ENVIRONMENTAL HEALTH SERIES
Air Pollution
CLIMATOLOGY
OF STAGNATING ANTICYCLONES
EAST OF THE ROCKY MOUNTAINS
1936-1965
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
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CLIMATOLOGY OF STAGNATING ANTICYCLONES
EAST OF THE ROCKY MOUNTAINS, 1936 - 1965
Julius Korshover
Environmental Science Services Administration
U.S. Department of Commerce
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
Bureau of Disease Prevention and Environmental Control
National Center for Air Pollution Control
Cincinnati, Ohio
1967
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Public Health Service Publication No. 999-AP-34
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ABSTRACT
Since stagnating anticyclones are often associated with incidents of heavy air
pollution in urban areas, a 30-year climatology is presented to delineate occur-
rences of stagnating anticyclones in the eastern United States. Occurrences of
stagnation are determined on the basis of pressure-gradient values considered with
other meteorological factors. Affected areas are depicted on maps by use of a
system of grid points. Data showing cases of stagnation (i. e. , episodes) and total
days of stagnation indicate that stagnation is most likely to occur in autumn months.
111
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CLIMATOLOGY OF STAGNATING ANTICYCLONES
EAST OF THE ROCKY MOUNTAINS, 1936-1965
INTRODUCTION
This study combines the results of two separate investigations by the author.
The first study (1) covers a period of 2 1 years (1936-1946); the second, a period of
9 years (1957-1965). The combined results of these studies are consolidated in a
30-year synoptic climatology of stagnating high-pressure systems in the eastern
part of the United States.
Our chief interest in these stagnating anticyclones is their association with
incidents of heavy air pollution in urban areas. Such anticyclones usually linger
over an area for a protracted period (4 days or more). During this period surface
wind speeds may be very low, and circulation at the surface is therefore insufficient
to diffuse the accumulated pollutants of the local atmosphere. The resulting accu-
mulations cause distressful and possibly hazardous conditions for inhabitants of the
area.
In recent years, because of a rapid increase in urban living, the air pollution
problem has become more and more aggravating and has commanded considerable
attention. Our population is growing, and heavy industries are concentrated in
relatively congested areas. Thus the possibilities that polluted air can affect
human life adversely are also increasing. Since the major air pollution episodes
are usually associated with stagnating anticyclones in or near urban areas, the
synoptic climatology presented in this report may be useful in efforts now under
way to prevent harmful effects of air pollution upon human life in the future.
PRESSURE GRADIENT
There are few studies of air pollution climatology on record. After consid-
erable preliminary experimentation, it was decided to employ a technique based on
pressure gradient for determining the occurrence of cases of stagnation. This pro-
cedure is desirable because the pressure gradient gives a more representative
picture of the general flow near the surface of the earth than do individual surface
wind measurements. Willett evaluated the significant surface wind during the
extreme smog development at Donora, Pennsylvania, in October 1948. At no time
during a 4-day period was the maximum surface wind greater than 8 mph (2). In
this study, therefore, the limiting wind speed taken at anemometer level is assumed
to be 7.5 knots. The authors of Dynamic Meteorology and Weather Forecasting
show (in Table 12. 35. 1) the ratio between surface wind and geostrophic wind speed.
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At 30°N latitude over land, this ratio is 0. 31 and at 40°N latitude the value is 0. 38
(3). Willett maintains that the surface winds are observed to average about 2/5 of
the gradient wind speed (4). Brunt states that the value for surface wind speed
should be about 0. 7 times the geostrophic value for light winds (5). In this study,
the value of 0. 5 has been adopted as the ratio between surface wind speed and geo-
strophic wind speed. Usually the difference between the geostrophic and the gradient
wind speeds is small, and for this reason these two terms are used interchangeably
by many authorities. As added confirmation for the 15-knot geostrophic value, J. J.
George found that the average maximum gradient (geostrophic) wind speed that allows
the formation of fog is close to 15.0 mph (6).
METHOD AND PROCEDURES
All the apparent stagnant anticyclones that have occurred between 1936 and
1965 east of the Rocky Mountains were selected for this study. The following pro-
cedures were used in determining cases of stagnation with the pressure gradient
method:
Step 1. The United States Weather Bureau Daily Map (7) containing synoptic sea-
level analysis was used exclusively to determine the pressure gradient. Before
1936, available upper-air information was sparse and inconclusive. The period of
this survey extends for 30 years, 1936-1965.
All anticyclones that showed a possibility of stagnation for 4 days or more
were recorded. These cases were carefully checked to determine whether portions
of areas of stagnation as defined below overlapped for periods of 4 days or more.
In Figure 1, the technique for determining stagnant areas is shown. For simplicity
in this figure, the effect of latitude on the geostrophic wind is not considered, and
therefore the geostrophic wind is constant for a given pressure gradient. In the
numerical evaluations latitude was considered, and the pressure gradient corre-
sponding to a 15-knot geostrophic wind was determined at each 5 degrees of latitude
from 27. 5 to 47. 5. In the isobaric channel ST of Figure 1, the geostrophic wind
changes from more than 15 knots to less than 15 knots at points C and D, with the
stronger winds to the northeast. In the isobaric channel RS, the geostrophic wind
speed is everywhere more than 15 knots. Therefore, the dashed curve delineating
area of stagnation, as defined above, is drawn coincident with the isobar approxi-
mately up to the points C and D. Within the isobar T the geostrophic wind is
everywhere less than 15 knots. Therefore, the dashed curve delineates the area
of stagnation as drawn coincident with the isobar T approximately up to the points
C and D. The dashed line is "faired-in" across the isobaric channel at points C
and D. The region enclosed by the dashed line represents the area of stagnation.
Step 2. A map of the United States from the Rocky Mountains to the east coast was
divided into a set of grid points at every 2 degrees of latitude and of longitude
(Figure 2). An overlay pattern was then used to facilitate the locating of the grid
points that lay within stagnant areas. The grid points of the "squares" thus located
were then numbered. The daily records of areas of stagnation were recorded by
means of these points on a worksheet. It was then possible to read off areas of
stagnation for 4, 5, 6, and 7 or more consecutive days.
STAGNATING ANTICYCLONES
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Step_3.^ Wherever frontal areas overlapped areas of apparent stagnation, the latter
Were excluded because of changing air masses, cloudiness, wind speed, and other
elements .
p_4. ^ Areas of precipitation were excluded because of the cleansing properties
of precipitation.
Step_5^ It was discovered that only warm highs meet the criteria for stagnation
conditions, since cold highs do not persist long enough. Warm anticyclones are
deep, stagnant systems; the high pressure prevails far up into the troposphere and
may extend into the stratosphere. Each case was checked for either a trough or a
ridge in the area of interest at approximately 10,000 feet.
SUMMARY OF RESULTS
The 30-year study yields results so similar to those of the earlier survey
that most of the summary remarks are confined to the latter.
Table I is a chronological presentation of all cases of stagnation. Because of
the strict criteria, some cases may be ignored that might have resulted in potentially
serious pollution situations.
In Figure 3, the upper values (above each grid point) are the total number of
stagnation cases (4 or more days) and the lower values are the total number of days
of stagnation. Dashed isopleths show the distribution of the number of stagnation
cases; solid isopleths, the distribution of the number of days of stagnation. They
are distributed similarly. This chart suggests a strong influence of the Bermuda
High on the east coast. A maximum frequency in both the number of cases and the
number of days stagnation occurs in an area that covers parts of Georgia, South
Carolina, and North Carolina. This region is frequently dominated by the western
extension of this semipermanent high-pressure area.
Figure 4 shows the geographical distribution of annual occurrences of cases of
stagnation for 7 or more days. The values given are the total for the entire 30-year
period, 1936-1965. These can be compared with Figure 3, the total annual number
of stagnation cases for 4 or more days (dashed isopleths).
A monthly distribution of the number of cases of stagnation for 4, 5, 6, or
7 or more days and of the total number of days of stagnation is given in Table II.
Figure 5 depicts the distribution by months of the number of cases of stagna-
tion for 4, 5, 6, and 7 or more days. This figure illustrates Table II graphically.
The primary peak for 4 or more days occurred in October; a secondary maximum
occurred in May. Figure 5 also shows the percentage distribution by months of
cases of stagnation for 4 or more days; Table III presents these data in numerical
form.
Geographical distribution by months of the occurrences of stagnation
episodes of 4 or more days is given in Figure 6. Each month was analyzed, and
isopleths were drawn for equal numbers of cases. The 4 months with the maxi-
mum frequency of cases at any one grid point are May, September, October,
EAST OF THE ROCKY MOUNTAINS
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and November. In May, 14 cases of stagnation occurred over South Carolina. In
the last 9 years during this month the maximum number of cases shifted to north-
ern Florida. In September, the maximum number of cases, 16, occurred in
Pennsylvania, near 40°N and 75°W; the secondary maximum, 11 cases, occurred
over eastern Tennessee. The two peak areas tend to combine. In October, 21
cases of stagnation occurred over northern Georgia; in November, 12 cases
occurred in central South Carolina. Wexler (8) also finds October the month most
favorable for the occurrence of deep anticyclones and hazardous smog episodes.
Klein (9) depicts the number of different highs for 5-degree squares for each month
during 20 selected years of the original Historical Map Series, 1904-1914 and 1924-
1937. In Klein's paper, charts 21, 22, and 23 for September, October, and Novem-
ber are somewhat similar to those for the same months shown in Figure 6, but
with centers of maximum frequency displaced somewhat northward. He also
gives the number of days with highs (both migratory and stationary) during the
40 years from 1899-1939. September and October apparently are crucial months
when stagnation is most likely to occur.
Data for additional years have not affected the original location and distri-
bution of stagnation anticyclones in the eastern United States. Thus, these data
appear to furnish a reliable climatology of such events.
ACKNOWLEDGMENTS
Dr. Lester Machta initiated the project, established the criteria for classi-
fying the stagnating wind speeds, and suggested the procedures to be followed in
analyzing the weather charts. The author is also grateful to Donald H. Pack, Dr.
James K. Angell, and Fred D. White for their many suggestions and constructive
criticism. The diagrams were drafted by Marguerite M. Hodges; Roland V.
Goulait assisted in tabulating the data.
STAGNATING ANTICYCLONES
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Table I.
Feb Mar
Stagnation Cases in the United States,
East of the Rocky Mountains, 1936-1965.
1936
1937
1938
1939
1940
1941
1942
1943
1944
1945
1946
1947
1948
1949
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
TOTAL
11-14
27-30
D31-J5*
12-16
,13-16
J26-F1
11-15
23-27
9-12
9
23-26
17-21
23-28
13-17
4
16-20
M28-A1
*.-•>
F28-M3
12-19
23-26
6
17-20
12-17
l-i
8-12
14-20
A23-M1
14-17
23-26
25-29
18-24
3-6
22-28
17-20
5-10
14-18
24-29
11-17
23-26
22-27
M30-A4
17-22
17-23
22
6-11
23-27
17-21
4-7
5-9
19-22
A29-M3
A30-M5
12-18
13-16
14-18
10-15
7-10
M29-J1
14-17
1-5
27-30
3-7
14-21
16-20
8-11
7-10
16-20
1-8
15-20
25
24-28
2-6
18-22
M30-J3
M30-J5
9-16
M30-J3
10-13
22-27
20-25
1-4
6-9
13-20
23-28
M31-J6
15-18
13-19
23-27
13-17
J28-J4
8-12
8-13
25-30
L-6
24
18-21
4-7
27-30
2-6
9-12
23-20
10-14
J26-A1
1-5
11-14
16-20
11
14-17
9-12
13-17
19-24
3-7
19-26
A29-S2
A27-S1
11-18
21-30
7-12
22-27
23-28
A17-S2
1-6
27-30
6-9
11-16
20-23
17-20
23-26
A29-S2
3-6
23
21-25
S27-01
23-26
3-7
17-21
19-22
S28-01
8-11
12-15
26-29
3-7
13-20
26-29
1-8
S26-03
9-14
S27-01
1-,
11-14
26-29
12-16
23-26
4-10
19-26
6-9
22-25
9-13
21-24
7-11
16-20
24-27
1-4
4-9
19-23
34
8-13
15-20
25-28
4-10
10-13
23-28
12-16
r-10
3-11
18-21
16-19
027-N3
2-7
14-18
20-25
1-8
24-31
9-12
19-22
16-19
029-N3
22-27
030-N2
1-6
20-27
?-13
n-16
6-10
11-16
12-17
3-7
8-14
6-13
11-14
1-4
14-20
23-26
11-15
24-27
25-29
40
23-26
13-18
18-21
18-21
11-14
6-9
6-9
20-25
N28-D3
030-N3
13-20
12-17
11-14
8-1 1
12-16
18-23
25-28
N27-D4
16-19
19
21-27
4-7
3-11
1-5
13-19
8-11
14-17
2-7
17-20
D30-J2**
10
4
5
8
5
8
8
9
11
7
5
10
4
6
5
8
5
9
5
4
9
7
12
7
16
16
6
5
11
6
6
227
* Dec 31 - Jan 5
** Dec 30 - Jan 2
EAST OF THE ROCKY MOUNTAINS
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Table II. Number of Stagnation Cases for Indicated Period and the
Total Number of Days by Month, in the United States,
East of the Rocky Mountains, 1936-1965.
Month 4
Jan 9
Feb 4
Mar 6
Apr 22
May 25
Jun 24
Jul 11
Aug 23
Sep 34
Oct 40
Nov 19
Dec 10
Total 227
Table
Month
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
Days
Total
5 6 7 8 9 10 17 Days
532
3 1
3111
16 12 6 1
16 7 3 1
20 11 7 2
6221
15 11 4 42
19 8 6 3
27 20 9 61
9622
53311
144 85 45 22 4
III. Percentage Distribution of Cases of
for 4 or More Days by Month in the
States, East of the Rocky Mountains
1936-1965
Day
4
9
4
6
22
25
24
11
23
34
40
19
10
142
46
20
30
123
127
136
55
2 1 131
172
223
95
53
2 1 1211
Stagnation
United
Percent
4.0
1.8
2.6
9.7
11. 0
10. 6
4.8
10. 1
15. 0
17.6
8.4
4.4
100. 0
STAGNATING ANTICYCLONES
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N
Figure 1. Schematic illustration of technique
for determining stagnation areas.
EAST OF THE ROCKY MOUNTAINS
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Figure 2. Geographical data reduction grid.
STAGNATING ANTICYCLONES
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- TOTAL NO. OF STAGNATION CASES
- TOTAL NO. OF STAGNATION DAYS
Figure 3. Distribution of stagnation cases and stagnation days,
1936-1965.
EAST OF THE ROCKY MOUNTAINS
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Figure 4. Total number of stagnation cases (7 or more days),
1936-1965.
10
STAGNATING ANTICYCLONES
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Number
of
cases
N
MONTHS
IIHIB>4 days
11 i > 5 days
i^ 6 days
i£ 7 days
Figure 5. Number of stagnation cases for 4, 5,
6, and 7 or more days by month in the
United States east of the Rocky Mountains,
1936-1965.
EAST OF THE ROCKY MOUNTAINS
11
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JANUARY
FEBRUARY
Figure 6. Monthly distribution of number of cases of atmospheric
stagnation (4 days or more), 1936-1965.
12
STAGNATING ANTICYCLONES
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JUNE
Figure 6 (continued). Monthly distribution of number of cases of
atmospheric stagnation (4 days or more), 1936-1965.
EAST OF THE ROCKY MOUNTAINS
13
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DECEMBER
Figure 6 (continued). Monthly distribution of number of cases of
atmospheric stagnation (4 days or more), 1936-1965.
14
STAGNATING ANTICYCLONES
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REFERENCES
(1) Korshover, J. , I960; Synoptic Climatology of Stagnating Anticyclones East of
the Rocky Mountains in the United States for the Period 1936-1956. Technical
Report A60-7, U.S. Public Health Service, R. A. Taft Sanitary Engineering
Center, Cincinnati, Ohio.
(2) Willett, H. C. , 1949: The Meteorological Conditions Related to the Occur-
rence of Smog at Donora, Pennsylvania. Department of Meteorology,
Massachusetts Institute of Technology, Cambridge, Massachusetts, for the
Industrial Hygiene Foundation of America, Incorporated, Pittsburgh,
Pennsylvania. (Unpublished)
(3) Godske, C. L. , et al. , 1957: Dynamic Meteorology and Weather Forecasting.
American Meteorological Society and Carnegie Institution of Washington,
p. 454.
(4) Willett, H. C. , 1944: Descriptive Meteorology. New York Academic Press,
Inc., p. 114.
(5) Brunt, D. , 1941: Physical and Dynamical Meteorology, Cambridge University
Press, p. 260.
(6) George, J. J. , 1939: Fog: Its Cause and Forecasting. Atlanta, Georgia,
Eastern Air Lines, Inc. (Meteorological Department), p. 10.
(7) Anon: Daily Weather Map, Washington, D. C. , U.S. Weather Bureau.
(January 1936-December 1965).
(8) Wexler, H. , 1949: Why is October the Optimum Month for Smog at Donora?
Air Pollution in Donora, Pa. Washington, D. C. Public Health Bulletin,
No. 306, pp. 171-173.
(9) Klein, W. H. , 1957: Principal Tracts and Mean Frequencies of Cyclones and
Anticyclones in the Northern Hemisphere. Research Paper No. 40. Wash-
ington, D. C. , U.S. Weather Bureau, 60pp.
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