COMPARISON OF THE RESULTS OF 2 MEASURING METHODS DETERMINING THE HORIZONTAL STANDARD VISIBILITY WITH THE VISUAL VISIBILITY RANGE

During tests lasting several weeks the horizontal standard visibility range was determined by two different methods: by means of integrating nephelometers and by means of contrast measurements of natural visibility targets. The three nephelometers used were situated at three different places: one on the roof of a heated building and the other two at distances of 20 and 250 m from this building. To determine the horizontal standard visibility range by contrast measurements of natural visibility targets, edges of forests, at distances of 0.6, 2.3, 6.2 and 6.8 km were used. Values for the horizontal standard visibility range thus determined were compared with the visual visibility range. The latter is the distance at which observers with normal sight can just see the edge of a forest i.e. they can see it with the contrast threshold of the human eye. It could be shown that the nephelometer on the top of the heated building mainly measured standard visibilities which were too large. The reason may have been the vertical air interchange and the lower humidity of the air. The measured values of the two other nepbelometers and those received by contrast measurements of four natural targets at different distances showed good conformity with the visual visibility range in nearly all cases. A few discrepancies between measured and observed values were due to inhomogeneities in atmospheric opacity. At those times the measured and the observed visibility ranges were fluctuating. © 1979.