Visibility deterioration may have negative impacts on aviation safety, particularly in landing and take-off procedures. Beside traditional weather observation methods which are based on human visual estimations, automatic weather observation systems (AWOS) are recently used in airports in order to provide instant measurements of the relevant meteorological parameters. One of the most important sensors is the RVR sensor. In this study, the variability of human visibility and optical instrumental visibility, including the meteorological optical range (MOR) and Runway visual range (RVR), along with the probable differences between their values are investigated from December 2019 to February 2020 at Payam Airport, Karaj, Iran. Results indicate that in different weather conditions, the values of MOR and RVR are predominantly greater than human visibility. In this investigation, MOR is about 99% and 96% more than observed visibility for daytime and nighttime, respectively. The probable reasons for such discrepancies between these two sources of data have argued in this paper. However, to overcome the differences between the two methods, an optimized parameter is introduced as visual optical range (VOR). This parameter simulates human’s visibility through MOR to gain the benefits of both methods. Using darkness coefficient, the new parameter depends not only on extinction coefficient but also on sky brightness. The value of 0.07 is proposed for VOR calculation instead of 0.05 in the calculation of MOR. Meanwhile, in the new method, RVR value is also taking the greater values of the Allard’s visibility and VOR rather than MOR. Applying this method, it is observed that VOR values align between traditional and instrumental visual ranges. Furthermore, the new method gives more reasonable results considering minimum values of the Runway’s light intensity over nighttime.