Effects of nonstandard conditions on visibility measurement

The effects of nonstandard visibility observation conditions upon computed visual range (meteorological range) were investigated by using data from an eastern U.S. visibility study sponsored by the Electric Power Research Institute. Data collected by multiwavelength telephotometry were marked in the field when nonstandard conditions were observed. The nonstandard conditions explored were those that violate the assumptions implicit in routine visual range calculations. These conditions include bright and dark clouds behind the target, bright haze, shadowed targets, and snow-covered targets. The errors caused by nonstandard conditions were evaluated by comparing derived visual ranges and observed luminances along affected and nonaffected viewing paths. The factors introducing error into visual range calculations during bright haze and bright cloud conditions were found to depend strongly upon the relative degrees of light scattering in front of and behind the target. Large errors were noted in visual ranges derived for snow-covered targets if an intrinsic contrast of -1.0 was assumed; a method for approximating intrinsic sky-target contrast was applied to correct these errors. Recommendations are provided for recognition of nonstandard conditions and field flagging of data.