Homogeneity properties of runway visibility in fog at Chicago O’Hare International Airport (ORD)

Spatial correlations of extinction coefficient (ó) are demonstrated along selected Category II and Category III runways for a number of fog events at Chicago O’Hare International Airport (ORD) between 1998-2000. The events were selected on the basis of standard METAR surface reports at the airport and on ó measurements derived from the runways’ forward scatter Runway Visual Range visibility sensors. Event durations ranged from about 3.5-20 hours. Most of the selected events had peak values of ó greater than 30 km-1. ORD has four Category II and two Category III runways. Entire event time cross-correlations were determined as well as cross-correlations for shorter time periods within the same events. In this study, a measure of the spatial homogeneity of fog events is taken as the percentage of time the three runway sensors (touchdown, midpoint and rollout) reported values of ó that are within varying fractions of each other, ranging from 90% to 10%. The results suggest considerable variation in the homogeneity of events. The duration of the various classes of events was also evaluated. Significant variations between the two parallel Category III runways for the same event and within shorter time intervals during an event are often evident. The results not only demonstrate that many low-visibility fog events are inhomogeneous but also confirm the need for the three visibility sensors employed at Category II and III runways.