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EPA 450 5-79-010
Sppternber 1979
Air
Existing Visibility Levv,,^
in the United States
Isopleth Maps of Visibility
in Suburban/Nonurban
Areas During 1974-76
REGION VHJ9";'v?r'
U-S-ENVIRON^iNTALFROTECTlGf
AGENCY
1445 ROSS AVENUE -
OAUAS, TEXAS 7520?
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EPA-450/5-79-010
: Existing Visibility Levels
I in the United States
Isopleth Maps of Visibility
in Suburban/Nonurban
Area During 1974-76
by
John Trijonis and Dawn Shapland
Technology Service Corporation
Route 3, Box 124-K
Santa Fe, New Mexico 875J1
Grant No. 802815
EPA Project Officer: John Bachmann (OAQPS)
William E. Wilson and Thomas G. Ellestad (ESRL)
Prepared for
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Air, Noise, and Radiation
Office of Air Quality Planning and Standards
Research Triangle Park, North Carolina 27711
September 1979
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This report has been reviewed by the Strategies and Air Standards Division
of the Office of Air Quality Planning and Standards, EPA, and approved for
publication. Approval does not signify that the contents necessarily reflect
the views and policies of EPA. Mention of trade names or commercial products
is not intended to constitute endorsement or recommendation for use. Copies
of this report are available through the Library Services Office (MD-35),
U. S. Environmental Protection Agency, Research Triangle Park, N. C.
27711, or from National Technical Information Services, 5285 Port Royal
Road, Springfield, Virginia 22161.
Publication No. EPA-450/5-79-010
11
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ABSTRACT
Maps are prepared which illustrate median, mid-day visibility levels
in suburban/nonurban locations of the continental United States. Median
visibilities at 94 locations are determined from cumulative frequency dis-
tributions of quality-checked airport observations. Seven locations in
the Southwest with photographic photometry or nephelometry data (which
agree quite well with the airport data) are also included.
The spatial pattern of visibility is demonstrated with isopleth maps
for both the annual medians and summertime medians during the years 1974-
1976. The consistency of the spatial gradients in visibility and the
agreement between neighboring locations attest to the quality of the
visibility data. The isopleth maps reveal that the best visibility (70+
miles) occurs in the mountainous Southwest. Visibility is also quite good
(45-70 miles) north and south of that region, but sharp gradients occur
to the east and west of that region. Most of the area east of the Mississippi
and south of the Great Lakes exhibits median visibilities of less than 15
miles annually and less than 10 miles during the summertime.
This report was submitted in partial fulfillment of Grant 802815 by
Technology Service Corporation under the sponsorship of the U.S. Environ-
mental Protection Agency. Work on this project started in May 1978 and
was completed in September 1978.
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IV
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CONTENTS
ABSTRACT
FIGURES.
TABLES .
1.
2.
3.
INTRODUCTION AND SUMMARY
DATA BASE ASSEMBLY AND DATA ANALYSIS METHODS
Selection of Study Locations and Acquisition of Data. .
Computation of Median Visibilities
VISIBILITY ISOPLETH MAPS
Median Yearly Visibilities, 1974-1976
i i i
vi
vi i
1
3
3
5
17
17
Median Third-Quarter Visibilities, 1974-1976 ..... 21
REFERENCES ............................. 26
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FIGURES
Number Page
1 Locations used to characterize visibility in suburban/
nonurban areas 6
2 Examples of cumulative frequency distributions,
(mid-day data, 1974-1976) 7
3 Median yearly visibilities at suburban/nonurban locations . . 18
4 Median yearly visibilities and visibility isopleths for
suburban/nonurban areas 19
5 Shaded isopleth map of yearly visibilities for
suburban/nonurban areas 20
6 Median summer visibilities at suburban/nonurban locations . . 22
7 Median summer visibilities and visibility isopleths for
suburban/nonurban areas 23
8 Shaded isopleth map of summer visibilities for
suburban/nonurban areas 24
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TABLE
Page
Tiber
I List of Study Sites and Median Yearly Visibilities 9
vn
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CHAPTER 1
INTRODUCTION
A growing air quality concern in the United States is the issue of
visibility or haze. In the.west — where the high mountians, rugged ter-
rain, and exceptional visual range often produce exceptional panoramas --
there is apprehension that increased haze may deteriorate both scenic vistas
and the blueness of the sky. In the east, where visibility is usually
quite low, the concerns involve not only aesthetics but also hindrance of
aviation (Miller et al., 1972) and possible climate modifications (Bolin
and Charleson, 1976; Husar and Patterson, 1978; Trijonis and Yuan, 1978b)
produced by large-scale haze. Some of these concerns are reflected in the
visibility provisions of the 1977 Clean Air Act Amendments.
Because-of the growing interest in the visibility issue, there is a
need to address the very basic question "What are existing visibility
levels throughout the United States?" Some partial answers to this question
have already been provided. Trijonis and Yuan (1978a, 1978b) examined exist-
ing visibility levels and long-term visibility trends in urban and nonurban
areas, but these studies only covered the Rocky Mountain Southwest and part
of the Northeast. Husar et al. (1976) have cor.structed a very interesting
isopleth map for the United States, but this map involves only summertime
data (June-August in 1975), is stratified for relative humidity (thus being
more concerned with air quality impacts on visibility rather than typical
or average visibility), and provides no resolution above 16 miles (thus
characterizing the east very well but having essentially no resolution in
the west).
The purpose of this report is to prepare an isopleth map illustrating
*
large-scale patterns in median visibility throughout the United States.
In this report, the terms "visibility" and "visual range" will be used in-
terchangeably, both referring to the distance at which a black object can
just be distinguished against the horizon sky. Actually, at many times and
in many places, there are no objects within line of sight at distances as
great as the prevailing visibility. The term visibility actually refers to a
hypothetical case -- the distance at which a black object would just be dis-
tinguished against the horizon sky, vf such an object were within a line of
sight
1
-------
Three recent years (1974-1976) of mid-day airport observations are used in
order to provide a robust data set. Maps of visibility are prepared for
both the yearly median and the third quarter (July-September) median. The
data are quality checked and processed in a way that permits resolution of
visibility from 0 to 90 miles. Only suburban/nonurban airports are included
to avoid the perturbations in visibility that are sometines produced by
major urban centers.
The remainder of this report is organized in two chapters. Chapter 2
discusses the data bases used and the methods of data analysis. Chapter 3
presents visibility maps and includes a brief narrative describing the
nationwide patterns in visibility.
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CHAPTER 2
DATA BASE ASSEMBLY AND DATA ANALYSIS METHODS
The objective of this report is to characterize existing visibility
levels in suburban/nonurban locations of the continental United States.
Before presenting our findings, it is worthwhile to review the data bases
and statistical methods which serve as the foundation for those findings.
This chapter describes the data bases used and the analysis methods applied.
SELECTION OF STUDY LOCATIONS AND ACQUISITION OF DATA
*
Most of the data presented in this report consists of mid-day "pre-
vailing visibility" observations made by airport meteorologists from 1974
through 1976. A single, near-noon hour is used each day because much of
the data were obtained in hard copy, and limiting the analysis to one hour
per day saved considerable work in hand-processing such data. Three years,
1974-1976, are included in order to provide a robust data set for determin-
ing existing visibility levels.
Before sites were selected for the study, telephone surveys pertaining
to data quality were conducted with the meteorologists at approximately 150
suburban/nonurban airports. The purpose of these surveys was to insure that
each airport had an adequate set of visibility markers for estimating visual
range. In particular, we attempted to select airports that had farthest
markers located at distances at least as great as the visibility levels
typical of the surrounding area. Most of the airports selected do have good
markers. In some cases, however, we were forced to use airports with less
than optimal markers in order to attain good geographical coverage in the
study; for these cases we had to extrapolate the cumulative frequency dis-
tributions in order to estimate median visual range (see discussion in next
section).
*
The National Climatic Center computerized data base contains observations
for every third hour. We selected the 1:00 PM (Standard Time) measurement
for the Eastern Time Zone, the 12:00 AM measurement for the Central Time
Zone, the 11:00 AM measurement for the Mountain Time Zone, and the 1:00 PM
measurement for the Pacific Time Zone.
-------
Suburban/nonurban locations are chosen for the study because we intend
to characterize large-scale patterns in visibility and to avoid the localized
perturbations that are sometines produced by major metropolitan areas. Some
readers may question whether a few of our study sites — in particular Long
Beach, Tucson, Oklahoma City, Memphis, Columbus, Dayton, and Charlotte --
are really suburban or nonurban. Our response to this question is as follows:
t The Long Beach airport is actually located in a major metropolitan
region. We included this site so that we would have at least one
location representing the large area covered by the Los Angeles basin.
t The Tucson airport is located about 9 miles south of downtown Tucson,
and all of the principal visibility markers are located to the south
and east of the airport. Furthermore, a previous study (Trijonis and
Yuan, 1978a) demonstrated that median visibility at Tucson tends to be
slightly higher than median visibility at Fort Huachuca, a nonurban
location about 50 miles to the southeast of Tucson.
t The other five sites in question are all located in the eastern half
of the country where nonurban visibility is low, and where even large
metropolitan areas do not appear to produce great perturbations in
visibility (Trijonis and Yuan, 1978b). Also, all of these airports
are located several miles from the center-city and outside the
urbanized area.
Ninety-four airports are included in the present study. To provide
better coverage in the Southwest, we have also added photographic photometry
and/or nephelometry data taken in special field studies at 7 nonurban
locations. That the various types of data are fairly comparable (at least
on the average) is evidenced by the following median noontime visibilities
obtained in the vicinity of the northern Colorado-Utah border:
Airport Data: Grand Junction, CO 84 miles
Rock Springs, WY 76 miles
Photographic Photometry Data: Cedar Mountain, UT. ... 74 miles
Piceance Creek, CO. ... 70 miles
Nephelometry Data: Bonanza, UT 81 miles
Cedar Mountain, UT 75 miles
There is a question, however, concerning the time coverage of the field study
data. The photographic photometry and nephelometry data usually are taken
-------
intermittently over a one-year period. We have included special field data
only if they cover at least part of 1974-1976, but there is no guarantee
that these data adequately represent the entire 3-year period.
The 101 study locations are illustrated in Figure 1. The full site
names are given later in the next section (Table 1). The locations of the
special field studies are indicated by a superscript, "p" for photographic
photometry and "n" for nephelometry.
COMPUTATION OF MEDIAN VISIBILITIES
Airport visibility measurements are most appropriately summarized by
cumulative frequency distributions of the form "percent of time visibility
is greater than or equal to X miles". This is because, in practice, an air-
port visibility observation of X miles often means that visibility is ^t_
least X miles rather than visibility is exactly X miles. It is also very
important, in determining the cumulative frequency distributions, to use
only those visibilities that are routinely reported by the airport obser-
vation team; otherwise, artificial "kinks" will be produced in the cumu-
lative frequency distribution. Summarizing the data this way should also
make the data consistent from airport to airport, even if the various air-
ports have visibility markers at different distances.
Figure 2 presents examples of cumulative frequency distributions for
four locations which vary widely in observed visibilities. As is the case
with the four locations illustrated in Figure 2, most of the median visi-
bilities are determined by linear interpolation of the cumulative frequency
distribution. In some cases, however, the frequency distribution required
extrapolation in order to reach the median visibilities. The form of these
extrapolations, linear or nonlinear, was based on a comparison of the dis-
tribution for the site in question with distributions for other sites
located in the same region of the country. For sites where these extrapo-
lations involve significant uncertainties, the uncertainties are noted
throughout this report by asterisks placed on the estimated median
visibilities.
The visibility data reported here are based on all observations, with
no sorting for meteorology. Thus, the spatial patterns in visibility are
-------
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Colorado Springs, CO Median
Kalispell, MT
Albany, NY
l i i r i i i
10% 20 30 40 50 60 70 80 90 100%
Cumulative Frequency (percent)
Figure 2. Examples of cumulative frequency distributions,
(mid-day data, 1974- 1976).
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due to both climatological variations and air quality variations. Because
a previous report (Trijonis and Yuan, 1978b) noted a substantial increase
in haze within the Northeast during the summer quarter (July, August, and
September), visibility maps have been prepared for the third quarter as well
as for the entire year.
Table 1 lists the 101 study locations and the median visibilities for
1974-1976. Special remarks are made to note locations which involved photo-
graphic photometry data, nephelometry data, or extrapolation of the frequency
distribution. The remarks also note those locations which significantly
changed reporting practices during 1975 in response to the National Weather
Service directive that visibility could be estimated out to a distance twice
as far as the farthest marker. For those locations changing reporting
practices in 1975, an overall median visibility is determined by averaging
the medians for the 1974 and 1976 data.
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
ALABAMA
Huntsville (HUN)
Montgomery (MNT)
ARIZONA
Holbrook (HOL)
Page (PAG)
Palo Verde (PVE)
Prescott (PRS)
Tucson (TUC)
Winslow (WIN)
Yuma (YUM)
ARKANSAS
Fort Smith (FTS)
CALIFORNIA
Arcata (ARC)
Bakersfield (BAK)
Bishop (BHP)
Blue Canyon (BLC)
China Lake (CHL)
17
9
68F
72F
55F
•*
80
60
*
80
59
22
15
14
90*
46
50
Photographic photometry data for 1973-
1974 provided by Arizona Public Service
Company. Data are described by Roberts
et al. (1975).
Photograohic photometry data for 1974
provided by the Salt River Project.
Photographic photometry data for 1975-
1976 provided by Arizona Public Service
Company. Data are described by APSC
(1976).
Based on uncertain nonlinear extrapolation
of frequency distribution.
Based on uncertain nonlinear extrapolation
of frequency distribuiton.
N
Involves uncertain nonlinear extrapolation
of the frequency distribuiton.
Nephelometry data for 1975-1976 provided
by Tom Dodson of the China Lake Naval
Weapons Center.
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, -Site
Fresno (FRS)
Long Beach (LNB)
Red Bluff (RDB)
Sandberg (SAN)
COLORADO
Colorado Springs (CSP)
Grand Junction (GRJ)
Piceance Creek (PI)
Median
Mid-Day
Visibility
(miles) REMARKS
13
11
61
49
90
84
p
70 Photographic photometry data for 1975-
Pueblo (PUB)
CONNECTICUT
Bridgeport (BPT)
Windsor Locks (WLS)
DELAWARE
Wilmington (WM)
FLORIDA
Tallahassee (TAL)
West Palm Beach (WPB)
GEORGIA
Augusta (AUG)
Rome (ROM)
1976 provided by Occidental Shale Oil
(C-b Shale Oil Venture, 1977).
77
15
19
15
12
15
11
12
Reporting practices change in 1975. Non-
linear extrapolation of 1974 distribution
yields a 13 mile median; interpolation of
1976 distribution yields an 11 mile median.
Reporting practices change in 1975. Non-
linear extrapolation of 1974 distribution
yields a 16 mile median; interpolation of
1976 distribution yields a 15 mile median.
Reporting practices change in 1975. In-
terpolation of 1974 distribuiton yields
an 11 mile median; interpolation of 1976
distribution yields a 14 mile median.
10
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
Savannah (SAV)
IDAHO
Pocatello (POC)
ILLINOIS
Moline (MOL)
Springfield (SPF)
INDIANA
Evansville (EVS)
IOWA
Dubuque (DUB)
KANSAS
Dodge City (DOC)
KENTUCKY
Lexington (LEX)
LOUISIANA
Baton Rouge (BTR)
Shreveport (SHV)
MAINE
Caribou (CRB)
16
39
13
11
10
19
*
29
11
9
11
33
Reporting practices change in 1975. Non-
linear extrapolation of 1974 distribution
yields a 13 mile median; interpolation of
1976 distribution yields a 19 mile median.
Reporting practices change in 1975. Linear
extrapolation of 1974 distribuiton yields
a 9 mile median; interpolation of 1976
distribution yields a 13 mile median.
Involves uncertain linear extrapolation
of frequency distribution.
Data available only for 1975 and 1976.
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
an 11 mile median; interpolation of 1976
distribution yields a 12 mile median.
Involves linear extrapolation of frequency
distribution.
11
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
MASSACHUSETTS
Worchester (WOS) 18
MICHIGAN
Muskegeon (MUS) 11
Sault St. Marie (SSE) 17
MINNESOTA
Rochester (ROC) 16
MISSISSIPPI
Jackson (JAK) 13
MISSOURI
Springfiled (SPR) 17
MONTANA
Billings (BIL) 60
Great Falls (GRF) 65
Kalispell (KAL) 41
NEBRASKA
Grand Island (GRI) 22
Reporting practices change in 1975. Linear
extrapolation of 1974 distribution and in-
terpolation of 1976 distribution both
yield 11 mile medians.
Reporting practices change in 1975. In-
terpolations of 1974 and 1976 distributions
both yield 17 mile medians.
Reporting practices change in 1975. Linear
extrapoation of 1974 distribution yields a
16 mile median; interpolation of 1976 dis-
tribution yields a 17 mile median.
Involves linear extrapolation of frequency
distribution.
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 64 mile median; interpolation of 1976
distribution yields a 67 mile median.
Reporting practices change in 1975. Linear
extrapolation of 1974 distribution yields
a 24 mile median; interpolation of 1976
distribution yields a 21 mile median.
12
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
NEVADA
Ely (ELY) 65*
NEW HAMPSHIRE
Concord (CO) 37
NEW MEXICO
Farmington (FAR) 80
*
Truth or Consequences 70
(TRC)
*
Tucumcari (TUI) 45
NEW YORK
Albany (ALB) 22
Binghampton (BMT) 17
Syracuse (SYC) 13
NORTH CAROLINA
Asheville (ASH) 18
Charlotte (CHR) 12
Wilmingotn (WIL) 10
OHIO
Columbus (CLB) 11
Based on uncertain nonlinear extrapolation
of frequency distribuiton.
Involves linear extrapolation of frequency
distribution.
Involves nonlinear extrapolation of
frequency distribution.
Involves uncertain nonlinear extrapolation
of frequency distribution.
Involves uncertain linear extrapolation
of frequency distribution.
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 17 mile median; interpolation of 1976
distribution yields a 20 mile median.
Reporting practices change in 1975. In-
terpolations of 1974 and 1976 distributions
both yield 10 mile medians.
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 10 mile median; interpolation of 1976
distribution yields a 12 mile median,
13
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
Dayton (DAY)
Youngstown (YUN)
OKLAHOMA
Oklahoma City (OKC)
OREGON
Astoria (AST)
Burns (BRN)
Medford (MED)
Pendleton (PEN)
Salem (SAL)
PENNSYLVANIA
Harrisburg (HSB)
Wilkes-Barre (WBS)
Williamsport (WIM)
SOUTH CAROLINA
Columbia (COL)
SOUTH DAKOTA
Huron (HUR)
Pierre (PIR)
10
10
17
17
65
25
29
23
13
19
17
14
20
•i
45
Reporting practices change in 1975. Linear
extrapolation of 1974 distribution yields
a 9 mile median; interpolation of 1976
distribution yields a 12 mile median.
Reporting practices change in 1975. Linear
extrapolation of 1974 distribution and in-
terpolation of 1976 both yield 17 mile
medians.
Based on linear extrapolation of frequency
distribution.
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 17 mile median; interpolation of 1976
distribution yields a 16 mile median.
Reporting practices change in 1975. Un-
certain nonlinear extrapolations of 1974
and 1976 distributions yield 40 and 50
mile medians, respectively.
14
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
TENNESSEE
Bristol (BRS)
Chattanooga (CHT)
Memphis (MEM)
TEXAS
Abilene (ABL)
Corpus Christi (CC)
Del Rio (DLR)
Port Arthur (PTA)
Waco (MAC)
UTAH
Bonanza (BON)
Cedar Mountain (CMT)
14
13
11
36
15
34
18
16
74
PN
Wendover (WEN)
VERMONT
Burlington (BUR)
80
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 10 mile median; interpolation of 1976
distribution yields a 12 mile median.
Reporting practices change in 1975. In-
terpolation of 1974 distribution and
linear extrapolation of 1976 distribution
both yield 36 mile medians.
Reporting practices change at end of 1975.
Interpolation of 1974/1975 distribution
yields a 35 mile median; interpolation of
1976 distribution yields a 33 mile median.
Nephelometry data for 1975-1976 provided
by Aerovironment Inc. Data are described
by Tombach and Chan (1977),
Photographic photometry and nephelometry
data for 1976-1977 provided by NOAA En-
vironmental Research Laboratories (Allee
et al. 1978). The photographic photometry
data yield a 74 mile median; the nephel-
ometry data yield a 75 mile median.
Involves uncertain nonlinear extrapolation
of frequency distribution; data are
available only for 1974 and 1975.
23
15
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TABLE I. LIST OF STUDY SITES AND MEDIAN YEARLY VISIBILITIES (Cont'd)
STATE, Site
Median
Mid-Day
Visibility
(miles)
REMARKS
VIRGINIA
Dulles (DUL)
Lynchburg (LYB)
Richmond (RIC)
Roanoke (ROA)
WASHINGTON
Olympia
Spokane (SPK)
Yakima (YAK)
WEST VIRGINIA
Charleston (CHS)
WISCONSIN
Green Bay (GBY)
Madison (MAD)
WYOMING
Cheyenne (CHY)
Rock Springs (RSP)
17
21
10
31
29
36
47
10
16
16
64
76
Reporting practices change in 1975. In-
terpolation of 1974 distribution yields
a 10 mile median; interpolation of 1976
distribution yields an 11 mile median.
Involves linear extrapolation of frequency
distribution.
Reproting practices change in 1975. In-
terpolation of 1974 distribution yields
a 9 mile median; interpolation of 1976
distribution yields a 10 mile median.
16
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CHAPTER 3
VISIBILITY ISOPLETH MAPS
This chapter presents maps which illustrate the nationwide patterns
in yearly and summertime visibility. Three types of map are presented:
maps indicating median visibilities at individual locations, maps illus-
trating isopleths drawn to these data, and shaded isopleth maps. This
chapter also provides a brief analysis of the maps; this analysis, however,
is only of a descriptive nature. To perform a causal analysis explaining
the visibility patterns would require considerably more work, such as ex-
amination of spatial emission patterns, scrutiny of climatological patterns,
study of nonurban air quality data, and stratification of the visibility
data into fixed meteorological classes.
MEDIAN YEARLY VISIBILITIES, 1974-1976
Figure 3 presents the median yearly visibilities for the 101 study
sites, plotted at the locations of those sites. Figure 4 illustrates
isopleths drawn to the data, and Figure 5 provides a shaded isopleth map.
The quality of the visibility data is evidenced in Figures 3 and 4
by the monotonic trends that often exist in passing from areas of poor
visibility to areas of good visibility, and by the consistency of the read-
ings at many neighboring stations even at distances of a few hundred miles.
As prime examples of monotonic spatial trends, we note the following: on a
straight line from Concord, NH to Charleston, WV, one encounters a sequence
of data points of 37 miles, 22 miles, 19 miles, 12 miles, and 10 miles;
continuing on another straight line from Charleston, WV to Rock Springs, WY,
the sequence of data points proceeds as 10 miles, 11 miles, 22 miles, 64
miles, and 76 miles. As examples of consistency, we note that the 8 stations
within 300 miles of the central Indiana/Ohio border all record visibilities
of 10 or 11 miles, and that the 8 stations within 200 miles of Grand
Junction, CO all record visibilities of 70 to 84 miles. There are some
sites where visibility appears to be anomalous compared to the general pat-
terns, but these sites are few in number, and the aberrations are usually
minor.
17
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The isopleths in Figures 4 and 5 have been drawn to distinguish six
basic ranges in median visibility: greater than 70 miles, 45-70 miles, 25-
45 miles, 15-25 miles, 10-15 miles, and less than or equal to 10 miles. The
best visibility (70 miles) in the country occurs in the mountainous South-
west -- an area comprised of Utah, Colorado, Nevada, northern Arizona,
northern New Mexico, and Southwestern Wyoming. Visibility is also quite
good to the north and south of this region. Passing westward or eastward,
fairly sharp gradients occur. Median visibility falls to less than 25 miles
in a narrow band along the northern Pacific coast, to less than 15 miles in
the central valley of California and the Los Angeles basin, and to less than
15 miles at the Mississippi.
Most of the area east of the Mississippi has a median visibility of
15 miles or less. In fact, three areas — the Ohio River Valley, the
Louisiana/Mississippi/Alabama Gulf coast, and the North Carolina/Virginia
coast -- exhibit visibility of 10 miles or less. Visibility improves toward
the northern Atlantic States, exceeding 15 miles in eastern Pennsylvania and
eastern New York, and exceeding 30 miles in New Hampshire and Maine. A long,
narrow region of 15 miles visibility also extends down the entire eastern
slopes of the Appalachian Mountains.
MEDIAN THIRD-QUARTER VISIBILITIES, 1974-1976
Figures 6, 7, and 8 are similar to Figures 3, 4, and 5, except they
represent median summertime (third quarter) visibilities. Again, the quality
of the data is evidenced in Figures 6 and 7 by the monotonic trends appearing
in many of the spatial patterns, and by the good agreement that often exists
between nearby locations. In fact, there is one continuous region — the
Ohio Valley and areas to the south and east of the Ohio Valley — that in-
cludes 22 stations, all of which exhibit median visibilities of 8 to 10 miles.
Comparing Figure 7 to Figure 4, or Figure 8 to Figure 5, it is obvious
that summertime visibility is significantly lower than yearly visibility in
the entire area east of the Mississippi and south of the Great Lakes. This
phenomenon is also apparent, but to a lesser degree, in the south-central
states (i.e. Texas, Arkansas, and Louisiana). Most of the western United
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States show little change in the summer, with some stations increasing
slightly and other stations decreasing slightly compared to the yearly
medians. The only exception in the West is the Pacific Northwest, where
visibility increases significantly during the summer.
The nationwide spatial gradients in visibility during the summer are
fairly similar to those during the entire year. As shown in Figures 7 and
8, the same large region in the mountainous Southwest still experiences
median visibilities of 70 miles. Visibility also remains good north and
south of this region, and strong gradients again exist toward the west and
east of this region. Visibility falls to less than 25 miles along a narrow
strip on the northern Pacific coast, to less than 15 miles in California's
central valley and coastline, and to less than 10 miles in the Los Angeles
basin. Visibility also falls to less than 10 miles as one moves east across
the southern half of the Mississippi River.
A very large area east of the Mississippi and south of the Great
Lakes exhibits summertime visibilities of less than lOmiles. The spatial
trend is such that visibility becomes better than 10 miles in central
Pennsylvania, western New York, and a narrow strip on the eastern slopes
of the Appalachians. Summertime visibility exceeds 15 miles in eastern
New York and southern New England and 25 miles in northern New England.
25
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REFERENCES
Allee, P.A., et al., "Air Quality Studies in Carbon and Emery Counties,
Utah, December 1976 - July 1977", Draft Report by NOAA Environmental Re-
search Laboratories, 1978.
APSC (Arizona Public Service Company), "Visibility and Particle Analysis
for the Palo Verde Nuclear Generating Station", Reports for July 1975-
March 1976, 1976.
Bo!in, B. and R.J. Charlson, "On the Role of the Tropospheric Sulfur Cycle
in the Shortwave Radiative Climate of the Earth", Ambio. Vol. 5, No. 2,
1976.
Husar, R.B., et al., "A Study of Long Range Transport from Visibility
Observations, Trajectory Analysis and Local Air Pollution Monitoring
Data", Presented at the 7th International Technical Meeting on Air Pol-
lution Modeling and its Application, NATO/CCMS, Airlie, Virginia,
September 7-10 1976.
Husar, R. and D. Patterson, Report in preparation for EPA Office of Re-
search and Development under Grant No. 802815, Washington University,
St. Louis, Missouri, 1978.
Miller, M.E., et al., "Visibility Changes in Ohio, Kentucky, and Tennessee
from 1962 to 1969", Monthly Weather Review, Vol. 100, No. 1, pp. 67-71,
January 1972.
Roberts, P.M., et al., "Visibility Measurements in the Painted Desert",
Paper No. 75-26.1, Presented at the 68th Annual Meeting of the Air Pol-
lution Control Association, June 1975.
Tombach, I.H. and M.W. Chan, "Physical, Chemical, and Radiological Charac-
terization of Background Particulate Matter in Northeastern Utah", Paper
No. 77-48.6, Presented at the 70th Annual Meeting of the Air Pollution
Control Association, June 1977.
Trijonis, J. and K. Yuan, "Visibility in the Southwest, An Exploration of
the Historical Data Base", EPA-600/3-78-039, April 1978.
Trijonis, J. and K. Yuan, "Visibility in the Northeast", Prepared at
Technology Service Corporation under Grant ilo. 802815 for EPA Office
of Research and Development, 1978.
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
EPA-450/5-79-010
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
Existing Visibility Levels in the United States
5. REPORT DATE
September 1979
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
John Trijonis and Dawn Snap!and
8. PERFORMING ORGANIZATION REPORT NO
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Technology Service Corporation
Route 3, Box 124-K
Santa Fe, New Mexico 87501
10. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
Grant No. 802815
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Research Triangle Park, N. C. 27711
13. TYPE OF REPORT AND PERIOD COVERED
Interim Mav-Seot. 1978
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
Maps are prepared which illustrate median, mid-day visibility levels in suburban/
nonurban locations of the continental United States. Median' visibilities at 94
locations are determined from cumulative frequency distributions of quality-checked
airport observations. Seven locations in the Southwest with photographic photometry
or nephelometry data (which agree quite well with the airport data) are also
included.
The spatial pattern of visibility is demonstrated with isopleth maps for both the
annual medians and summertime medians during the years 1974-1976. The consistency
of the spatial gradients in visibility and the agreement between neighboring
locations attest to the quality of the visibility data. The isopleth maos reveal
that the best visibility (70+ miles) occurs in the mountainous Southwest. Visibility
is also quite good (45 - 70 miles) north and south of that region, but sharp
gradients occur to the east and west of that region. Most of the area east of the
Mississippi and south of the Great Lakes exhibits median visibilities of less than
15 miles annually and less than 10 miles during the summertime.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. cos AT I Field/Group
Air pollution
Visibility
Haze
United States
18 DISTRIBUTION STATEMENT
Release to public
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
35
20 SECURITY CLASS (Thispage)
22. PRICE
EPA Form 2220-1 (9-73)
27
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