The USDOT Volpe Center has for several years monitored the performance of forward scatter-based RVR systems at a number of major US airports. The results often show great variability in visibility along the same runway. This paper examines this phenomenon for a number of RVR events along selected Instrument Landing System (ILS) runways at Denver International Airport (DEN), Portland International Airport (PDX) and O'Hare International Airport (ORD). These airports were selected because of their dissimilar climates. PDX is located next to a river and not very far from the Pacific Ocean; ORD is inland but near Lake Michigan; DEN is well inland and about 1.6 km above sea level. PDX has six visibility sensors (VS) on three runways; ORD has 12 VS on seven runways; and DEN has 13 VS on five runways. Both fog and snow events are considered. The events are selected on the basis of their operational impacts such as severity, timing and duration. Duration, spatial homogeneity and propagation of events across selected ILS runways at each airport are examined. The conditions of fog or snow were determined from official surface weather observation data at each site. Characteristics of the runway such as length, location and orientation are also considered. The results show that significant variation in visibility along runways occurs regularly at all three airports. Several display options are used to demonstrate the phenomenon. In a related matter, the decorrelation time was used as a measure of the persistence of RVR events at each VS site. The results show that visibility during RVR events often experiences significant change over a period of minutes. For instance, approximately 75 % of sampled events appear to decorrelate within 5 minutes, and only around 4 % remained correlated for durations of 15 minutes or longer. This temporal variability is also examined relative to the spatial variability observed along the runways.