Christopher S. Horne
National Weather Service Office
The accurate forecast of the onset and dissipation of low clouds and fog is a major factor contributing to the safety of general aviation. Another concern to the aviation community is the additional financial cost incurred to flight operations when functioning in regions where poor or marginal weather exists, and/or is predicted.
Decisions to carry extra fuel, determine an alternate destination airport or even modify a flight plan are often made based on information in terminal forecasts. The importance of accurate prediction of weather conditions is very important to smooth aircraft operations and to the National Weather Service's mission and reputation.
This paper will present the climatology of fog development and dissipation at Heilig Field, and along the North Platte River Valley near Scottsbluff, Nebraska. The intent of this work is to provide helpful information to forecasters predicting critical ceiling and visibility values at Scottsbluff.
An in-depth study of surface observations taken at Heilig Field was conducted for 1989-1994. During this six-year period, a total of 172 fog events occurred, with 52 of them dense (1/4 mile or less visibility). These numbers are for events that occurred without significant precipitation; however, those accompanied by flurries, drizzle or freezing drizzle was included. In addition, a general overview of all weather records since January 1943 was conducted. Some personal observations of fog events at Scottsbluff were included as well. A fog event is any period the horizontal visibility was reduced to less than 7 miles, with the primary obstruction to vision being fog (NWS 1994). Among the variables examined were surface wind direction and speed, precipitation occurrence before the incident, fog onset time and dissipation.
Scottsbluff averages 33 fog events each year, eight of these resulting in dense fog. Distribution of these occurrences, as shown in Figure 1, is spread somewhat evenly throughout the year, with a noticeable minimum in summer and maxima during early spring and late autumn. These findings are closely related to the average duration of dense fog events (Figure 2), where the minimum occurred during the summer months and maxima in early autumn and mid winter.
Figure 1. Average number of fog days per month, 1943-1994.
Figure 2. Average length of dense fog events, 1989-1994.
The average fog development and dissipation time, Figures 3 and 4, show that the typical development times throughout the year occurred around 0500 or 0600 local time, with the average dissipation time around 0900 and 1000 local time. During the warm season (April through September), the majority of events occurred between the typical development and dissipation times. However, in the cool season (October through March), events tended to be more variable, often developing as early as midnight and sometimes lasting past noon. The noticeable maximum of fog duration time during the winter months likely relates to the lack of significant surface heating, temperature inversions and snow cover.
Figure 3. Warm season fog development and dissipation times, 1989-1994.
Figure 4. Cold season fog development and dissipation times, 1989-1994.
Incidences of both upslope and radiation fog were observed during the year. Upslope fog is defined as a type of fog that forms when air flows upward over rising terrain and is, consequently, adiabatically cooled to or below its dew point. Radiation fog is a type of fog produced over a land area when radiational cooling reduces the air temperature to or below its dew point (Huschke 1989). The semi-arid climate of Scottsbluff along with it's High Plains location, favors upslope fog, which accounted for most of the events during the period 1989-94.
Radiation fog was less common, mainly due to the infrequency of a shallow surface layer of moist air, which is a major factor favoring the formation of radiation fog. However, this variety was observed after significant rain or snowfall. The increased strength of surface-based temperature inversions and the presence of strong areas of arctic high pressure was two factors that tended to favor radiation fog during the winter. During radiational events, fog tended to be more shallow, accompanied by mostly clear skies and an average wind speed of five knots or less. In addition, several times during this period radiational fog resulting from snowcover of 3 inches or less produced visibilities less than 1/4 mile with only a partial obscuration.
One of the most important factors in fog development appeared to be surface wind direction. The most favorable direction, as shown in Figure 5, was east-southeast (personal observations seemed to confirm this). Because the majority of fog events are the upslope type, this direction provides the best upslope component and is aligned with the North Platte Valley that extends from east-southeast to west-northwest.
Figure 5. Prevailing wind direction during dense fog evens, 1989-1994.
Near Scottsbluff, the North Platte Valley is about 20 miles wide with hills projecting to both the north and south, parallel to the river. Some projections to the south of the North Platte River have rises of nearly 1000 feet, so it is assumed that localized downslope south or southwest winds prevent any significant fog with these conditions. On the other hand, hills to the north of the river are more gentle, with rises less than 400 feet. North or northeast surface winds are also somewhat unfavorable for fog development, but cases have been observed where these directions supported short duration events. With north or northeast winds, fog will often be confined along the river itself and in the city of Scottsbluff which lies nearly 100 feet lower in the valley than Heilig field.
As shown in Figure 5, a second maximum was noted with northwest winds. Of the 172 events observed during the period 1989-1994, 27 (16%) occurred with northwest winds. The fog usually developed after the passage of a cold front, in the northwest or north surface flow on its poleward side. The timing was typically 2-4 hours after frontal passage, and of much shorter duration. This post-frontal fog development was often the result of precipitation falling into cold stable air raising the dew point temperature (Huschke 1989).
While fog occurs infrequently in the High Plains, accurate forecasts can be made if certain synoptic and mesoscale conditions are recognized. Surface wind direction is the major factor in contributing to fog events at Scottsbluff. East to southeast winds was present in 89 (52%) of all fog cases over the past six years and surprisingly around 27 (16%) were supported on northwesterly winds. The winter months were more favorable for development as well, for both the number of occurrences and fog duration. It appeared the longest and most numerous fog events were associated with arctic anticyclones building southward from the Northern Plains into the Western and Central Plains. This pattern resulted in an east to southeast upslope flow across the Nebraska panhandle.
The author wishes to express his appreciation to Steve Byrd (SOO, NWSFO Omaha) and Pat Spoden (SOO, NWSO Paducah) for their review and valuable input on this paper.
Huschke, R.E., 1989: Glossary of Meteorology. American Meteorology Society, Boston, MA, 638 pp.
National Weather Service, 1994: National Weather Service Observing Handbook No.7, Surface Observations. Government Printing Office (GSA), Silver Spring, MD, 361pp.