WESTERN REGION TECHNICAL ATTACHMENT
NO. 04-01
MARCH 3, 2004
STUDY OF DENSE FOG AT THE
Jonathan
Slemmer
National Weather
Service (NWS) Center Weather Service Unit (CWSU)*
(*currently affiliated with NWS Aviation Weather Center, Kansas City, MO)
This study will investigate dense
fog at the Salt Lake City International Airport (SLC) and its impacts on aviation.
For the purposes of this paper, "dense fog" is defined as a reported
surface visibility equal to or less than one-quarter statue mile (SM) without
precipitation other than snow grains. SLC lies at the bottom of the
Many dense fog studies have concentrated on detecting
dense fog events using satellite imagery. There is a comprehensive paper on
dense fog climatology along the
The history of observations and climatology of dense fog at SLC will be discussed
first. A comparison of surface visibility versus Runway Visual Range (RVR) and
surface visibility versus tower visibility follows. Some interesting cases of
the conditions leading to the formation of dense fog during the 2000-01 and
2001-02 Great Basin winters will then be presented.
The SLC dense fog climatology was
derived from 30 years of surface observations beginning in July 1971 and ending
in June 2001. Even though the surface observing site has moved several times
since 1971 (Alder et al. 1998), the sites have been located relatively close
together in the southeastern part of SLC, thus allowing
for generally consistent observation locations. The station was initially located
at 174 North and 2300 West at the southeastern side of the airport with reporting
by a human observer. On
The average number of hourly dense fog occurrences per season is 58 (Fig. 1). Hourly dense fog occurrences have been as high as 266 in 1980-81 and as low as 0 in 1977-78. Four seasons had more than double the average occurring in 1975-76, 1980-81, 1984-85, and 1991-92. Eleven seasons had less than 15 hours of dense fog including 1971-72, 1973-74, 1974-75, 1976-78, 1978-79, 1981-82, 1993-94, 1994-95, 1995-96, 1996-97, 1998-99. Every dense fog season since 1993-94 has had below normal hourly dense fog occurrences. Hill (1988) suggests that the size of GSL may influence the amount of moisture in the air resulting in more dense fog when GSL levels are higher and areal coverage is greater. However, the greatest and least seasonal frequency of dense fog at SLC in this study occurred in 1980-81 and 1977-78, respectively, when GSL levels were near normal historical levels(USGS 2004; see http://ut.water.usgs.gov/infores/gsl.intro.html). There are probably more significant contributions to the development of dense fog such as the presence of Deep Stable Layers (DSL=s; Wolyn and McKee 1989). It's important to note that the 2000-2001 season had 106 observations with dense fog reports although only 49 of these observations (46 percent) were reported at the surface. This is due to dense fog being reported at the tower and not at the surface sensor. Similarly, the 1999-2000 and 2001-02 seasons had numerous observations reporting dense fog at the tower but not the surface. This will be discussed later in this paper.
Dense fog is most common in January and December when insolation is at its weakest (Fig. 2, Fig. 3). The average frequency of dense fog reported at the surface in the hourly observations is 3.4 percent in January. Even though insolation typically increases in February, enough cold air and snow cover may be present to create dense fog. November and March have had rare episodes of dense fog while all other months have had statistically insignificant occurrences.
Dense fog occurrences per day (Fig. 4) rapidly increase in the middle of December and fluctuate significantly through the middle of February. These data was smoothed over 9 days to reduce the fluctuations. Three notable peaks occur in mid-December, around the first of the year, and the later part of January. Two big dips are evident in late December and mid-January. The dense fog occurrences from the middle to later part of January gradually decrease through late February. The daily dense fog fluctuations during winter are also supported by Alder (1998) when he looked at the frequency of daily dense fog occurrences from 1928 to 1998. The 1968-96 daily climatology of several upper-air fields (e.g., 500-mb heights, 250-mb winds) obtained from the National Climate Data Center web-site (see http://www.cdc.noaa.gov/cgi-bin/Composites/printpage.pl) were studied corresponding to the dense fog Apeaks and valleys@ in December and January. No significant correlations were found, such as stronger ridging with higher occurrences of daily dense fog. The fog climatology data set may not be large enough to naturally smooth these peaks and valleys.
Dense fog is most common from 0600 to 0800 LST (Local Standard Time) and is least common from 1200 to 1600 LST (Fig 5). A significant decrease in dense fog frequency occurs between 0900 and 1000 LST. Dense fog frequency gradually increases from 1700 through 0700 LST. From 0600 to 0800 LST, the typical year has an average of 4 to 5 occurences of dense fog per hour. From 1200 to 1600 LST, the typically year has an average of one dense fog occurrence per hour.
When wind speeds are not calm (less than 3 kt), dense fog most commonly occurs with a wind direction from the southeast or northwest (Fig. 6). These wind directions are common at SLC nearly any time of the year because of local mountain-valley circulations. When the synoptic scale has little impact on the Salt Lake Valley=s weather and diurnal circulations dominate, the southeast winds last approximately two times as long as the northwest winds during the winter (wind directions of 120 to 190 degrees true occur 49.8 percent of the time while wind directions of 290 to 360 degrees true occur 27.5 percent of the time during the period of December through February). Thus, there is a higher probability of dense fog occurring when there is a northwesterly wind since this tends to advect low level air masses that are colder and have higher relative humidities.
The majority of dense fog occurs when wind speeds are 6 kt or less (Fig. 7). Dense fog is rare when wind speeds are 9 kt or greater, with the highest frequency near 4 kt. This may be a reflection of dense fog advecting in from GSL. The greatest frequency of dense fog occurs when the temperature ranges between 23 to 27 oF (Fig. 8). Approximately 90 percent of dense fog occurs between 13 to 32 oF. Dense fog is rare when temperatures get much above freezing.
The average dew point depression when dense fog occurs is 2.2 oF (Fig. 9). Dew point depressions should be close to zero when dense fog is present, except when fog forms as a result of saturation with respect to ice (Frisbie 1995). Approximately only 3 percent of the dense fog observations in this study were cold enough for conditions to be saturated with respect to ice. However, the climatology revealed some unusually high dew point depressions. Most winters had dew point depressions of 2 oF or less, but the winters of 1972-73, 1980-81, 1983-84, 1984-85, and 1985-86 had a much greater frequency of high dew point depressions (Fig. 10). It=s surmised that the equipment was poorly calibrated or of low quality during these periods, particularly in the mid 1980=s.
The greatest frequency of dense fog occurs when the altimeter ranges between 30.20 to 30.29 in Hg (Fig. 11). Dense fog usually does not form with an altimeter reading below 29.90 in Hg.
Most dense fog events last for only
four consecutive hours or less (Fig. 12). A significant
number of dense fog events occur over a period of 5 to 10 hours. One extreme
event had 28 consecutive hours of dense fog which began at 0600 LST on
When the visibility decreases to four miles or less and the tower and surface visibilities differ, the lowest visibility becomes the prevailing visibility in the observation while the higher visibility is mentioned in the remarks.
Prior to November 1999, the SLC tower was approximately 100 feet above the ground and located near the middle of the airport property. The frequency of dense fog reports at the tower with no dense fog reported at the surface occurred approximately 9 percent of the time. In these cases, low clouds or dense fog may have reduced the visibility to less than one-quarter mile at the tower, while none was occurring at the surface station.
Since November 1999, the tower has been located approximately 374 feet above the ground on the northern side of the airport [2] . During the winters of 1999-2000 through 2001-2002, the tower reported dense fog 46 percent of the time, while at the same time the surface had no dense fog. This increase in dense fog occurrences at the tower and not at the surface may be due to the increased height of the tower and the movement of the tower further north. The increased tower height makes the tower more susceptible to reporting low clouds as dense fog. The movement of the tower further north may also make the tower more susceptible to shallow cold pools advecting from GSL.
An aviation forecaster shall forecast a visibility that is representative of expectant surface conditions when composing a Terminal Aerodome Forecasts (TAF). Since the tower visibility may not be representative of conditions at the surface, aviation forecasters should be aware that the tower visibility may be reported as the prevailing visibility in the surface observation. Researchers should also realize that a visibility climatology for some sites may be a mixture of surface and tower reports.
Though surface visibility is fairly
representative of RVR reports, this is not always the case. One of the main
reasons for the difference between surface and RVR visibilities is the location
of the observations. The ASOS is located at the south central end of SLC, while
the RVR=s cover the northern, middle, and southern sections of all the
runways. During the winter, dense fog associated with shallow cold pools occasionally
advects from around the
In these cases, RVR sensors on the northern and western ends of the runways frequently report low visibilities before they are reported by ASOS. In some of these cases, dense fog never develops at the southeastern end of the airport. This has produced situations where RVR readings are below 1400 feet for an extended period of time towards the northwestern side of the airport, and surface visibilities in excess of 3 miles at the southeastern end of the airport.
The Federal Aviation Administration (FAA) uses airport RVR thresholds when issuing air traffic ground delays and stops. When dense fog is reducing RVR values enough to have impacts on air traffic operations at SLC, the surface observation occasionally reports no dense fog, and in unusual cases visibilities in excess of 3 SM.
Though RVR data would be of potential use to an aviation forecaster, who can be alerted to the potential development of dense fog at the ASOS, the forecaster is currently unable to access RVR data.
The dense fog reports occurred primarily between
When dense fog was reported at the surface, the RVR=s frequently reported visibilities at or below 1200 feet. The surface high pressure was relatively strong in both cases with altimeter ranges of 29.96 to 29.99 in Hg during the 10 January case, and 30.39 to 30.47 in Hg during the 20 January case. Even though an approaching storm system might be able to weaken an inversion, it may not be strong enough to eliminate the cold pool. If light precipitation falls into the cold pool, the additional moisture may lead to the formation of dense fog.
Shallow Cold Pool Advecting from GSL Cases - Several of the dense fog events, particularly during December 2001 and January 2002, occurred as a result of radiation fog developing over GSL that eventually advected over SLC. Because of slight elevation gains from north and west of SLC to the south and east, the dense fog was sometimes slow to move to the southeast and over the airport.Larry Burch (Meteorologist in Charge at NWS CWSU SLC) participated in several fog dispersion flights during some of these dense fog advection events from the GSL. On these flights, dry ice was crushed and emitted from the plane around the airport. The dry ice is made of hydroscopic condensation nuclei that cause suspended water droplets to condense and precipitate from the atmosphere as snow grains when temperatures are sufficiently cold. The fog dispersal activities can be effective at significantly increasing visibilities for a period of time. On a couple of these flights, the fog was observed to form Afingers@ near and over GSL (an undulating surface pattern of fog, then clear skies). This shows the development of dense fog can be very sensitive to slight atmospheric differences within a small area.
When a shallow cold pool with dense fog moves over
the airport, a dramatic temperature drop typically occurs. Although these dropping
temperatures and visibility reductions usually occur together, their relationship
and dependency is unclear. A recent case of a shallow cold pool advecting over
the airport from GSL has shown that the temperature can drop dramatically without
a great reduction in visibility. An observation from
These cases are particularly difficult to forecast as the shallow cold pools may Aslosh@ (progress and retreat) over the airport several times per event. Operational models have a very difficult time predicting the movement of these shallow cold pools, even in weak flow situations. Typically in these cases, models will forecast a diurnal wind pattern (southeast drainage during the night and morning, and northwest during the afternoon and early evening).
The period of December 2000 through January 2001 had 54 percent of the dense fog observations reported at only the tower, 43 percent reported at the tower and surface, and 3 percent at only the surface. These statistics were greatly influenced by the DSL event in late December and early January.
The period of December 2001 through January 2002 had much shorter lived fog events and mostly resulted from shallow cold pools advecting from GSL. As a result, dense fog observations as reported at the tower only were much lower than the previous winter at 23 percent. Dense fog was reported at the tower and the surface 54 percent of the time, and 23 percent at the surface only.
This study examined dense fog and its impacts to aviation at the Salt Lake City International Airport. The 30-year dense fog climatology showed the highest occurrence of dense fog to be during the winter when there is a minimum of solar radiation. Dense fog typically formed during the early morning hours when temperatures are on average at their daily minimum. There are an average of 58 hourly occurrences of dense fog per winter. Several winters have had very little dense fog while the winter of 1977-78 had no dense fog reports at all. Dense fog occurrences are most likely when the surface temperature is in the 20=s oF, the dew point depression is 3 degrees or lower, the wind is 6 kt or less, and the altimeter is 29.90 in Hg or higher. Dense fog most frequently occurs with a southeast or northwest wind, although a northwest wind is preferred. Prolonged dense fog events usually do not exceed 12 consecutive hours in duration, typically beginning in the evening hours and lasting through the morning. In one rare event, dense fog lasted 28 consecutive hours.
Surface observations were used to develop the dense fog climatology. When dense fog is reported in the surface observation, commercial aircraft operations are usually impacted. However, the FAA uses RVR visibility thresholds for issuing airport ground delays and stops. Surface observation visibilities and RVRs are usually consistent, but can differ significantly due to local effects such as when dense fog advects in from the GSL and to the location of the equipment.
Another potential problem with using surface observations arises when the tower visibility replaces the surface visibility. In these cases, the visibility at the tower becomes less than 4 SM and is less than the surface visibility. Until November of 1999, when the tower visibility was taken approximately 100 feet above the ground, dense fog reported without dense fog at the surface occurred approximately 9 percent of the time. Since then, the tower visibility is reported at 374 feet above the surface, significantly increasing the frequency of dense fog occurring at the tower while not occurring at the surface to 46 percent. This large discrepancy is due to the tower sometimes reporting dense fog while in low clouds. Thus, tower visibility would not be a good indicator of dense fog impacts to SLC air traffic.
Dense fog has also been observed to occur at SLC
when deep stable layers form, when precipitation has fallen into a weakened
and shallow inversion, and when shallow cold pools advect from the
Alder, W., Nierenberg, L., Buchanan,
S., Cope, W., Cisco, J., Schmidt, C., Smith A., and Figgins, W., 1998: Climate
of
Alder, W., 1998:
Frisbie, P., 1995:
Fog Climatology at
Hill, G., 1988: Fog Effect of the
Hogan, D., 1998:
Leipper, D., 1994: Fog on the
USGS, 2004: Great Salt Lake web page (see http://ut.water.usgs.gov/infores/gsl.intro.html)
Wolyn, P. and McKee, T., 1989: Deep Stable Layers
in the Intermountain
[1] Federal Aviation Administration, Administrators Fact Book, April 2000. (return to text)
[2] Federal Aviation Administration, Approach
Plates for Salt Lake International Airport, April 18, 2002.