Slant and Runway Visual Range Relationships

A study was conducted to determine if any significant relationships existed between horizontal and slant visibility  during fog conditions.  Approximately 11,000 sets of atmospheric transmittance observations in fog were obtained at six horizontal levels from 5 to 155 feet and from 5- to 155-foot slant path by extinction-type transmissometers mounted on two airfield towers separated by 250 feet.  Each observation set was classified into one of seven vertical profile classes according to the transmittance difference between the top (155-foot) and bottom (5-foot) transmissometers.  The implications of the various fog structures for aircraft landings are discussed.  Average 5- to 155- foot slant visual versus 5-foot runway visual range (SVR-RVR) ratios and standard deviations of ratios were computed by profile class.  An analysis showed that results are definitive and could form from the basis of a procedure for estimating SVR from RVR through fog profile measurement.  A multiple linear regression analysis to predict SVR from RVR, surface temperature, windspeed, and atmospheric stability showed RVR to be the best predictor of SVR, while the other variables showed some effect only with dense fog.  The results are considered tentative, since they are based on a limited data sample.  Fifteen minute changes in SVR by profile class were examined for certain SVR ranges.  Results showed that knowledge of the profile class would not significantly improve 15- minute persistence forecasts of SVR.