A significant discontinuity in the calculated values of runway visual range (RVR) can occur when the ambient light measurements cross the 2-fL threshold used to discriminate between daytime and nighttime conditions as recommended by international aviation standards. The computations are governed by Allard's Law, and the ambient light is a measure of the estimated background luminance a pilot experiences when making a landing approach. The effects are quantified, illustrated and seen to be strongly related to airport runway light conditions.

In order to determine when occurrences of RVR discontinuities might occur, an investigation of a year's data of background luminance at the Otis Weather Test Facility on Cape Cod, Massachusetts was performed. The results show that this threshold is usually crossed during twilight time, often within 20 minutes or less before sunrise and 30 minutes or less after sunset. The data consist of one-minute running averages reported at intervals of 10 seconds. The time difference between threshold crossing and sunrise or sunset tends to be somewhat longer in the summer months than in other seasons. The time between threshold crossing and sunrise is somewhat shorter during January than the remaining months. The threshold crossing also tends to occur somewhat closer to sunrise than to sunset. The duration of threshold crossings is usually around one minute or less.