purpose of modeling and for tracking progress. States should work with their EPA Regional

Office and Federal Land Managers to determine how to estimate baseline visibility for these


Another issue that is a more specific problem for modeling occurs when data is missing

during the meteorological time period that was modeled. It is likely that most States will only be

modeling a single year (or less). Therefore it is possible that some or all of the ambient data at

one or more Class I areas is missing during that year. Without ambient data, it is impossible to

identify the 20% best and worst days (used to calculate modeled RRFs).

80The f(rh) values in the Tracking Guidance were developed for the “old” IMPROVE

algorithm and should not be used to calculate visibility using the “new” IMPROVE algorithm.

81Bering Sea Wilderness (Alaska) is the only Class I area that has no IMPROVE monitor

and no representative IMPROVE site. On-site IMPROVE or representative IMPROVE data can

be found for the other 155 sites.

82It may be informative to compare model response at the representative IMPROVE site

versus the actual location of the Class I area. Large differences in the model response may

indicate that the “representative” site may not adequately represent visibility at the Class I area.


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