Problems Associated With Atmospheric Visibility Measurement Techniques

AbstractA brief review of the psychophysical relationships which influence an observer's ability to discern barely perceptible changes in apparent contrast is presented with comments related to meteorological observations and instrument calibrations. Examples of how uncer­ tainties in the specification of the atmospheric optical state might influence slant path contrast transmittances are illustrated.IntroductionThe title of this note should really be "The Trouble With Determining Visibility.....", in that determining visibility is in some ways similar to making love, that is, everyone can do it, only some do it better than others. Now it is not within the scope of this short note to tell you all there is to know about either of these two commonly practiced activities, however, we can make a few comments regarding the former, which may eliminate a few of your problems and thus greatly improve your attitude toward the latter.As one might expect, there are a number of ways one might proceed with the determination of visibility, or the sensor equivalent of visibility. A sample of the overall procedural variety is illustrated in Table 1, which runs the gamut from the traditional visual determina­ tions made by each of us from time to time to the more modem attempts made by our mechanical and electro-optical surrogates high in the sky.Table 1. Procedural Techniques for the Determination of Visibility1. Visual Determinations using fixed targets.2. Transmissometer measurements over fixed paths.3. Scatter meter measurements from small illuminated volumes.4. Computations from measured aerosol particle size distributions.5. Lidar Visometer backscatter measurements.6. Satellite remote sensing of tropospheric optical properties.Each of the procedures outlined in Table 1 has its adherents, and each has its place in the hierarchy of experimental and operational scenarios that are addressed in technical proceedings such as those being held here today. This note is not intended to examine all in equal detail, but will comment briefly upon several in which some of our experience may be illuminating, and for which we can present some useful examples and illustrations.Visual DeterminationsThe literature on visibility and its determination is extensive extending from the 179 entries in the annotated bibliography by Grimes (1969),l through the 130 excellent references contained in the comprehensive review by Douglas and Booker (1977).2 It is not totally surprising to find this rich body of resource material since human observer estimates of daytime visibility have been made and recorded routinely by professionals and amateurs alike for both many years and an infinite variety of uses.As we are all aware, the human observer estimates of visibility are ruled by an abundance of physical vagueries related to his obser­ vational scene, all of which tend to influence his perception of that scene and his ability to assess its atmospheric clarity. The touchstones of quantitative determinations of visual range include the theoretical concepts set forth by Koschmieder (1924).3Since the determination of luminance contrast is the fundamental problem underlying this entire discussion, its definition, though well known to most, is specified below: