Mark A. Ratzer
National Weather Service Forecast Office
Chicago/Romeoville, Illinois




Fog is but one of the many meteorological phenomena which have a significant impact upon aircraft operations. All airports have minimum requirements regarding ceiling and visibility, below which approaches to the airport are not allowed. Dense fog can reduce visibility below these minimum requirements, resulting in flight delays and cancellations that are costly to the airline industry and irritating to passengers. Because of the large volume of air traffic that operates into and out of Chicago's O'Hare International Airport (ORD), dense fog there can result in flight delays across the nation. By studying the climatology of dense fog and its characteristics at O'Hare, forecasters may be able to produce more accurate fog forecasts for the airport and the surrounding area.



Fog forms when condensation occurs in a moist layer of air at or near the ground. Unlike clouds above ground, which usually form by a process of adiabatic cooling due to lifting of air, fog usually forms in air near the ground as it cools (Houghton 1985). Common types of fog are radiation fog, that often forms on clear, calm nights when the ground cools by radiating infrared heat energy away to space, and advection fog, that forms when a relatively warm, moist air mass is advected over a cold surface such as snow cover (Wallace and Hobbs 1977).



Dense fog, as defined by the National Weather Service (NWS), is fog that reduces horizontal visibility-to-less than 5/16 of a statute mile (NWS 1988). For purposes of this particular study, a dense fog day is defined as a 24-hour day having at least one hourly observation of dense fog, and a dense fog event is used to describe a series of consecutive hourly observations of dense fog, with breaks of no more than four hours between reports of visibility below 5/16 of a mile.

Hourly WBAN observations from O'Hare for the years 1983 through 1995 were obtained from the National Climatic Data Center. This data was re-formatted for use in a spread sheet program, where observations which included both a visibility of less than 5/16 of a statute mile, and which had fog indicated as the primary obscuring phenomena were compiled. Some observations with fog were not used, however, because other obscuring phenomena such as heavy snow or rain were at least partially responsible for the reduction in visibility.



O'Hare Airport is located 18 miles northwest of downtown Chicago, in a generally open, level area surrounded by light industry and commercial development. Lake Michigan lies approximately 15 miles to the east and north of the airport. On the airfield, the terminal complex is located at the center of the airport, with several large aircraft maintenance hangers and air freight buildings on the northwest and southeast sides of the airfield. A small reservoir, Lake O'Hare, is located just to the southeast of the terminal complex. The field elevation is 668 feet above mean sea level. (Figure 1a and b).



In the 13-year span of this period, dense fog with visibilities less than 5/16 of a statute mile was found to have occurred at O'Hare an average of 8 days per year. The year with the greatest number of dense fog days was 1984, when 16 such days occurred. The fewest number of dense fog days occurred in 1994, with dense fog reported on only 1 day that year (Figure 2). Dense fog occurs at O'Hare a relatively small number of days a year, but when it does, it can have a significant impact on the nation's air transportation system.

Figure 1a. A diagram of O'Hare International Airport (ORD).

Figure 1b. Map depicting relative location of ORD to downtown Chicago.

Figure 2. Annual days with at least one hourly observation of dense fog, 1983 - 1995 at Chicago O'Hare.

Winter and Spring (December through May) have the highest percentage of hourly dense fog observations, accounting for nearly 85 percent of the total dense fog hours at O'Hare (Figure 3). This is not surprising however, considering that the low sun angle and short sunlit hours of the winter and early Spring months tend to limit insolation that would otherwise help to dissipate fog. Cold ground conditions and snow cover during this time of the year are also much more conducive to the development of advection fog. In fact, when the snow months (December through March) were compared to the non-snow months of the year, it was found that over 60 percent of the total dense fog hours occurred during the snow months (Figure 4). Because the distribution of dense fog observations from O'Hare seems to be well correlated to the seasons, the remainder of this paper will concentrate on the relationship of each season to the occurrence of dense fog observations.

Figure 3. Distribution of hourly dense fog observations by meteorological season. (Winter: December, January, February; Spring: March, April, May; Summer: June, July, August; Fall: September, October, November).

Figure 4. Distribution of hourly dense fog observations for snow cover and non-snow cover months of the year. Snow cover months for Chicago are typically December through March.



Figure 5 shows the relationship between the time of day and dense fog occurrences from December through February. Although the peak time occurred just after sunrise (0800 L), it should be noted that there were also significant percentages of dense fog observations at other times of the day. Again, the fact that less incoming solar radiation is available to dissipate fog during the winter months is likely a major factor in the pattern shown in this graph, with a much smaller diurnal dependence than during the other seasons of the year. An additional conclusion that could be drawn from this small diurnal variation of winter dense fog observations, is that advection fog, which is far less dependent on the time of day than radiation fog, was prevalent during these months as well.

Figure 5. Distribution of hourly dense fog observations by individual hour, for winter months.

Wind data also indicates that advection plays a large role in winter time dense fog days at O'Hare. The most prominent wind directions found in dense fog situations during the winter were southeasterly and southwesterly (Figure 6). This was to be expected, as warm-air advection with southerly flow over cold ground or snow cover often leads to dense fog in the Chicago area. Interestingly, northeasterly winds accounted for over 16 percent of the winter dense fog observations, which may suggest some influence from Lake Michigan, such as an advection of moist air moving from the lake surface. Occasionally, a cold moist northeasterly flow off of the lake at the surface, combined with warm air advection, aloft, results in a strong inversion layer, trapping saturated air at the surface and resulting in persistent dense fog. This is usually the case when a warm front stretches west to east to the south of Chicago, with low pressure centered to the southwest. The majority of winter fog observations had wind speeds of seven knots or less, and no winter season dense fog was reported with a wind speed of 15 knots or greater.



Characteristics associated with dense fog observations during the seasons of Spring and Fall, were found to be very similar in nature, and will be described together. It should be noted, however, that Spring accounted for substantially more total dense fog observations than did Fall, as indicated in Figure 3. This is likely due to the ground remaining cold as winter transitions into early Spring, resulting in more cases of advection fog.

When comparing Spring and Fall dense fog observations to the time of day, a distinct diurnal pattern becomes evident. The percentage of dense fog observations increased quickly after midnight, peaking just after sunrise (0700 L). Within approximately two hours after sunrise, dense fog observations decreased rapidly, indicating that perhaps the majority of these events were linked to radiational cooling during the overnight hours (Figures 7 and 8).

Figure 6. Distribution of hourly dense fog observations by wind direction for winter months.

Figure 7. Distribution of hourly dense fog observations by individual hour for spring months.

Figure 8. Distribution of hourly dense fog observations by individual hour fall months.

Northerly and northeasterly winds dominated both Spring and Fall dense fog observations at O'Hare, with over 60 percent of the Spring observations, and 45 percent of the Fall observations coinciding with winds from these directions (Figures 9 and 10). 70 percent of all Spring events, and over 90 percent of all Fall dense fog events also had wind speeds of seven knots or less.

Figure 9. Distribution of hourly dense fog observations by wind direction for spring months.

Figure 10. Distribution of hourly dense fog observations by wind direction for fall months.



The summer months had the most distinct diurnal pattern of dense fog observations of any of the seasons (Figure 11). The only dense fog observations found during the summer months, occurred between midnight and 7a.m. The peak time of the day for dense fog during the summer was around 5a.m., that is just after sunrise. This would seem to indicate that almost all of the summer dense fog events were due to radiational cooling.

Figure 11. Distribution of hourly dense fog observations by individual hour for summer months.

Wind direction during summer dense fog cases, as with Spring and Fall cases, showed a distinct trend toward the north and northeast, with over 65 percent of the observations having winds from these directions (Figure 12). Often, a weak cold front will bring precipitation to the area during the afternoon, with winds becoming light from the northeast behind the front. High pressure then builds in with light winds and clearing skies, and along with moist ground conditions left from the preceding precipitation, sets up excellent conditions for nighttime radiational cooling. Further supporting radiational cooling as the greatest contributor to summer dense fog events at O'Hare, 92 percent of the observations had wind speeds of seven knots or less.

Figure 12. Distribution of hourly dense fog observations by wind direction for summer months.



From this study, it can be concluded that dense fog occurring at Chicago's O'Hare International airport can be generally classified into cold season, and warm season events. During the cold months of the Winter and early Spring, the advection of relatively warm, moist air over cold ground or snow cover seems to play an important role in the development of dense fog at the airport. In the warmer months of the year, radiational cooling is likely responsible for most dense fog events. The proximity of Lake Michigan may play a role in fog development at O'Hare throughout the year, as northeast winds tend to be common with dense fog observations. These winds off of the lake are thought to bring in low level air of higher moisture content, as well as aid in the strengthening of temperature inversions across the Chicago area.

To summarize the major conclusions, and to provide some insight into forecasting dense fog at O'Hare International Airport, here are some points to consider:

A. Dense fog is most common at O'Hare in the cold season, from December through April.
Meteorological seasons of Winter and Spring account for almost 85 percent of all dense fog observations at O'Hare.
"Snow months" from December through March account for almost 65 percent of all dense fog observations. Warm advection over cold ground or snow cover is likely responsible for the majority of Winter and early Spring dense fog events at O'Hare.
Half of all dense fog observations are between the hours of 3a.m. and 9a.m., with a maximum near sunrise. The least likely time for dense fog observations at O'Hare is 3 p.m.
The most common wind direction associated with dense fog at O'Hare is northeast. This may be attributed to the low level moistening and cooling effects of Lake Michigan. In Winter, the southerly winds and warm advection are more common with dense fog.



Houghton, D. D., 1985: Handbook of Applied Meteorology. John Wiley & Sons Inc., New York, NY, 1461pp.

National Weather Service, 1988: Federal Meteorological Handbook No. 1. Handbook of Surface Observations. Government Printing Office (GSA), Silver Spring, MD, pp. A7-6-A7-7.

U.S. Weather Bureau, 1954: Terminal Forecasting Reference Manual, Midway Airport Chicago, Illinois. U.S. Government Printing Office, Washington D.C., 14pp.

Wallace, J.M., Hobbs, P.V., 1977: Atmospheric Sciences: An Introductory Survey. Academic Press, Inc., New York, NY, 467pp. is the U.S. government's official web portal to all federal, state and local government web resources and services.