A Climatology and Case Study of Continental Cold Season Dense Fog Associated with Low Clouds

A Climatology and Case Study of Continental Cold Season Dense FogAssociated with Low Clouds

NANCY E. WESTCOTT AND DAVID A. R. KRISTOVICH

Illinois State Water Survey, Institute for Natural Resource Sustainability, University of Illinois

at Urbana–Champaign, Urbana, Illinois

(Manuscript received 27 March 2008, in final form 17 April 2009)

ABSTRACT

This study focuses on dense fog cases that develop in association with low clouds and sometimes precipi-

tation. A climatology of weather conditions associated with dense fog at Peoria, Illinois, for October–March

1970–94 indicated that fog forming in the presence of low clouds is common, in 57% of all events. For events

associated with low pressure systems, low clouds precede dense fog in 84% of cases. Therefore, continental

fogs often do not form under the clear-sky conditions that have received the most attention in the literature.

Surface cooling is usually observed prior to fog on clear nights. With low cloud bases, warming or no change in

temperature is frequent. Thus, fog often forms under conditions that are not well understood, increasing the

difficulty of forecasting fog. The possible mechanisms for fog development under low cloud-base conditions

were explored for an event when dense fog covered much of Illinois on 7 November 2006. Weather Sur-

veillance Radar-1988 Doppler (WSR-88D) and rawinsonde observations indicated that evaporating precip-

itation aloft was important in moistening the lower atmosphere. Dense fog occurred about 6 h following light

precipitation at the surface. The surface was nearly saturated following precipitation, but relative cooling was

needed to overcome weak warm air advection and supersaturate the lower atmosphere. Surface (2 m)

temperatures were near or slightly cooler than ground temperatures in most of the region, suggesting surface

sensible heat fluxes were not important in this relative cooling. Sounding data indicated drying of the at-

mosphere above 800 hPa. Infrared satellite imagery indicated deep clouds associated with a low pressure

system moved east of Illinois by early morning, leaving only low clouds. It is hypothesized that radiational

cooling of the low cloud layer was instrumental in promoting the early morning dense fog.

1. Introduction

In the Midwest fog can be a significant safety hazard,

greatly impacting ground (e.g., Goodwin 2002; Westcott

2007) and air (e.g., Keith and Leyton 2007) trans-

portation. Continental fogs are generally thought to

occur when air is cooled to the point of saturation by

radiation under clear-sky conditions, or when moist air

is advected over a cold surface, such as snow, resulting in

cooling of the overlying air to saturation (Roach 1995).

In fact, much of the current understanding of conti-

nental fogs is based on field studies of radiation fogs

under clear-sky conditions (e.g., Roach et al. 1976;

Meyer and Lala 1990; Jiusto and Lala 1980; Mason 1982;

Turton and Brown 1987). Based on observations taken

in the October–March cold season from 1970 to 1994,

Westcott (2005) found that precipitation is often present

at the onset of dense fog in the Midwest. Tardif and

Rasmussen (2007) likewise found for New York City,

and the nearby metropolitan areas, that fog often occurs

with precipitation or low cloud bases. Fogs forming in

association with or between low pressure systems or

nearby fronts were often observed by early researchers

(e.g., George 1951; Byers 1959) and more recently by

Westcott (2005), Croft and Burton (2006), and Tardif

and Rasmussen (2008). While forecasters are aware that

continental fogs can be associated with low clouds and

precipitation (George 1940; Byers 1959; Croft et al.

1997), few case studies examining processes leading to

the development of fogs over land (e.g., Tardif 2006)

have been reported.

A climatology of fog occurrence is presented for a site

in the continental Midwest. Furthermore, a case study is

presented to illustrate some of the physical processes

Corresponding author address: Nancy E. Westcott, Illinois State

Water Survey, 2204 Griffith Dr., Champaign, IL 61801.

E-mail: [email protected]

VOLUME 48 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y NOVEMBER 2009

DOI: 10.1175/2009JAMC1999.1

� 2009 American Meteorological Society 2201

that may be important for the development of dense fog

under low cloud conditions, or following the passage of

a precipitation system.

2. Methods

a. Climatology

A climatology of surface conditions, before and at the

time of dense fog formation, was developed for all dense

fog events observed at Peoria, Illinois, using hourly

surface airways data for the period 1951–96 (Westcott

2005, 2007). These hourly observations consist of hori-

zontal visibility, as well as standard surface observations

including cloud-base height and prevailing weather. The

areal extent of each fog event was based on surface

airways data collected at National Weather Service

(NWS) first-order stations in the Midwest region. Dense

fog events were defined as having at least one hour of

visibility #400 m (1/4 mi) when fog was reported, simi-

lar to the definition of fog events by Meyer and Lala

(1990). Peoria, located in central Illinois, was found

by Westcott (2007) to be representative of the Midwest.

The Champaign, Illinois, area is about 110-km southeast

of Peoria, and the location of some of the surface ob-

servation sites used in the current case study is similar to

Peoria in that topographic features and urban influences

do not greatly affect the surface observations.

The majority (.80%) of widespread, long-duration

events in the Midwest occur during the October–March

cold season period. These events are the basis of the

present climatology (Westcott 2007). During the 1970–94

cold seasons, 302 dense fog events were identified. Each

of these events was classified by synoptic type based on

3-h surface weather maps. Ten synoptic types were

considered in Westcott (2005). The most notable feature

was that precipitation often occurred within one hour of

the onset of dense fog events associated with low pres-

sure systems and approaching or nearby fronts. Because

of the similarity in the distribution of surface conditions

and prevailing weather at dense fog onset and for sim-

plicity of analysis, the 10 original classifications were

combined into 5 synoptic types for this study. The syn-

optic classification was made with respect to Peoria, at

the time of dense fog onset (Table 1). Examples of the 5

synoptic types are presented in Fig. 1, based on images

from the U.S. Daily Weather Map Series. [Available on-

line at http://docs.lib.noaa.gov/rescue/dwm/data_rescue_

daily_weather_maps.html archived by the National

Oceanic and Atmospheric Administration (NOAA)

Central Library Data Imaging Project for dates prior to

2003, and by the National Centers for Environmental

Prediction (NCEP; available online at www.hpc.ncep.

noaa.gov/dailywxmap/pdffiles.html) for later dates.] Of

the 302 dense fog events, 23% occurred when weather in

Illinois was dominated by a low pressure system cen-

tered in Illinois or one of the surrounding states at

the time dense fog formed, as was the case for the event

on 7 November 2006. Other synoptic categories in-

cluded events associated with approaching or nearby

fronts (35%), with high pressure system (26%) events

that occurred following the passage of a front (7%), or

were between synoptic features (termed transition

events, 9%).

The original climatology was expanded upon for this

study by the inclusion of upper-air data from Peoria. The

current study focuses on available rawinsonde pairs for

those events when the 0000 UTC (1800 LT) sounding

was launched before dense fog onset, and when the 1200

UTC (0600 LT) sounding launch occurred at fog onset

(21%), during dense fog (73%), or within 2 h of the end

of dense fog (6%). There were 120 sounding pairs that

characterized about 40% of the dense fog event sample.

These were employed to characterize the depth of the

moist layer near the time of dense fog events, and de-

termine the possibility of cold or warm air advection at

low levels.

TABLE 1. Synoptic classification of 302 dense fog events occurring at Peoria, during the Oct–Mar period of 1970–94.

Pattern Definition based on 3-h surface maps near time of dense fog onset Sample

Percent

of sample

High pressure High pressure dominates IL weather, often with a low in Canada, off the east coast

or in the southwestern United States, or with a cold front in the high plains states.

78 26

Postfrontal Behind warm front (front north of Peoria, often in south or central WI),

or behind cold front (front east or south of Peoria, often in eastern IL or IN).

22 7

Transition Peoria between some combination of fronts, lows or troughs with the

primary synoptic condition ambiguous; sometimes with an identifiable col.

27 9

Low pressure Low pressure center to southwest or west of Peoria, sometimes ahead of a cold front,

a warm front, or both.

68 23

Frontal Ahead of a nearby or approaching front. Cold fronts often slow moving, to the north

or northwest of IL; warm or stationary fronts often positioned across southern IL.

107 35

All 302 100

2202 J O U R N A L O F A P P L I E D M E T E O R O L O G Y A N D C L I M A T O L O G Y VOLUME 48

FIG. 1. Examples of the five synoptic weather pat-

terns employed in this study: (a) high pressure, (b)

postfrontal, (c) transition, (d) low pressure, and (e)

frontal, adapted from the U.S. Daily Weather Map

Series valid at 1200 UTC. Shown are isobars (black

lines), and precipitation during the previous 24 h ending

at 0600 UTC (shading). Dense fog onset was within

several hours of 1200 UTC.

NOVEMBER 2009 W E S T C O T T A N D K R I S T O V I C H 2203

Precipitation data for the 302 dense fog events at

Peoria were obtained from 3 sources: 1) daily precipi-

tation totals at Peoria, ending at midnight; 2) the NOAA

Daily Map Series daily precipitation analyses for the

surrounding region, ending at 1200 UTC (0600 LT); 3)

hourly observations of prevailing weather (precipitation

type) at Peoria. The midnight observation at Peoria and

the Daily Weather Map Series data provide an indication

of availability of moisture for fog development at Peoria

and in the general region. Peoria is not one of the sites

analyzed on the Daily Weather Maps, so there is not

perfect agreement between the maps and precipitation at

Peoria. In all cases in which precipitation was observed

near the time of dense fog onset and in most cases (.90%)

in which precipitation was observed to occur within the

previous 24 h at Peoria, an area of precipitation was ob-

served on the Daily Weather maps. In cases in which

precipitation was not specifically observed at Peoria, it

usually was observed within about 100 km. This suggests

that there may be cases where precipitation was in the

immediate area, but not reaching the ground.

b. Case study

Operational observations taken by NOAA facilities

and local field experiment observations in central Illi-

nois were examined to obtain as complete a description

as possible of the 6–7 November 2006 dense fog event

(Fig. 2). Surface observations from NWS Automated

Surface Observing System (ASOS) and Federal Aviation

Administration (FAA) Automated Weather Observing

System (AWOS) locations in Illinois and Indiana, taken

at time intervals of 20 min to 1 h, were obtained from

NOAA’s Midwestern Regional Climate Center (avail-

able online at http://mrcc.isws.illinois.edu) at the Illinois

State Water Survey. These sites record visibility, cloud-

base height, and precipitation, as well as standard me-

teorological variables. Hourly-averaged data from the

Illinois Climate Network (ICN) gave valuable addi-

tional information on standard meteorological variables,

precipitation, net radiation, ground temperatures, and

10 cm (4 in.) soil temperature and moisture at 19 sites

throughout Illinois. In addition, data taken at the Uni-

versity of Illinois Willard Airport AWOS (KCMI) about

5 km south of Champaign gave detailed information on

the evolution of the fog. An AmeriFlux site (Billesbach

et al. 2004), located about 4 km south–southwest of

KCMI, provided information on surface sensible heat

fluxes, and 2-m air and ground temperatures; a duplicate

set of most instrumentation was located at a second tower

about 4 km south of KCMI. The two AmeriFlux towers

were separated by 2 km. While visibility was not mea-

sured by AmeriFlux, the temperature, humidity, wind

measurements, and temporal trends were quite similar to

those at KCMI and at the ICN sites at Bondville, Illinois,

and Champaign.

For fog development in cases of low clouds and pre-

cipitation, it is anticipated that variations in atmospheric

conditions throughout the lower troposphere might be

important. High-resolution (6 s) rawinsonde observa-

tions taken near the center of the area of dense fog

development were obtained from the NOAA Weather

Service Forecast Office in Lincoln, Illinois (KILX). These

observations were taken at 0000 and 1200 UTC (1800 and

0600 LT) on 6–7 November 2007 and examined using the

Universal Rawindsonde Observation Program (RAOB)

software (Environmental Research Services 2006).

Weather Surveillance Radar-1988 Doppler (WSR-88D)

observations taken at Lincoln, were examined to under-

stand the potential impacts of precipitation on fog de-

velopment in central Illinois. Archived level II data

were obtained from the National Climatic Data Center

(NCDC; available online at http://hurricane.ncdc.noaa.

gov/pls/plhas/has.dsselect) and viewed using the software

program GRLevel2Analyst (Gibson Ridge Software

2006). Satellite observations gave critical information on

the spatiotemporal evolution of cloud systems in the re-

gion of fog development. Visible and infrared imagery

taken by Geostationary Operational Environmental

Satellite (GOES-8) were examined. This imagery was

obtained from the National Aeronautics and Space

Administration (NASA) Langley Cloud and Radiation

Research Group Web page (available online at http://

www-angler.larc.nasa.gov/).

FIG. 2. Location of AWOS/ASOS (labeled METAR) and ICN

surface data sites and the Lincoln, (ILX) radar and rawinsonde site.

Inset indicates location of AmeriFlux, AWOS site, and ICN sites

in the Champaign (CMI) area. Vertical line indicates location of

radar cross section of Fig. 5.


3. Climatology

In a number of ways, fogs forming under differing

synoptic conditions in Illinois are quite similar (Table 2).

Dense fog is usually a nighttime event. It often occurs

with low wind speeds and when surface flow is from the

south or east. Further, it often occurs when snow is not

present within 100 km of Peoria. There also are some

obvious differences. Winds are usually lighter when high

pressure dominates Illinois (median 2.6 m s21), or when

the weather is not clearly dominated by a major surface

feature (median 2.1 m s21). In comparison with events as-

sociated with high pressure systems, when a low pressure

system is nearby fog is more likely during daytime hours

and with higher (although still light) surface winds (me-

dian 3.6 m s21). However, within any particular synoptic

class, fog forms under a wide variety of surface conditions.

Clouds are known to impact fog development by

delaying its onset or in leading to the rapid termination

of a fog event (e.g., Saunders 1960). Examination of

cloud-base heights (Table 3), however, suggests that fog

in the Midwest often occurs under cloudy conditions.

Employing the cloud-base height threshold of 1 km used

by Tardif and Rasmussen (2007), low cloud-base events

are common, particularly when low pressure centers

(84%) dominate Illinois weather. Low cloud bases also

are common when fronts are approaching or nearby

(67%). The majority of all dense fog events (57%) were

associated with low cloud bases in the 6 h prior to the

onset of dense fog. The impact of cloudiness on changes

at the surface and in the moist layer is examined in the

next two sections.

a. Cloud-base height with surface temperature andmoisture changes

Typically under clear-sky conditions during nighttime

hours, divergence of radiation fluxes cools the surface and

lowest region of the atmosphere. In cases where the sky

was clear, or where cloud bases were higher than 1 km in

the 6 h prior to dense fog onset, made up only 34% of the

dense fog events (Table 3) and were usually associated

with high pressure or followed the passage of a warm or

cold front, or a front without precipitation. Figure 3 re-

veals that temperature falls of 0.68C or more in 6 h can be

expected when cloud bases are greater than 1 km. Dew-

point temperatures were also observed to decrease, but

not as frequently as did temperature. Relative humidity

in the 6 h prior to dense fog onset often increased by

more than 5% when cloud bases were higher than 1 km.

This suggests that radiational cooling occurred, allowing

the relative humidity to increase perhaps without the

addition of moisture. By 3 h prior to dense fog onset,

however, while cooling still frequently occurred, the at-

mosphere was nearly saturated, and so only small changes

in relative humidity were seen. It should be noted that

particularly at nearly saturated conditions, relative hu-

midity changes of 5% may be within sensor accuracy, and

TABLE 2. Percentage of dense fog events by synoptic type at Peoria: occurring when snow was present at 1200 UTC (0600 LT); during

daytime hours 1600–2300 UTC (1000–1700 LT); above or below wind speed thresholds; the median wind speed; and occurring within

direction categories (1358–2258S, 2258–3158W, 3158–458N, 458–1358E), for the 302 events during the Oct–Mar period of 1970–94.

Percent occurrence Speed (m s21) Percent occurrence direction

Synoptic category Snow Daytime ,2 m s21 .4 m s21 Median S E N W

High pressure 27 1 35% 19% 2.6 47 15 17 21

Postfrontal 18 5 28% 34% 2.9 28 22 22 28

Transition 26 11 15% 30% 2.1 46 27 12 15

Low pressure 26 28 13% 41% 3.6 41 28 22 9

Frontal 17 14 18% 32% 3.1 54 32 6 8

All 22 13 22% 31% 48 26 13 13

TABLE 3. Percentage of dense fog events at Peoria, with cloud-base heights .1 km AGL, variable cloud bases, or cloud base ,1 km in 6 h

prior to dense fog onset, for the Oct–Mar period of 1970–94.

Synoptic category Cloud-base height within 26 h of fog onset

Sample Percent of sample .1-km Variable ,1-km no precipitation at onset ,1-km precipitation at onset

High pressure 78 26 62% 11% 23% 4%

Postfrontal 22 7 50% 9% 23% 18%

Transition 27 9 22% 26% 15% 37%

Low pressure 68 23 10% 6% 25% 59%

Frontal 107 35 28% 5% 18% 49%

All 302 100 34% 9% 21% 36%


thus variations at these high values are likely within

sensor noise. When cloud bases are variable in the 6 h

prior to dense fog onset, a weaker but similar pattern of

temperature and relative humidity change is observed.

For low cloud-base dense fog events, small or positive

changes in temperature, dewpoint temperature, and rel-

ative humidity are common in the 6 h (Fig. 3a) and 3 h

prior to dense fog onset (Fig. 3b). In the majority of cases

a warming of $0.68C or no temperature change occurred

over the 3 and 6 h prior to dense fog onset. If precipitation

was present at or near dense fog onset, warming was most

common. Note that hourly temperature was recorded in

FIG. 3. Frequency of relative humidity, temperature, and dewpoint temperature change in the (a) 6 and (b) 3 h

prior to dense fog onset for fog events for cloud-base heights .1 km AGL, variable cloud-base heights, and for cloud-

base heights ,1 km by precipitation occurrence at fog onset for Peoria, during October–March 1970–94. Temper-

ature and dewpoint temperature were recorded in increments of 0.68C (18F).


whole degrees Fahrenheit (0.68C), so more subtle changes

in temperature may have gone undetected. An exami-

nation of dewpoint temperature by cloud-base height

indicated that under low cloud conditions with and

without precipitation at fog onset, the dewpoint temper-

ature often increased by $0.68C in the pre-fog hours. This

occurred with more frequency than did increasing tem-

perature cases, suggesting the addition of moisture into

the surface layer.

For low cloud cases with precipitation at dense fog

onset, the atmosphere often was nearly saturated, with

only 15% of the cases exhibiting increasing humidity by

more than 5% in the 3 h prior to dense fog onset, and

30% increasing more than 5% in the 6 h prior to dense

fog onset. Similar results were presented in Tardif and

Rasmussen (2008) for precipitation fog events. Within

3 h of dense fog onset, in the majority of cases, the at-

mosphere was saturated or nearly saturated no matter

the cloud-base height.

b. Vertical structure of the moist layer fromsounding data

Sounding pairs were examined for 120 events when

dense fog onset occurred after the 0000 UTC (1800 LT)

sounding, with the 1200 UTC (0600 LT) sounding

generally representative of the fog event (94% of the

1200 UTC soundings occurred when dense fog was pres-

ent). The data for this subset of dense fog events showed

some similarity in vertical structure among the events. It

was found that an inversion was nearly always present in

the early morning, no matter the synoptic type or cloud

base presence or height. Of all soundings, the inversion

generally (86%) lowered and/or strengthened from what

was found 12 h previously. The inversion layer was typ-

ically (86%) within or corresponded to the moist layer.

At 1200 UTC (0600 LT), 50% of inversion layers were

less than 172 m AGL (970 hPa), 90% ,500 m AGL

(935 hPa), and 95% ,1000 m AGL (850 hPa).

At 1200 UTC, the depth of the moist layer (T 2 Td #

18C) differed by synoptic type and by cloud-base height

(Table 4). The events with no or high clouds had the

shallowest moist layers. These were dominated by high

pressure and post frontal cases, but many of the frontal

cases likewise showed shallow moist layers. When low

clouds were present, the moist layer was generally deeper.

While it might be expected the moist layer would be

deepest when precipitation was observed near the time of

dense fog onset, this was not always the case (Table 4).

This suggests that observed precipitation sometimes may

have resulted from settling within the fog layer. The depth

of the moist layers found here for no/high cloud events

and for low cloud events, are similar to those for radiation

and coastal advection fog events, respectively, reported

by both Jiusto (1981) and Croft et al. (1997).

Other differences could be observed between synop-

tic type and cloud-base categories regarding changes

in conditions overnight and in conditions present at

1200 UTC. The 0000 and 1200 UTC sounding pairs were

compared to determine the percentage of cases when

cooling or moistening predominated in the moist layer

during the intervening 12-h period. The results were

similar to those found examining surface observations.

Cooling or no change in temperature predominated

in the no and high cloud cases (100%), and in the vari-

able cloud-base cases (.90%). Cooling or no change in

temperature, however, was also often present in the

low cloud cases when precipitation was not occurring

at dense fog onset (;70%), but was less frequent when

precipitation was present (38%). When precipitation

was occurring at onset, temperatures often increased

(62%). Overnight, dewpoint temperature was often ob-

served to increase or remain constant in cases where

precipitation was occurring at dense fog onset (;90%),

and when lows or fronts were present (;75%). This

overnight increase, or constant dewpoint temperature,

suggests that warm moist advection and/or the occurrence

of precipitation preconditioned the atmosphere for fog

formation. In addition, it should be noted that wind pro-

files exhibiting a veering (suggesting low-level warm air

advection) or no obvious shear pattern predominated; no

TABLE 4. Median height of the moist layer (m AGL) from 1200 UTC soundings for dense fog events at Peoria with cloud-base heights

.1 km AGL, variable cloud bases, or cloud base ,1 km in 6 h prior to dense fog onset, for the Oct–Mar period of 1970–94. The sample

size for each category is in parentheses.

Synoptic category Cloud-base height (m) within 6 h of fog onset

Sample 1200 UTC RAOB .1 km Variable ,1 km, no rain at onset ,1 km, rain at onset

High pressure 36 192 (22) 233 (3) 409 (11) (0)

Postfrontal 10 307 (8) (0) 767 (2) (0)

Transition 12 326 (3) 396 (4) 1143 (2) 647 (3)

Low pressure 22 740 (2) 22 (1) 814 (7) 595 (12)

Frontal 39 309 (19) 562 (2) 595 (5) 1267 (13)

All 119 286 (54) 395 (10) 595 (27) 893 (28)


matter the cloud-base height, the synoptic type, or

the height of the moist layer at 1200 UTC (0600 LT).

Backing occurred infrequently and was not associated

with any particular synoptic type, moist layer height, or

synoptic type.

It was suggested by Croft et al. (1997) that radiational

cooling was important in both radiation and coastal

advection fog cases. They found T 2 Td differences of

;208C above the layer of dense fog in their study cases

(excluding cases where precipitation occurred at dense

fog onset), and a rapid warming and drying above the

fog layer (i.e., ‘‘goal post’’ structure). In the Peoria fog

sample, approximately 60% of the soundings showed a

difference of 208C or more above the near-surface moist

layer at 1200 UTC. A T 2 Td difference of 208C or more

was found for 75% of no cloud or high cloud-base events

and for 70% of nonprecipitating low cloud-base cases.

Only 35% of precipitating low cloud-base cases showed

a 208C temperature difference. When cooling rates at

the surface and at 950 hPa were examined over the 12-h

period with regard to the presence of a drying region, no

discernable difference could be found within synoptic or

cloud-base categories. This may not be surprising, as

over the 12-h period between soundings, temperature

change rates could result from changes in air mass, from

storm outflows, and advection, as well as the rate of

radiation flux divergence, or other processes. Nearly

all of the 1200 UTC soundings (95%) had a T 2 Td

difference of at least 128C above the fog layer.

c. Precipitation

Precipitation fogs were described by George (1940),

Byers (1959), and recently by Tardif and Rasmussen

(2008). Light precipitation often occurs at the onset of

dense fog events (Westcott 2005; Tardif and Rasmussen

2007, 2008). This precipitation may be from deep clouds,

or the result of settling from shallow clouds. While the

available datasets do not allow for determination of

precipitation fog processes, the current study quantifies

the frequency of occurrence of regional precipitation

and thus, the potential for precipitation to precondition

the atmosphere for later fog development (Westcott

2004; Westcott 2005; and for the New York City region

by Tardif and Rasmussen 2008).

The Peoria climatology indicated that some 71% of all

dense fog events occurred where precipitation had been

observed within the previous 24 h (Table 5). Precipita-

tion generally occurred during the prior 24 h for low

pressure (;95%) and frontal system (81%) synoptic

categories, and more than half of low pressure or frontal

system events had precipitation at the time of dense fog

onset. In most cases (76%) 24-h precipitation totals were

small (,0.63 cm, ,0.25 in.), and in only four cases were

totals more than 2.54 cm (1 in.). When precipitation was

observed at or near dense fog onset, it was almost always

light (#0.25 cm, 95%), and usually unfrozen (86%).

The frequent presence of regional precipitation sug-

gests that it may aid in preconditioning the low levels of

the atmosphere for fog development, and thus would

have an important impact on forecasts of fog occurrence

and coverage.

4. Case study

To illustrate how some of the events with low clouds and

prior precipitation may give rise to dense fog, as well as

limitations in operational observations for understanding

and predicting fog formation, the widespread dense fog

event of 6–7 November 2006 is explored. In this event,

dense fog developed over much of eastern Illinois. This

event exhibited many of the features found to be common

in the climatological analyses, such as small temperature

changes prior to fog onset, a warm–dry layer above the low

clouds, and precipitation prior to dense fog development.

TABLE 5. Percentage of dense fog events at Peoria, with precipitation observed within 100 km of Peoria on the daily weather maps,

within the previous 24 h at Peoria, or at dense fog onset at Peoria, by synoptic type and cloud-base height categories, for the Oct–Mar

period of 1970–94.

Synoptic category Precipitation

Sample Percent of sample Precipitation within 100 km PIA precipitation Precipitation at onset

High pressure 78 26 42% 33% 5%

Postfrontal 22 7 68% 59% 18%

Transition 27 9 74% 81% 45%

Low pressure 68 23 97% 94% 66%

Frontal 107 35 81% 81% 53%

All .1000 m 102 34 38% 37% 5%

Variable 28 9 64% 64% 29%

250–1000 m 43 14 95% 86% 67%

All ,250 m 129 43 95% 91% 62%

All 302 100 73% 71% 41%


a. Synoptic-scale processes

During the 18-h time period preceding the develop-

ment of widespread dense fog, the atmosphere was

preconditioned by the vertical redistribution of moisture

and horizontal advection into the region by processes

associated with an approaching cyclone. A weakening

area of surface low pressure moved from Oklahoma to

northern Kentucky from 6 to 7 November 2006, trans-

porting a deep layer of moisture northward and north-

westward into the relatively dry Illinois region. As

surface winds turned from southwesterly to southeast-

erly at lower altitudes, precipitation developed to the

south and east of the study region. As the cyclone de-

parted, marked drying was evident at higher levels.

Rawinsonde observations that were taken at Lincoln,

(Fig. 4) show changes in the atmospheric column that

occurred as a result of these processes.

During the nighttime hours of 6–7 November, an im-

portant redistribution of moisture in the atmosphere

took place. Large increases in atmospheric moisture

from 1200 UTC 6 November to 0000 UTC 7 November

(0600–1800 LT on 6 November) were found between

about 3- and 10-km height (Fig. 4). Somewhat smaller

increases were observed below about 1.5 km. Despite

the increase in moisture at higher altitudes, air below

about 850 hPa was unsaturated at 0000 UTC (1800 LT).

Observations from the Lincoln, WSR-88D site (KILX,

Fig. 5), indicated that precipitation moved northward

into the region at altitudes between about 3 and 10 km.

Widespread precipitation was observed in this layer

over much of central Illinois, which evaporated as it

fell into the subsaturated air below. Precipitation

reached the surface about 40–60-km south of Peoria and

briefly in Champaign. Evaporation of precipitation, as

well as continued weak moisture advection from the

southeast, allowed for important increases in relative

humidity. During the period leading to the onset of

precipitation, ceilometer observations at many central

Illinois ASOS/AWOS sites indicated a lowering cloud

base that was accompanied by falling surface air tem-

peratures and rising dew point temperatures and rela-

tive humidities. An example of this is presented for

KCMI (Fig. 6). As the precipitation and deep cloudiness

moved northeastward and away from the region, con-

siderable drying of the atmosphere above ;2 km took

place (Fig. 4).

b. Evolution of weather conditions and densefog development

Fog formed in the early morning of 7 November 2006

in an area of overcast skies and with an absence of

cooling at the surface. Figure 7 shows the area where

FIG. 4. High-resolution soundings taken at Lincoln (a) on

1200 UTC 6 Nov 2006, (b) 0000 UTC 7 Nov 2006, and (c) 1200

UTC 7 Nov 2006. Soundings are shown using the RAOB software

program.


visibilities of 400 m (1/4 mi) or less were observed during

the early morning hours of 7 November, overlaid on an

IR satellite image from 0445 UTC (2245 LT). This shows

that the area of eventual dense fog development was

located in the wake of, and was closely oriented with, a

departing widespread area of deep clouds associated

with the low pressure system. Fog began to develop in

south–central Illinois between 0800 and 0900 UTC (0200

and 0300 LT) and developed to the northeast throughout

Illinois between 1100 and 1400 UTC (0500 and 0800 LT).

Most dense fog dissipated between 1400 and 1530 UTC

(0800 and 0930 LT), except at sites close to Lake Mich-

igan. Sunrise was at 1230 UTC (0630 LT) on this day.

During the evening of 6 November, little or no pre-

cipitation was observed at the surface in the northern

and western portions of the state, while continuous light

precipitation was observed in the southern third of Illi-

nois. After about 0600 UTC (0000 LT), precipitation

became much lighter and more scattered. It was ob-

served at several locations in east–central Illinois until

about 1000 UTC (0400 LT). Virtually no precipitation

was indicated by the WSR-88D in the central Illinois fog

region after 1000 UTC (0400 LT). While precipitation

occurred overnight at many of the sites that later

reported fog, no precipitation was reported within 4 h of

the fog formation or during the fog event at these sites.

In the hours after the movement of precipitation

across the region and before initial development of

widespread dense fog, surface relative humidities in

excess of 90% and low cloud bases were observed over a

very large region, in Illinois and surrounding states.

Despite this, fog only developed in a rather narrow

band stretching much of the length of the eastern and

central regions of Illinois (Fig. 7). Examination of the

evolution of surface and higher-level atmospheric

conditions both in and out of the area of dense fog

formation gives important clues as to the processes that

may have led to fog.

c. Possible processes related to the development ofdense fog

In order for fog to develop in the region after 1200 UTC

(0600 LT), atmospheric moisture must have remained at

near-saturated levels, and a mechanism was needed

to cause the air to become supersaturated. Examination

of the surface temperature field and wind velocities re-

veals that modest warm air advection was present

over most of Illinois with the exception of a small area in

southwestern Illinois. Surface dewpoint and wind fields

indicate that most regions had weak positive moisture

advection, except for a few areas with negative moisture

advection in far southern and far northern Illinois.

In the absence of any other processes (i.e., vertical

mixing, condensation on the surface, etc.), the balance

between moisture advection and changes in the amount

of moisture needed for saturation would determine

temporal trends in the surface relative humidity field.

Figure 8 shows the difference in horizontal moisture and

saturation moisture fluxes (determined by temperature

advection) at 1200 UTC (0600 LT), near the time of

widespread dense fog development. Regions with neg-

ative difference values, where less moisture was ad-

vected than needed to keep the air saturated, are seen in

much of northern and western Illinois and in a small

portion of southeastern Illinois. A comparison between

the remaining areas of positive differences shown here,

and the overall area of dense fog development shown in

Fig. 7, show reasonably good correspondence despite

the small advection values. It should be noted that cal-

culation of small differences between moisture and

temperature advection with available operational data

will have significant errors. Such errors could result in

local changes in the sign of the differences. However, the

overall pattern of horizontal winds, temperature, and

moisture fields suggest that the presence of a narrow

band of positive values in the region is a reasonable

conclusion.

While moisture advection can give rise to saturated

conditions, another process, such as mixing between two

saturated air masses of different temperatures or cooling,

must be present to allow fog to form. In this case, weak

shear was evident in the Lincoln, sounding near the top of

the moist layer. Weak and variable winds within the

moist layer, however, suggest that shear-driven turbulent

FIG. 5. North–south cross section of smoothed radar reflectivity

observed by the WSR-88D at Lincoln, (KILX), at 2245 UTC

(1645 LT) 6 Nov 2006. The horizontal distance is approximately

55 km. Maximum effective reflectivity factor values, within the

area of lightest shading, are approximately 11 dBZ. Radar data are

shown using the GR2Analyst software program. Cross-sectional

location is indicated in Fig. 1.


mixing was unlikely to have been an important process

on this date. Theoretically, horizontal mixing of air

masses could also give rise to regions of supersaturation.

However, no mechanism has been proposed to allow for

a region of horizontal mixing on the size scale of Illinois.

Supersaturated conditions also could be generated

through cooling of the near-surface air relative to ad-

vective warming by the presence of external heat sinks.

Based on horizontal sensible heat fluxes (used in devel-

opment of Fig. 8), surface temperatures would be ex-

pected to increase by 0.18–0.58C throughout most of the

region with dense fog formation. However, temperatures

remained constant or decreased at all sites in the dense

fog region between 0400 and 0700 LT. In central Illinois,

where low clouds were observed, the decreases were

0.18–0.58C. In far northwestern Illinois, where no deep

FIG. 6. Horizontal visibility, cloud-base height, and precipitation from KCMI. Temperature, winds, momentum

flux, and relative humidity from AmeriFlux site for 6–7 Nov 2006. See Fig. 1 for site locations. The period of dense fog

(vertical gray bars) and the period of light fog (thin vertical lines) are shown.


clouds had been present and where horizontal moisture

advection was negligible, temperature decreases were on

the order of 1.58–2.08C where fog developed.

One possible source for cooling in central Illinois is

from the ground surface. Throughout Illinois, observa-

tions of ground temperatures are obtained hourly by the

ICN, allowing for an estimate of whether regional sur-

face fluxes were negative. At 1200 UTC (0600 LT), ob-

served ground temperatures were the same as or warmer

than surface (2 m) air temperatures for nearly all ICN

sites within the region where dense fog formed. Obser-

vations at the AmeriFlux sites in the vicinity of Cham-

paign, Illinois, were somewhat inconclusive. The two

sets of instrumentation indicated weak downward heat

fluxes at 10 m AGL (,5 W m22 within 6 h of fog for-

mation). However, they differed on whether the air

was warmer than the surface (but both with ,18C dif-

ference in air and ground temperatures). The heat flux

10 cm below the ground surface was directed upward

(;5 W m22) at the one site taking these observations.

Given the tendency for air temperatures to be near or

colder than the ground temperatures in most of the re-

gion of fog development, and small and inconsistent es-

timates of surface-air heat exchanges at the AmeriFlux

sites, it seems unlikely that cooling from the surface

played an important role in fog development in this case.

An alternative source of cooling is radiation divergence

between cloud top and the surface. As seen in both the

satellite image (Fig. 7) and soundings at Lincoln, (Fig. 4),

deep clouds departed the region during the early morning

hours of 7 November, leaving behind a shallow cloud

layer of approximately 2-km depth. This gave rise to

between 2 (far eastern and southern Illinois), 6–8 (east–

central Illinois), and 12 (northwestern Illinois) nighttime

hours without deep clouds in the region of dense fog

formation. Since the cloud layer was at the lowest

FIG. 7. Satellite image of GOES-8, channel 4, 10.7-mm cloud-top

temperature K at 0445 UTC (2245 LT), with location of dense fog

(400-m visibility; solid line) and less dense fog (800-m visibility;

dashed) indicated. Colder temperatures are shown by lighter

colors. Satellite image was obtained from the NASA Langley

Cloud and Radiation Research Group Web page.FIG. 8. Difference between horizontal moisture advection

and changes in the amount of moisture needed for saturation

(g kg21 s21) computed at 1200 UTC (0600 LT) from ICN surface

data.


measureable altitude for 4 h before dense fog formation,

and was therefore apparently linked to the surface, it

might be speculated that convective motions driven by

cloud-top radiative processes could distribute the cooling

throughout the layer as suggested by previous studies.

As a fog layer forms, the level of maximum radiation

flux divergence is known to migrate to the top of the fog

layer (e.g., Jiusto and Lala 1980; Brown 1987; Fitzjarrald

and Lala 1989). Similarly, a maximum in net radiation

flux divergence has been found at the top of stratiform

cloud layers (e.g., Caughey et al. 1982; Slingo et al. 1982;

Frish et al. 1995; Nakanishi 2000). Curry (1986) dem-

onstrated that negative net radiation fluxes can pene-

trate downward, well into shallow stable cloud layers.

While radiational cooling is likely important in fog de-

velopment and maintenance, with an existing cloud

layer, it often is not observable at the surface. On 7

November, the net radiation flux measured at the

AmeriFlux site was negligible in the 3 h prior to and at

dense fog onset. The cloud layer and weak warm air

advection likely masked any radiational contribution

toward cooling at the surface.

Drying of the atmosphere above about 800 hPa (Fig. 4),

with the departure of deep clouds in satellite imagery

(Fig. 7), would allow for radiational cooling at the top of a

fog (e.g., Jiusto and Lala 1980; Brown 1987; Fitzjarrald

and Lala 1989; Croft et al. 1997). With vertical mixing,

radiational cooling at fog or cloud top may generate su-

persaturated conditions (Jiusto and Lala 1980). In stratus

clouds, buoyancy fluctuations may occur when the mixed

layer beneath is destabilized as radiationally cooled

air from above sinks and is replaced by warmer air

from below (e.g., Caughey et al. 1982; Frish et al. 1995;

Nakanishi 2000). Fluctuations in wind speed, direction,

and momentum flux noted in the surface data (Fig. 6)

suggest that this process may be acting. However, the

exact cause of the fluctuation in winds could not be

determined.

Without additional measurements above the surface,

including information on the cloud structure and depth,

the exact mechanism leading to dense fog when clouds

are present is ambiguous. As noted by Curry (1986) and

others, the heat budget of low-level clouds is very com-

plex. The profile of all sources and sinks of heat is required

to understand the balance of radiative cooling, latent heat

exchanges, entrainment, and surface heat fluxes.

Regardless of the specific mechanism, it is hypothe-

sized that cooling of the layer between the top of the low

clouds and the surface, in combination with a region of

favorable moisture advection, allowed the near-surface

air to cool at a rate slower than expected because of

warm air advection, but sufficient to allow a wide region

of supersaturation and dense fog to develop. Qualita-

tively, this is consistent with the close correspondence

between the southwest–northeast orientation of the fog

region and the western edge of the departing deep

clouds.

5. Conclusions

Cold-season fogs in the Midwest are often associated

with low pressure systems or fronts. Based on a clima-

tology of dense fog events in Peoria, Illinois, the majority

of these continental fog events form when precipitation

has occurred in the region and sometimes when it is

still occurring, and when cloud bases in the 6 h prior to

dense fog formation are less than 1000 m. When fog

forms in the presence of a low cloud base, an unchanging

or increasing surface air temperature is not uncommon.

The processes by which saturation and supersaturation

occur in these cases are unclear. Despite the frequency

of these prefog conditions, little quantitative work has

been reported on low cloud-base continental fog events.

The variability found in the frequency of surface changes,

precipitation occurrence, and synoptic classifications

suggests that low cloud-base fogs form under a variety

of conditions. Clearly, further case studies would be in-

valuable in determining the processes by which super-

saturation occurs in these cases.

The 6–7 November 2006 case study observations fit

well within the Peoria climatology of dense fog events

associated with low cloud bases and with low pressure

systems. For this event, precipitation and continued

moisture advection associated with a nearby cyclone

played a critical role in lowering of the cloud base. Pre-

cipitation developed above about 3 km over a wide re-

gion of central Illinois, but based on ceilometer, radar,

and surface observations, evaporated before reaching

the surface for several hours. The preconditioning helped

saturate the lower atmosphere prior to fog formation.

After precipitation ended, the surface temperature

remained nearly constant, and the relative humidity was

saturated or nearly saturated. AmeriFlux observations

taken near Champaign, Illinois, suggest that surface

sensible heat fluxes were small, and results from various

other methods of estimating the direction of heat fluxes

were inconsistent. Therefore, processes at the surface

likely played a minimal role in fog development in this

case. With small differences between the ground and the

2-m air temperature and the low wind speeds, this would

not be considered an advection fog event in any case.

The drying and clearing of the upper-level cloudiness

following precipitation and prior to fog formation,

however, suggests that radiational cooling at cloud top

played an important role leading to supersaturation and

the development of fog.


Thus, it is hypothesized that the primary mechanism

for cooling of the surface layer relative to sensible

heat advection was radiative cooling of the cloud and

subcloud layer. However, verification of this hypothesis

is not possible with the datasets available on this day.

With upper-air observations available only at 0000 and

1200 UTC (1800 and 0600 LT), timing of the changes

in the cloud, temperature, and moisture structure is

unknown. A radiative transfer model could shed further

light on processes involved in fog formation, and would

be most useful if supported by intervening nighttime

soundings, a profiling radiometer, above-cloud aircraft

observations, and/or a cloud radar to document the

changes of the temperature and moisture fields, and the

presence and depth of clouds aloft.

Acknowledgments. The authors thank Robert Scott

at the Illinois State Water Survey for providing ICN

observations, and Carl Bernacchi for providing the

AmeriFlux observations. We also thank the personnel

at the National Weather Service Forecast Office at

Lincoln, Illinois, and in particular James Auten for

useful discussions and Dan Kelly for providing high-

resolution rawinsonde data. We also appreciate the

advice of James Angel, Michael Palecki, and three anon-

ymous reviewers, which led to important improvements

in this manuscript. This research was partially supported

by NOAA Cooperative Agreement NA67RJ0146. Any

opinions, findings, conclusions, or recommendations are

those of the authors and do not necessarily reflect the

views of the Illinois State Water Survey or NOAA.

REFERENCES

Billesbach, D. P., M. L. Fischer, M. S. Torn, and J. A. Berry, 2004:

A portable eddy covariance system for the measurement of

ecosystem–atmosphere exchange of CO2, water papor, and

Energy. J. Atmos. Oceanic Technol., 21, 639–650.

Brown, R., 1987: Observations of the structure of a deep fog.

Meteor. Mag., 116, 329–338.

Byers, H. R., 1959: Fog. General Meteorology, McGraw Hill, 480–510.

Caughey, S. J., B. A. Crease, and W. T. Roach, 1982: A field study

of nocturnal stratocumulus. II. Turbulence structure and en-

trainment. Quart. J. Roy. Meteor. Soc., 108, 125–144.

Croft, P. J., and A. N. Burton, 2006: Fog during the 2004–2005 winter

season in the northern mid-Atlantic states: Spatial characteristics

and behaviors as a function of synoptic weather types. Preprints,

12th Conf. on Aviation Range and Aerospace, Atlanta, GA,

Amer. Meteor. Soc., P3.3. [Available online at http://ams.

confex.com/ams/Annual2006/techprogram/paper_97843.htm.]

——, R. L. Pfost, J. M. Medlin, and G. A. Johnson, 1997: Fog

forecasting for the southern region: A conceptual model ap-

proach. Wea. Forecasting, 12, 545–556.

Curry, J. A., 1986: Interactions among turbulence, radiation, and

microphysics in Arctic stratus clouds. J. Atmos. Sci., 43, 90–106.

Environmental Research Services, cited 2006: RAOB version 5.7

for Windows. [Available online at http://www.raob.com.]

Fitzjarrald, D. R., and G. G. Lala, 1989: Hudson Valley fog envi-

ronment. J. Appl. Meteor., 28, 1303–1328.

Frish, A. S., D. H. Lenschow, and C. W. Fairall, 1995: Doppler

radar measurements of turbulence in marine stratiform cloud

during ASTEX. J. Atmos. Sci., 52, 2800–2808.

George, J. J., 1940: Fog: Its causes and forecasting with special

reference to eastern and southern United States. Bull. Amer.

Meteor. Soc., 21, 135–148.

——, 1951: Fog. Compendium of Meteorology, T. F. Malone, Ed.,

Amer. Meteor. Soc., 1193–1197.

Gibson Ridge Software, 2006: GRLevel2Analyst version 1.05.

[Available online at http://www.grlevelx.com/.]

Goodwin, L. C., cited 2002: Analysis of weather-related crashes on

U.S. highways. Mitretek Systems. [Available online at http://ops.

fhwa.dot.gov/weather/best_practices/CrashAnalysis2001.pdf.]

Jiusto, J. E., 1981: Fog structure. Clouds Their Formation, Optical

Properties, and Effects, P. V. Hobbs and A. Deepak, Eds.,

Academic Press, 187–235.

——, and G. G. Lala, 1980: Radiation fog formation and dissipa-

tion: A case study. J. Rech. Atmos., 14, 391–397.

Keith, R., and S. M. Leyton, 2007: An experiment to measure the

value of statistical probability forecasts for airports. Wea.

Forecasting, 22, 928–935.

Mason, J., 1982: The physics of radiation fog. J. Meteor. Soc. Japan,

60, 486–498.

Meyer, M. B., and G. G. Lala, 1990: Climatological aspects of radia-

tion fog occurrence at Albany, New York. J. Climate, 3, 577–586.

Nakanishi, M., 2000: Large-eddy simulation of radiation fog.

Bound.-Layer Meteor., 94, 461–493.

Roach, W. T., 1995: Back to basics: Fog: Part 2 - The formation and

dissipation of land fog. Weather, 50, 7–11.

——, R. Brown, S. J. Caughey, J. A. Garland, and C. J. Reading,

1976: The physics of radiation fog: I – A field study. Quart.

J. Roy. Meteor. Soc., 102, 313–333.

Saunders, W. L., 1960: The clearance of water fog following the

arrival of a cloud sheet during the night. Meteor. Mag., 89, 8–10.

Slingo, A., R. Brown, and C. L. Wrench, 1982: A field study of noc-

turnal stratocumulus: III. High resolution radiative and micro-

physical observations. Quart. J. Roy. Meteor. Soc., 108, 145–165.

Tardif, R., 2006: A climatological study of low ceiling and fog

events associated with the occurrence of precipitation in the

northeastern United States. Preprints, 12th Conf. on Aviation

Range and Aerospace Meteorology. Atlanta, GA, Amer.

Meteor. Soc., 5.1. [Available online at http://ams.confex.com/

ams/Annual2006/techprogram/paper_101782.htm.]

——, and R. M. Rasmussen, 2007: Event-based climatology and

typology of fog in the New York City region. J. Appl. Meteor.

Climatol., 46, 1141–1168.

——, and ——, 2008: Process oriented analysis of environmental

conditions associated with precipitation fog events in the New

York City region. J. Appl. Meteor. Climatol., 47, 1682–1703.

Turton, J. D., and R. Brown, 1987: A comparison of a numerical

model of radiation fog with detailed observations. Quart. J.

Roy. Meteor. Soc., 113, 37–54.

Westcott, N. E., 2004: Synoptic conditions associated with dense

fog in the Midwest. Preprints, 14th Conf. on Applied Clima-

tology. Seattle, WA, Amer. Meteor. Soc., P2.4. [Available

online at http://ams.confex.com/ams/pdfpapers/72954.pdf.]

——, 2005: Conditions associated with cold season dense fog in the

midwestern United States. Ph.D. dissertation, University of

Illinois at Urbana–Champaign, 117 pp.

——, 2007: Some aspects of dense fog in the midwestern United

States. Wea. Forecasting, 22, 457–465.


A Climatology and Case Study of Continental Cold Season Dense Fog Associated with Low Clouds

Documents

Transcript of A Climatology and Case Study of Continental Cold Season Dense Fog Associated with Low Clouds