Visibility variability at Seattle, WA and Portland, OR: Insights into the impacts of Runway Visual Range (RVR) measurements on aviation operations

The FAA's new generation Runway Visual Range (RVR) system was first placed into service in 1994 at several key airports in the United States. During the last three years, the Volpe National Transportation Systems Center has monitored RVR data on behalf of the FAA from a number of airports as part of a program to assess the performance of these systems. This paper utilizes data collected on RVR at airports in Seattle, WA and Portland, OR in order to establish the variability of Category I, II and III conditions at these sites as a function of time of year and time of day. The findings provide important insights into both the similarities and differences in RVR that occur at these two airports. The most severe RVR conditions tend to occur during the same periods of the year, starting in late summer and ending around mid-January of the next year. However, most cases of severe reduced visibility are not coincident in time. The variability in RVR values are shown to be dependent on the time of day; the most prevalent times for Cat II and III events begin in the late evening and extend on through to around 11:00 am LST of the next day. The greatest amount of time in Cat II and III conditions occur from around 5:00 am to 10:00 am. There is also an important tendency for these RVR events to occur in sequences that last for several consecutive days. Although Seattle and Portland are both in the Pacific Northwest and experience similar weather conditions, there are significant differences in the periods when Cat II and III conditions occur at each site. This is attributed to the location of Pacific storm tracks relative to the location of the airports. The insights obtained from such analyses of RVR data should prove valuable for airline planning and lead to more effective control of traffic at these airports.