For the purposes of this subchapter:

(1) "Federal Class I area" means those areas listed in ARM 17.8.806(1) and any other federal land that is classified or reclassified as Class I.

(2) "Adverse impact on visibility" means visibility impairment which the department determines does or is likely to interfere with the management, protection, preservation, or enjoyment of the visual experience of visitors within a federal Class I area. The determination must be made on a case-by-case basis taking into account the geographic extent, intensity, duration, frequency, and time of visibility impairment, and how these factors correlate with times of visitor use of the federal Class I area, and the frequency and occurrence of natural conditions that reduce visibility.

(3) "Visibility impairment" means any humanly perceptible change in visual range, contrast, or coloration from that which would have existed under natural conditions. Natural conditions include fog, clouds, windblown dust from natural sources, rain, naturally ignited wildfires, and natural aerosols.