In an ENVI training course we were advised to check initial visibility on the image acquisition date from the nearest airport or weather station. The problem is that often (especially at airports) horizontal visibility is only recorded up to 10-km or simply as "10-km and above." To an airport, this is "clear," but ENVI help documentation (http://www.exelisvis.com/portals/0/pd... p.24) suggests values of 40 to 100-km as 'Clear,' 20 to 30-km as 'Moderate Haze,' and values of 15-km or less as 'Thick Haze.' This leads to several questions. 1) If I typically work with Landsat 7 and 8, what is the best way I can guess at the initial visibility value I should use, particularly if clouds and haze aren't obvious from looking at the image in some band combination (NIR-R-G, SWIR-NIR-R, nature color, etc.)? 2) How much does having an accurate value for this paramater matter in getting good results from FLAASH with multi-spectral images (Landsat 7 and 8)? I've been told in the past to leave the default at 40-km. 3) What is the effect of low-balling initial visibility, eg. taking the airport station value of 10-km? How would this affect analysis of vegetation health, classifying crops or delineating flooded areas, for example? Thanks!!