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In 1995, the IMPROVE (Interagency Monitoring
of Protected Visual Environments) Steering Committee, including
the U.S. Forest Service, formed a consensus that five years
of scene monitoring yields sufficient examples of most visual
air quality conditions. To secure a representative set
of observed air quality conditions for each scene monitoring
site, a series of slides is selected from the period of record
and archived on CD-ROM. This series of slides makes
up the historical photographic archive and consists of:
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• A spectrum series of regional
haze visibility conditions observed at the site for
each monitored time of day.
• A selection of the cleanest
clear sky slides to represent seasonal pristine conditions
for each monitoring season.
• Selected visibility and
meteorological episodes or events observed during the
period of record (including wildfire, winter inversions,
and/or regional haze impacts).
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• Selected layered haze events
observed during the period of record.
• Specific slides that show
scenic views of the vista and observations of meteorological
interest during the period of record.
• Historical slide selections
requested by the U.S. Forest Service and/or other air
quality managers that depict visibility conditions for
project-specific reports or public presentations.
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The total number of slides selected for
each historical archive depends on the vista, the variability
in visual air quality, the period of record, and completeness
of the slide database. Final sets can vary from 50 to 150
slides. Master historical photographic archives produced
for each site contain 4 JPEG resolutions of each selected
image (Image Gallery); graphic images of the monitoring location
and site vistas (Site Specifications); as well as associated
cumulative frequency summaries, tables, and/or data listings
(Spectrum Frequencies).
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