The Federal Aviation Administration's (FAA) new generation Runway Visual Range (RVR) system was first placed into service in 1994 and is now operational at many major airports throughout the United States. During the last three years, the Volpe National Transportation Systems Center has monitored RVR data at a number of airports as part of a program to assess the operational performance of these systems. This paper utilizes data collected on RVR at O’Hare Airport in Chicago, IL in order to examine the variability of Category I, II and III conditions as a function of time of year and time of day. Chicago O’Hare Airport has 12 RVR sensors along its seven runways spread out over around ten square miles. Techniques used in a similar analysis of Portland and SeaTac airports are adapted for use in this study. The findings provide important insights into both the similarities and differences in extinction coefficient and RVR that occur across the airport surface domain. Category II and III conditions tend to occur around the same time periods of the year, beginning in early winter and ending around early spring of the next year. A preliminary examination of Category II and III events during one of the peak months reveal event durations ranging from several minutes to about 13 hours. The event onsets and decays did not exhibit a strong affinity for time of day as to morning, afternoon and evening periods. Occasional clustering of major RVR events tended to occur in sequences that last for several consecutive days and are most prevalent during winter months.