319
LOW-LEVEL INVERSION FREQUENCY IN THE CONTIGUOUS
UNITED STATES*
CHARLES R. HOSLER
U.S. Weather Bureau, Washlngton, D.C.
[Manuscript received May 3, 1961; revised June 19, 1961
ABSTRACT
A tabulation of percent frequencies of inversion and/or isothermal conditions, based
below 500 feet above station elevation, for Weather Bureau radiosonde stations in the
contiguous United States for four observation times, provides a sampling of daytime and
nightt,ime stability conditions for all time zones. Thcse data are analyzed with respect to
nighttime surface wind speed and cloud cover. Radiosonde data are compared to vertical
temperature gradicmt data obtained from meteorologically iwtrumented towers and valley-
ridge stations. Finally, an attempt is made to delineatc., geographically and climatologically,
the percent frequvncy of low-level stability for the entire country.
1. INTRODUCTION
Air pollution surveys of entire industrid regions :mtl
metropolitan areas are in demand by public and private
agencies for purposes of evaluating levels of air contnmi-
nation, air zoning regulations, and identification of
obnoxious sources. In recent' -cars meteorology h s as-
sumed an important role in the atom.ic energy industry for
engineering and operational applications com.m.on to the
industry generally, but in particu1:rr because of its uppli-
cation to the study of dispersion of airborne rndioactivit'y
by the atmosphere. Wcnt~ller may play :tn im.port:lnt role
in either conventiord or nuclear intlustries and the proper
appreciation of certain m.eteorologicul factors is pnrnmount
for an intelligent a n d y i s and cvduation of air pollution
phenomena.
In essence the dilution efficiency of the atmosphere
depends on the wind ~n d temperature gradients, both of
which vary verticdly, llorizontttllyv, and with time. Since
the general spatial prob1em.s of slow atmospheric dispersion
which affect large areas unquestionably arise at times when
a stilble stratified layer of air exists I W A ~ the S I I ~~~I C C , ,Z
knowledge of the frequency of low-level stability for geo-
gr:rphicul and/or climatologicnl areas will he helpful for
assessing the potential for air pollution inllerent to :L givm
region. This study involves the compilation of irlversion
frequencies and the :innlJ-sis of related meteorological
parameters in an attem.pt to delineate, geogr:lpllic:tlly and
climatologicull~*, the percent' frequency of low-levcl
stability for the contiguous United States.
2. INVERSIONDATA
The initial step for obtaining statistics of inversion
*Support for this study w t s jointly provided hy the C .S . Atomic Energy Commission
and the U.S. Public Heaith Service.
occurrence W:LS to wlect upper air (latit that most nearly
correspotltls to tlaytimc and nigllt,time periods. Table 1
gives t h e obscrv;ltion times for radiosonde soundings from
mllich vertical temperature gradient data could he ob-
ttLinetl. I t W:LS decided tllitt a 2-yttr sampling of both
0300-1.500 GMT ant1 0000-1200 GMT observation periods
wor1ltl he sufficient to give representative frequencies of
clnytimc it11tl nigl1ttim.e inversion occurrence for each time
zone. dcco~dingly, thc Xational Weather Records Center
a t Asllcville, hT.0., tabulated for selected stations a percent
frcquenvy of inversions alld/or isothermal layers based a t
or lwlow 500 feet ahow the station elevation. This level
w:ts ar1)itrarily selectrtl to define :t minim.um. vertical tlcptll
of air, a n t 1 it dso permitted conveniellt t:~hul:ttion of d a t u
from r:diosontle sountling plots. These frequencies, given
to the nctlrest' whole percent, :ire h s e d on all observiltiorls
:lc.tu:lll>- tilkcn for :L given time :~rltl season (Winter;
Drcernbrr, J:inu:lr>-, February; Spring: March, April,
?thy; ctc.). Tlwsr data are listed in the Appendix.
320 M O N T H L Y W E A T H E REVIEW SEPTEMBER 1961
TABLE 2.-Approximate n u m b e r o f n i g h t t i m e h o u r s (in dQ-how d a y ) TABLE 3.-Example of inzwsion percent frequency determination for
inland stations
__- Bismarck, N. 1)ak.
North 9 13
Soutl1 Central- 13
...............
recording of vertical temperature gradient such as that
obtained from towers gives such statistics, but the radio-
sonde data available do not permit such R tabulation
directly. Therefore, to prepare frequencies of low-level
st'abilit,y as a percent of total seasonal hours from radio-
sonde data, the following hypothesis was used for analyz-
ing data from inland stations: The entire nocturnal period
\vas considered stable if an inversion was observed at, any
time during the night, and t'he maxirnum percent, frequency
of inversions for any one observation time within a
nocturnal period was assumed applicable to the entire
night,time period; and, conversely, the entire daytime
period was assumed t'o be unstable, and low values of
inversion frequencies occurring at observation times during
the daytime heat,ing period were not considered in com-
putation of inversion frequency as R percent of total
hours.
A review of table 1 suggests that for the most, part
daytime observations are made either shortly after sun-
rise (transit,ion period of st,abilit'y to instability) or shortly
before sunset (instability to stability) ; consequently, the
"daytime" observations are not truly representative of
the unstable solar heating period. In general, it' is felt
that ignoring t'he low frequency values €or daytiInc obscr-
vat'ions cornpensated for the apparent overcstilnation
arrived a t from attributing the maximurn frequency value
to an entire nocturnal period.
,4 rough determination ol length of nighttin~e periods
was made froln the Smithsoninn Meteorological Tables
[9] and resulted in three stat'iorl categories: (a) t'he north-
ern tier of States; (b) an area roughly between 30' and
40°,N. latitude, conlprising tlle largest number of stations;
and (c) the soutllerrl regions alor~g the Gulf of lfesieo.
The number of nighttime hours in each category by
seasons is ~110~11 in table 2.
Maxi-
mum percent value (night-
time)
n
\\-inter .................. 67
Spring.. ................ 61
Summcr ................. 77
Fall ..................... G9
Ohservation time ([.ST)
0600
0600
2100 and 0600
2100
Nixht-
time
(hours) period
Fractiorl of
24-hour day
frequency Inversion
total hours) (percent of
A X B
G7X.625=42
61X.458=28
77X.375329
69X.542=37
The method €or computing a percent' frequency of low-
level stabilit,y for inland stations, defined as a percent of
total se:lson:tl (:lunual) hours, is exemplified in table 3,
using rdiosoude dtlta from Bismarck, K. Dak. The
results from this technique were conlptued t'o continuously
recorcled vertical temperature gradient dat'u from towers
(discussed lat'er), and appear to be quite adequate €or an
overall climatological andysis.
I n analyzing and computing the inversion frequency
(lata, it became apparent that t'here was a notable excep-
tion to this technique. Inversion frequencies obtained
from coastal stations reflect marine influences; that is, in
cotlstnl areas low-level stability may be either inhibited
or erhtnced by advwctiorl, in contrast to inland areas that,
for tlle m.ost part, exhibit radiation-t'ype inversions. The
coastal regions could be divided into (a) the Atlantic
co:lstal sections of t,lle Soutl~cr~stern States and t'lle ent>ire
co:mtal region of t'he Gulf of Mexico, where relatively
I V I L ~I I ~ w-ater exists offshore, and (b) coast'al sections
of Kew Eng1;md and the Pacific States w h r c ocean
wut,ers are relatively cold. The effects of air-water-land
temper:tture difierential and associated land-sea breeze
regimes on low-level stability are apparently reflect'cd in
the inversion frequency values of m.any coastal st n t ' lolls
that are affected by a frequent flow of air from over tllc
OCCLIII. 111 generd, for coastal :m:ts along the Soutll-
TABLE 4,"C'omparison of coastal and inland inuersion frequencies (percent)
I'aciJc yoa'oast ............................................... LS?' 16 18 04 07
Oak!and, Calif. (coastal)..-- ............................... IO 66 ii X8
hlcdford, Oreg. (inland).""" ............................
'ratoosh Island, IVash. (coastal)
Ili 7.5 65 i 0
17 22 11 1.5
Seattle. l\-;~sh. (inland) .................................... 24 13 68 31
...........................
.............................................
13urr\vood, La. (coastal) ................................... 46 63 lis 38
Jwko Charles, La. (in!and) ................................ 12 61 58 30
G,12fof.ve*ieo L S 7 ' 18 d l 06 09
Southeartern Atlantic- ...................................... LS7' 19 $2 07 10
Key \Vest, F!a. (marine) .................................. 1 2 6 26
Miami, Fla. (coastal) ...................................... 29 3Y 80 6
Jscksonvillc, Fla. (inland).-. .............................. 44 69 59 27
Northpastern and J l i d d l r Atlantic ........................... LAY?' 18 2 9 07 10
IIcmpstead, L.I., N.Y. (coastal) ........................... 18 21 24 7 Lakchurst, N.J. (inland) .................................. 41 17
Washington, U.C. (inland) ................................ 30 44 48 22
-~
18 d l 06 09 18 d l 00 09 1'9 21 00 09
1 X 57 51 11 4 26 11 1 13 38 33 3
3 53 58 5 5 62 71 3 9 75 GI 12
TABLE 5.-Example of inversion percent frequency determination f o r coastal stations
Representative LST ohscrva- Nipht fraction Representative 1,ST ohserva- Day fraction Inversion per-
1
nighttime
1
tion
I
of 24-hour day
1 1
daytimr vduc
I
tion
1
of 24-hour day
11
cent frequency v a l u e (p e r c e n t ) (pcrccnt) for total hours
_~___ - ___~ "_ ___ - _____
dfiami, H a .
I
Oakland, Calif.
I
3!J
27 2200
2200 27
2200
2200 41
0400
0400 0400 82 0400
-
east,ern Stnt'es and Gulf of Mexico, a. flow from over
warm wat>ers tends t o inhibit irlversion form-ation a t night',
while daytime sea breezes exhibit neutral stability. The
net result is lower inversion frequency for coast'ul arcas
than for inland trreas in these regions. The cold waters
off the Pacific coast and r~ortl~easterrl Atlantic coast
produce cool sea breezes which enhnnce low-levcl stability,
resulting in higher frequencies of inversions, particultrrly
during dayt'ime hours, for these areas. Examples of these
coastal regimes are given in table 4, where a list of i n -
version frequencies for coastal and ne:rrby inland st'atiorls
can be compared.
A review of these comparisons suggests that loc~ <L t' Ions
only a few miles inland from the coast exhibit :L conti-
nental-type frequency of low-levcl stabilitJ- (e.p., Lake
Charles and Jacksonville). ("onseqnently, in ~~nalyzing
the radiosonde dat'a for stat'ions in coastal areas, i t W:LS
necessary subjectively to c1:~ssify by season each st:ttion
as LLinl:lnd" or (' C O R S ~A ~" according to the rrlwrinc effects
reflected by the frequency data, as u d l as to corlsitlcr
station proxirnit,p to the water and prcv:Liling wind dire(.-
tions. For stations tlcsignat,cd coastd, both tl:~~-tirr~c a,nd
nightt'irne inversion frequency vnlues were considered for
computing :m inversion frequency as :I pcrccnt ol total
hours, instead of ncglccting daytilrle valucs, as was clonc
for inlmd stations. An example of attributing both :L
representative djt-time and nighttime percout, frequency
value t.0 the respective tliurn:J periods lor c:oruputing
inversion frequencics is shown in table 5 for the coastal
cities of Oddtmtl and Mianli.
Isopleths of inversion frequency, exprcssecl as :I pcrcent
of total hours, for t,lle contiguous Ullited States :Lrc show1-11
i n figure 1 &E.
Isopleths of t h e n l :~x i ~n u n ~ inversion frequeucJ- v d u c
reported t t t an^- of t'hc four observtbtion times :~ro show11
infigure 2 &E. Oor~scrv:~tively, these values sl~ow tllc
percent frequency of nights (or d:tys) t,ll:Lt 11:td at least o n e
hour or inversion. However, it is reasonahle to interprtrt
these st'atistics to show a percent frequency of nights (or
days, for some coastal areas) l~aving several hours of' inver-
sions, since an inversion report'cd at any one 11ourl~- obser-
vation during the night (or day) implies that' such stability
will continue (or h s coot'inuecl) for more than one hour
if i t already exists. This has been shown b\- :malysis of
I (, '1 '4 I+ nn,
0. 542 1000 0.458 21+3=24
. so0 1000 ,500
1000 ,542
3 1000 ,500
14+l= 15
12+1=13
20.5+1.5=22
cumulative frequencies of inversions obtained from
rnoteorological tower data [I, 4, 8, IO] at inland sites.
Such d a t a for four sites are shown in figure 3 . These
d:~ta are plotted on log-probability scnles to enhance the
k)r~:~ks" in the curves (abrupt changes in probability)
which reflect the lengt'h of stable nocturnal periods.
These nnulyses ol tower data also support the concept
uscd to tlct~orrninc inversion frequency for iulnnd s t a t' ions
as :L pcrccmt of t o t d t'irne. That is, R percent frequency
of invcrsions at night' for any one hour gives :t value that
is approxitllntcl?- representative for any hour during the
r~octurrld stable period.
The inherent shortcomings involved in this t-pe of
malysis nre obvious. The subjectivity in the analytical
rnethotls, differences of topogr:tplly, geograph~-, obscrva-
tion tilne (relntivc to the diurn:~I cycle), and sampling
periods arc' sources ol possible error. However, a more
exact qunntitntivc tLssessment of low-level inversion
frequellcy is not c:rsily obtained from existing radiosonde
data. I t is hoped that this study may provide a means for
co1npcLrirlg onc section of the country to anotlwr, relative
to inwrsion frequenc.y, as w ~l l a s furnishing irlforrrlation
hr:lpful for tlovcloping t~ diffusion climatology of the
Urlitctl States.
L (
4. CLOUD COVER AND LOW WIND SPEED
Xlet,corologists, from cxpcrience, would expect clear
skies and light surfaces winds :it night to favor t'he Corma-
tion ol nocturnal radiation-type inversions over land.
(:loud cover :md wind spced data for avdtlble st'at'ions,
t3ahul:ttcd for a 5-ycar pried in Xu~nrnnry sf Hour.Zy
Obsrnvxtiona by the U.S. Weatller Bureau were analyzed
with rospect t o t h e inversion frequency data. These
st:ltistic:s ttre t:lbulttted in the L\ppentlix.
The inherent suhjectivit!: involvect in nightt'inlc obser-
vatious ol cloud cover (c..g., thin cirrus), together with
tlifterctlt exposures and c\levations of t~nernonle,try, restricts
m y qu~tntitative andysis. However, :I st~;~t~isticnl study
W:LS trt:t,dr of t'he rela,t,ionship of nighttime cloud cover and
wind s p e d to inversion frequency, resulting in poor
correltition for d l seasons. For example, figure 4 indi-
c*:ttcs st poor relationship, dthough a trend is eviclcnt, of
observtd ttl:Lxirnurll inversion frequency to occurrence of
nighttime winds (I 7 rn.p.11.) and nighttime cloud cover
322
FIGVRE l.-Inversiorl frequency (percent of total hours) : (A)
Winter, (B) Spring, (C) Sumrner, (1)) Fall, (E ) Annual.
h
323
WINTER
u
FIGURE 2.-Inversion percent frequency (maxirnum observed) : (A)
Winter, (13) Spring, (C) Surnnler, (11) Fall, (E) A I I I ~I I :~.
SUMMER w
ANNUAL
324 MONTHLY 'WEATHEIZ REVIEW SEPTEMBER 1'361
40
a
- 30-
u)
u -
g 20-
W
>-
I- -
s 10-
8 7 -
2 5 -
a
u) 9- z 8-
6 - z
I- 4 4 - K 3 - 0 3-
2 z -
2 -
>- : W
5
HANFORD. WASHINGTON.
Ai! = 200" 3'
1945-1956
".
.......
- WINTER
-- SPRING
...... SUMMER
-.- FALL
-. - . - . .
PROBABILITY (PERCENT)
40
b - 30-
u - SHIPPINGPORT. PENNSYLVANIA.
u)
p 20- A2 = 430'- 30'
JUNE 1955 - MAY 1957 W
+ ?-
s 10-
s 7 -
4 5-
v) 9-
z 8-
6 -
K 4 4-
3 - 0
Z " WINTER
3-
" 2 SPRING
: 2
z -
- ...... SUMMER
W
> -.- FALL
PROBABILITY (PERCENT)
40r
- 30- In K -
g 20-
W
I-
3
5 -
: 10-
5 8:
0 7
4 5 -
u) 9-
6-
4 4 - n 3 - 0 3-
0 z -
u)
a W
2-
>-
5
d
IDAHO FALLS, IDAHO.
Ai! = 250" 5'
JUNE 1950-SEPTEMBER 1958
- WINTER
" SPRING
...... SUMMER
FALL
I I I I I I I I I I I I I I I I I I I
0.01 0.1 I IO 30 50 70 90 98 99.5 0.01 0.1 I IO 3 0 5 0 70 9 0 9 8 99.5
P R O B A B I L I T Y
(5 3/10) respectively, for winter. Although this indicat,es
that' precise frequency values of low-level inversions r~~:ty
not be obtained for a specific locat,ion from wind speed
and cloud cover data alone, isoplet>hs of percent frequcn-
cies of cloud cover and wind speed, shown in figures 5 and
6 respectively, indicate that the general areas having R
higher frequency of inversions are also c11aract)erized by a
higher frequency of relative1)- clear nights with light winds,
and vice versa. This relationship can be l'urther ilius-
trated by graphic addition of the cloud cover and light
wind frequency values. The sums of t'hese percent
frequencies were divided by 2 to give conventiorld vducs
within the range of 0-100. These isoplct'hs of nighttime
(cloud cover+wind speed)/2 are shown in figure 7.
5. CURRENT OBSERVATION DATA
Since t'wo of the observation times (0300 nnd 1500 GMT)
used in computing inversion frequency are no longer in
use, it is desirable to compare the computed frequency
values to those obtained from a currcnt observation time.
The 1200 GMT observation value WBS chosen for cornpari-
son, since this is the currcnt local observation time in ail
four t'irrle zones that most nearly represents night8tirnc
conditions. Such a comparison serves two purposes:
(I) if perccnt frequencies from four different local obser-
vation t,irnes compare favorably to the computed frequency,
this supports the hypothesis uscd for cornput'irlg the
percent, v:tlues; a n t 1 (2 ) additional 1200 GMT inversion
fr'rcqucbncies, obtained either from towers or other upper
air facilities, mtly be :mttlyzcd for relatively short sampling
periods (months) t'o provide estimates of low-level
inversion frequency for a given irllwncl site.
However, since the 1200 GMT observnt'ion d u e was
used more often than any ot,her value to compute an
inversion frequency, expresscd as n percent of total time
(;.e., of the available radiosonde data the ~n:~xi~nu~n
SEPTEMBER 1961 MONTHLY WEATHER KEVIETV 325
O/O
I O O y
O/O
n
* z 4 0 1 0 0
0 - tn
w > z
a
-
0
0
>
5 80
m 0
70
v)
0
0 0 0 0
0
g 401 0 0
0
0
0 0
0 0
30
20
0 w
g 20
lo% 20 30 40 50 60 70 80 90
NIGHTTIME CLOUD COVER (5 3/10) NIGHTTIME WIND SPEED (5 7 M P H )
FIGURE 4.-Itelationship of maximum observed nighttime irlvcxrsion pcrcrnt frcqucncy to (A) nighttime cloud covers 3/10 and (U) wind
speeds 7 m.p.11. Occurrencr. for winter.
percent value was observed most often a t 1200 GMT, in
any season, for a t least 60 percent of the stations), it was
necessary to analyze the unrelated data to justif.yr the
selection of the 1200 GMT observation value as being repre-
sentative of the nocturnal period, for all time zones.
This was done lor all inland stations by determining t'he
variattion of computed frequency, based on the 1200 GMT
value, from the computed frequency based on the muxi-
mum observed value, for all cascs when the maximum
value was a t a n observation time other than 1200 GMT.
This comparison showed that for any season t'he cornputcd
frequency determined from the 1200 GMT value, when
averaged for all inland stations, differed by less than 3.4
percent from the computed frequency value determined
from rnaxirnum frequencies at 0000, 0300, or 1500 GMT.
On the other hand, since the fract'ion (nighttime hours)/24,
attributed to the length of the nocturnal period is factored
into t'hc computed frequency value, it can bc r.sti1natt.d
that the 1200 GMT frequency value averages about 7 pcr-
cent less than the maximum value, when the rnaxi~~lurn
frequency occurs a t 0000, 0300, or 1500 GMT. This indi-
cates that of the available radiosonde data andyzed, the
1200 GMT percent vrtlue is the most represrntative 01 the
nocturnal period for all four timc zones. &o, i t was of
interest t'o note t'hat the 0300 GMT value yields a fairly
close approximation of nighttime inversion frequency for
fall and winter seasons; however, t h e 1500 GMT v d u r
appears t'o be unrepresentntivc ol t'he nighttime period,
except for western sect>ions of the country in winter.
Tile next step WAS to relate empirically tho computed
inversion frequency to the currrntly observed 1200 GMT
percent value. This is done in figure 8, which compares
the computed percent value on the ordinate (Y) to the
1200 GMT percent value on the abscissa (X). Each point
rcprcscnts a stntion, and a least square fit provides the
regression line. Coastal stations we not included in the
regression analysis, but they are designated in the graphs.
Their position relative to t,he regression line rnay provide
intlcx to continentdity. Since the orclirlat'e is deter-
n~irletl from tile abscissa value for about 60 percent ol the
c:~ses, no correlations were attempted. However, despite
the re1:Lted datu inherent in the st:ttistics, good relevance
is inc1ic:lted. In general, the relationships are suffic,iently
good to permit use of t'he 1200 GMT radiosonde data, for
inland stations in all time zones, for determining inversion
frequencies us a percent) of total time.
6. TOWER DATA
Tertlperatures obtained from rneteorologicnl towers and
vallcJ--ridgc field stations and an instrumented TV tower
[l , 2 , 4, 6, 7, 8 , 10, 121 provide inversion frequency statis-
tics t h t 8 can bo conlpared t,o inversion frequencies corn-
put,ctl from nc:~rby r>~diosontle stations. For d l nine such
stat'ions ten1per:tture gradient d a h are continuously
recorded m d thereby readily provide frequencies in the
form of percent of total hours. These statistics, which
can be directly corrlpttred to the computed frequencies,
are shown in table 6.
A4nnual
45
43
28
28
23
30
3 1
29
33 41
45
44
4x
32
34
37
35
43
29 21
Period of record Vertical height
Ut.)
from valley floor; for hIV, AZ=60 ft.; 5 ft. from the floor of a wooded valley; lor X-10, A.%=135 ft.; 5 It. abop a smel! ridge overlonking an industrial type valley, YO ft. below.
I Average of'rower Shielding Facility (TSF), Melton Valley (XI\7), and Oak Ridgr Sational Laboratnry sltr (X-Ill Sea.): For TSF, AZ=300 f t .; above a wooded ridgp top, 500 It.
While most of the towers give vertical temper' <l t ure
gradients within :t height interval of less than 500 feet,
this is not a serious obstacle for comparing the two set's
of data, because nearly all nocturnal inversions (btLskd
below 500 feet above the surftkce) report'ed by inland
radiosonde stations are based below 100 leet, above the
ground. In fact, most are surface based. The inherent
discrepancies involved in differences of (a) periods and
lengths of record, (b) geography, and (c) instrun~ent* 'L t)' ion
are obvious; consequently, it, is surprising to find that
the frequency values cornpare so closel_v. In general, the
tower data sugge,st' that the methods of computation for
determining inversion frequency from radiosonde data arc
sufficiently reliable for general climato1ogic:~l analJ-sis.
7. OTHERCONSIDERATIONS
Since most of the Weather Bureau r:tdiosonde stations
are located a t airports which are at tlle outskirts of cities,
the inversion frequencies for the most part' arc represent-
ative of semi-rural or suburhurl areas. Consequently, one
would expect higher frequencies of nocturnal inversions
in the open country; but', lower frequencies for cerlt'ral
city areas would be expected because of the mecl1:tnictrl
turbulence and heat effects imposed by large metropol-
itan areas [3]. In the Rocky Mountain and Appalachian
regions most of the weat'her stations :we located in valleys;
consequently, for these areas the percent' vtdues reflect
inversion occurrence for elevat,iorls lower than the average
for the region. Less frequent inversion occurre~~ce would
be expected on mountain slopes and tops.
The clim.atic-synoptic aspects of t h e west roast, partic-
ularly the cottstd areas of Citliforniti and southern 01~go11,
deserve special at'tcntion. The m.tlritim.e irlfluenccs itrltl
>L semipermanent high pressure pattern produce dominat-
ing effects. The large-scale subsidence of air at, alt'itudes
above the ocean-cooled surface layer results in a stable
layer which is semipermanent over the California and
Oregon coasts during spring, summer, and fall months.
The subsidence inversion acts a s a "lid", limiting the
vertical motions originating in the marine air below; con-
sequently, it is an important contributor to the gross ac-
cumulation ofrLir pollution since the inversion base limits
the vertical volume of air in which pollutants may
uceumulnte.
Because the subsidence inversion is oft'en based between
500 and 2000 feet above ground along the coast, it is ap-
parent that' the data used in this st'udy (inversion based
2 500 feet above ground) yield very conservative fre-
quency vttlues for the west coast' area, rchtive to any as-
sessment of trtmospheric pollut'iorl potential for that area.
Unlike studies for other areas of the country, a study of
the diffusion climatology of the coastal areas of Oregon
and California must consider a frequent' and persistent
occurrence of subsidence inversions based below 2000 feet
above tlle ground, :is well as radiation-type inversions
btised below 500 feet. I t is important' t'o not'e that t'lle
present study reflects for t'he most part only surface-based
radiation-type inversions or low-level stability resulting
from cool sct~ breezes. For further details of west coast
inversions, the re:der is referred to studies bJ- Stanford
Research Institute [11] and J. G. Etlinger [5 ].
8. CLIMATIC-GEOGRAPHIC DELINEATION
A review of t,he dist'ribution of inversion frequency over
the count,ry suggests tt general delineation into regions ac-
cording to climatological and geographictll factors, and
this classification is shown in figure 9. The factors relat-
SEPTEMBER 1961 MONTHLY WEATHER REVIEW 327
a -
WINTER
m
SPRING
I
C
SUMMER u
FALL
FIGURE 5.-Prrccnt frequency of Ilighttimc cloud cuvPr <:3/lO:
(A) n-inter, (B) Spring, (C) Surrlrnc.r, (I )) Fall, (E) Arn1n:tl.
r
ANNUAL w
328 MONTHLY WEATHER REVIEW
WINTER w
SPRING w
FIGIJRE 6.-Percent frequcncy of nighttime wind speed 5 7 rn.p.11.:
(A) Winter, (B) Spring, (C) Summer, (1)) Fall, (E ) A111111d.
SUMMER w
ANNUAL w?
329
FIGVRE 7.-Isopleths of nighttime [(percent cloud cover 53/10) 4-
(percent wind speeds7 m.p.h.)]/2: (A) Winter, (B) Spring,
(C) Summer, (D) Fall, (E) Annual.
SEPTEMBER 1961
2 60-
c a WINTER
0
- 50-
Y - 2 16 + 0 57X
r 0
. "
W
z
30-
3 0
1200 GMT INVERSION OCCURRENCE (p e r c e n t )
-
i b SPRING
r
I Y s 5 .6 3 +0 3 E X
8 50t
CHARLESTON
X
S I N DlEGO
1200 GMT INVERSION OCCURRENCE (p o r c o n t )
5 0 1
Y-8.15 + 0.3IX
k 40-
-
W
0 OAKLAND
X
TATOOSH IS
3 0
0
0
BROWNSVILLE
X
-
eo 90 100
1200 GMT INVERSION OCCURRENCE (PelC*nt)
Y =6.81 + 0 47X
U Y
> 5 lo ,b do ,b do $0 70 eo I 90 I IAO
1200 GMT INVERSION OCCURRENCE (p o r c o n t )
- i 6 0 r
e ANNUAL
- 5 0
Y = 4.36 + 0.45X
LOS ANGELESX
.ATOOSH I S 'IJRR
X HATTERASX
SEPTEMBER 1961
2 60-
c a WINTER
0
- 50-
Y - 2 16 + 0 57X
r 0
. "
W
z
30-
3 0
1200 GMT INVERSION OCCURRENCE (p e r c e n t )
-
i b SPRING
r
I Y s 5 .6 3 +0 3 E X
8 50t
CHARLESTON
X
S I N DlEGO
1200 GMT INVERSION OCCURRENCE (p o r c o n t )
5 0 1
Y-8.15 + 0.3IX
k 40-
-
W
0 OAKLAND
X
TATOOSH IS
3 0
0
0
BROWNSVILLE
X
-
eo 90 100
1200 GMT INVERSION OCCURRENCE (PelC*nt)
Y =6.81 + 0 47X
U Y
> 5 lo ,b do ,b do $0 70 eo I 90 I IAO
1200 GMT INVERSION OCCURRENCE (p o r c o n t )
- i 6 0 r
e ANNUAL
- 5 0
Y = 4.36 + 0.45X
LOS ANGELESX
.ATOOSH I S 'IJRR
X HATTERASX
winter months inversions n1ay bc expected about’ 30 t’o
45 percent of the time. The higher frequency during
fall and winter probably is a reflection of r~linir~lurrl
storminess in fall and rnaxirnurn length of a stable noc-
turnal period in winter. The opposite is t’rue for the
spring and sunmer mont,hs. During the colder rrronths,
particularly if snow cover cxist’s, warm southerly t1tlvt.c-
tion over an existing cold surface may enhance low-level
stabilit,y in these areas; however, such advection regimes
are usually t,he incipient, stages of war111 fronts :~nd
cyclones with their associated precipitation and stornri-
ness, which ultimately produce less stable lapsc ratcs
below 500-loot elevations above ground.
Rocky Mountains.-This area also htls nu~rlicd conti-
nentditp. With t’lle exception of the northern regions
during the winter and spring, storms occur infrcqucmtlp,
partJic,ularly over the southwestern areas. ITrllilcc the
Appalachian regions, dry air prevails most’ ol thc titrlc,
and inversion frequency is directly relatcd to t’he diurtld
cycle. Radiation inversions form near t#he surfaw on
most night’s, and they are dissipated shortly aftcr sunrise.
Since the longest, noct,urrlal st,able period and ~rrirrirrr~lrrl
dayt8irne solar heat’ing o c ~u r during the winter, it is this
season thtit is potentially t,lre most stable in this aroa. In
general, inversions occ,ur 40 to 55 percent of the time i n
fall and winter, and about, 30 to 40 percent of thc time i n
spring and summer.
West Coast.-Radiation inversions ovcr interior arc:~s,
including areas only t l few miles from t,llc coast, I I ~C most
prominent8 during the late fall and winter months. Along
the immediate coast radiat,ion inversions may rnergo with
subsidence invcrsions, and low-level stability rr~uy pcrsist
until noon; or, on occasion, invcrsions persist for scvcrd
days during which fog oftcn occurs [Ill. For the lnost
part’, the immedi:xte coast has sufficient' surface winds to
provide good vent,ilation; howcvcr, the persistence of t h e
subsidence inversion in dtlition to radint’ion inversions,
the daily reversal of wind direction (land-sca brccxr), and
topograpllical sheltering ol the valley t ~n d h s i n WYVLS
along coastal sections conlbirw to create a potentidly
adverse clirnat,c as far as air polhtion is conccrncd.
Pacijic Northwetst Coast.~Prccipitation, c.loutlinese, ant1
relat’ivcly high winds tlre dorrrinnnt feat,ures of the climate
of Washingt8on and Oregon coastal tlreas. This region is
situated far enough north of the high pressure cell which
persists off the Chlifornia cotst’ to permit most stortn
systems from t’he Pacific to pass inland over the arctl.
Consequently, storm activity is frequent, part~icularly
during the wint’er and early spring. The instability
associated with t’llcsc frequent, storrrl passages results in
a relnt~ively low frequency of radiation invcrsions. Radi-
ation inversions are rrlost frequent during the sunlnler
and early fall, when storrrl activit’; is at’ a nlinimurn.
ACKNOWLEDGMENTS
T h e aut>hor is indebted to Alessrs. Philip Jason and
Bcnjurrlirl Able for compilation of many of tho clirnato-
logical tlatn. Also, the writer wishes to thank Mrs.
Barbara Ritcllic for draft’ing the diagrarns.
REFERENCES
1. 5. E. 13owne and R. R. Soller, A Meteorological Survey of the
CANEL Sitae at Aliddletow-n, Connecticut, U.S. Weather
Burcau, Middletown, Conn., July 1958 (unpublished).
2. W . RI. Culkowski, Estimates of Accumulated Exposures and
Environmental Build-up of Radioactivity, U.S. Weather
Burrlau Research Station, Oak Ridge, T c k u n . (not dated)
(unpublishrd).
3 . G. .4. I)e hfarrais, “Vertical Temperat,ure Difference Observed
Over an Urban Area,” U.S. Weather Bureau Research Sta-
tion, Cincinnati, Ohio. (Paper presented at 190th National
Alerting of American hleteorological Society, Yew York,
K.Y., J m . 2G, 1961.)
4. G. A. I k hlarrais and N. F. Islitzcr, “Diffusion Climatology of
the ?jat,ional Reactor Testing Station," II>O-12015, U.S.
\\’c>ather I3ureau llesrarch Station, Idaho Falls, Idaho,
April 19GO.
5 . E. G. Edinger, “Variability of Low-Love1 Thernml Stratification
Over Co:tstal Terrain in Southern California,” University of
California at Los Bngclcs, report, on Contract Cwb-SGGG,
6. E. 31. Frisby, “Iceport, on a Secord ?r’rar of Hourly Clirnato-
logical Obscrvations Taken at the P:Lt,hfinder i\tornic Power
Plaut Site Sear Sioux Falls, South Dakota,” South Dakota
State College, Rrookings, 8. J):lk., Sept. 15, 1960.
7. E. \I*. IIcwson et al., “M(lteorologica1 ilnalysis” (Fift,h Progress
Report) 2515-5-P, College of Engineering, University of
IIichigan, Ann Arbor, June 1960.
8. I). E. Jer~nc,, A Surnmary of thc I’crsistencc of Stable and Un-
stable T(~mpcrature Str:ltlfic:ations :It’ Ilanford for the Period
I94G through 195G, IIlV-t5321 7, IIanford Laboratories Oper-
ation, General Electric Co., Richland, W‘ash., Oct. 1957
(unpnblished).
9. R. J. T,ist, Editor, Smithsonicm Jfeteorological Tables, 6th I k -
viscy1 Edition, Smithsonix1 Tnst,itution, \Vashington, D.C.,
1951.
10. I). 11. I’ack, C. R. Hosler, and T. B. IIxris, h RIet,eorological
Survey of the PWlt Site at Shippingport, Pennsylvania,
Spccial Projects Section, U.S. Weather Bureau, Washington,
l).C,, Dee. 1057 (unpublished).
11. Stanford Ilrsearch Inst,itute, “The Use of Aleteorological T>ata
in L:Lrgc Scdc Air I’ollntion Survtlys,” California I k p t . of
Pnblic Tlenltll, 13ureau of Air Sanitation, June 1958, 110 pp.
12. I. T‘ar1 der Ilovcn, Personal Corrrspondcnce: Comparison of
hygrothermograph traces at Yucca Weather Statjion and the
500-foot ridge station at tlw southwestern end of Yucc:~ Dry
Lake for January 1959-34:xrch10G0, U.S. Weather Bureau
R.esc:trch Station, Las Vegas, Nev., ilrlg. 29, 1960.
May 1061.
SEPTEMBER 1961 MONTHLY WEATHER REVIEW
APPENDIX: CLIMATOLOGICAL DATA
333
Percent frcquency of- Station and pcriod of record used
Observations at 0000 and 1200 GUT
'Lt,hens, Ga., Junc 1Y5i-Mity 1959.
334 MONTHLY TV-EAYT'HEIZ REVIEW
APPENDIX- Continued
SEPTEMBER 1961
Station and geriorl of record used
Station
"
Carihou, Maine .......................
Charleston, S.C.---.-.- ...............
Charlotte,S.C .......................
Cliattanooga, T w n ...................
Cheycnnc, \\'yo .......................
Chicago,IIl- ..........................
Cincinnati, O l l i o ~.~~~~~~~~~~~~~~~.~~~.
Cleveland, Ohio. ......................
Columbk, &lo.. ......................
Columbia, S..... ....................
Columbus, Ohio ......................
Corpus Christi, ' P t x x .--------------
~
Dallns, l'es ...........................
Dayton, Ohio..- ....................
Denver, Cola..------..--. ............
Des Illoinps, Iowu ....................
Detroit, Mich .........................
L)odgeCity,Kan; .....................
Duluth, 12li nn .........................
E1 Paso, Tex .........................
E l y ,N c ~ .............................
Evonsvillc,Ind ......................
Fargo, N. I h k ......................
Observations a t 0000 and 1200 G m
Caribou, Maine, Juno 1957-May 1959.
Charleston, S.C., June 1957-May 1959.
Columhia, M a , June 1957-3Iay 1959.
Dayton, Ohio, June 1957-May 19.59.
Denver, Colo., June 195i-31ay 195Y.
Dodge City, Kan;., June 195i-Xlag
19.59.
Summer--.-'------ .......... 8 .
\Vintcr 1 71 91 3 84
V a l l ".I""-
Spring ...... 45 33 2 R i
Summer .... 34 11 1 61
Fall.- ....... 76 55 4 69
Winter ...... 79 91 22 81;
Spring ...... 52 52 0 78
S u n i m c r -.~~ 63 42 1 Y(i
Fall ......... Xli 89 Y 91
Spring Winter-..-..
S n m r u e r ~~.~' ~~~~~~~~~~~~~ Fa11 ........ -1
..............
..
...... ............
......
..............
......................
~
El Paso, Tex., June 195i-May 1959.
Ely, Ncv., Junc 195i-\\Iey 195Y.
I.
I
\Tinter ...... I....... ........
SEPTEMBER 1961 335
Station
I -
Fort \Vorth, Tcx., June I'J55-121ay
IY5i.
[nternational Falls, Mnn,, June 1955- \lay 1957.
Key \Vest (Navy), Fla., June 1955-
May 1957.
Ilata sparse
Ohserwtions at 0000 and 1200 G ~I T
Fort Worth, l'ex., Junc 1957-iVIay 1959.
Olasgow, Mont., June 1957-May 1959.
Grand Junction, Colo., JUIW 1Y57-May 195Y.
O r w t Falls, Mont., June 1957-May 1959,
Gre1~1 I3ny ,Wis., June 195i-XIay 1959.
Oremsboro, X.C., June 195i-May 1959.
International Falls, Minn., June, 1957- May 1959.
Jackson, Miss., June 1957-May 1959.
Peoria, Ill. June 195i-May 1959.
Kry West (Navy), Fla., Juno 1957-
M a y 1959.
rroughout period.
336
Station
-
Lacrosse, Wis ........................
LaGuardia, N.Y.. ...................
Lake Charles, La .....................
Lakehurst, N.J. ......................
Landrr, Wyo .........................
Lansing, SIich- ......................
Las Vegas, Nev. .....................
Little Rock, Ark .....................
Los Angclcs, Calif ....................
Louisdlle, Ky ........................
Madison, \Vis. .......................
Medford, Oreg.. .....................
Memphis, Tenn ......................
Mianri, Fla ...........................
Midland, Tex- .......................
blilwnukeo, \Vis ......................
Minneapolis, Minn ..................
Missoula, Mont .....................
Motrile, Ala--- ......................
>loline, I11 ......................
Montgomery, A41a ...................
Mount Clemens, Mich ..............
Nantucket, R.1 ......................
MONTHLY WEATHER REVIEW
APPENDIX-Continued
SEPTEMBER 1961
I
P e r c m t frvquency of-
~
O11scrv:ttions a t 0300 nnd 1500 GIMT
March 1954-
.! 1
.I
Olwrrations at 0000 and 1200 GMT
>aka Charles. La., Junc1957-SIay1959.
itation closed July 1955.
mlder, Wyo., June 1957-May 1959.
[,as Vrgas, Ncv., June 1957-May 1959.
Littlc Rock, Ark., June 1957-May 1959.
Snnta -Monica, Calif.. June 1957-May
1959.
>fedford, Oreg., June 1957-Slay 195Y.
-1
Miitmi, Ha., dune 1957-May 1959.
Midland, Ter., June 1957-May 195Y.
' 1 Flint, l l i c h ., Jnnrl 1957-Zf8.y 19.59.
World. (Sec Rrfcrcnces.)
MONTHLY 7/1I'EATHER REVIEW
APPENDIX-Continued
REPTEMRER l O G l 337
Percent frequency o -
..
Station and period of record used
Inrrrsion
S c a s m Station
Nashville, Tenn .....................
Newark, S .J . ....................
N e ~r Orlcans, I,%- ..................
New York, S ,Ir.. ...................
Xorfolk, Va .........................
Sorth Platte, Nebr .................
Oaklen(1, Calif-" ...................
Ogrlm, Utah ........................
Oklahoma City, Okla.." ............
Omaha, Nrhr--.---.- ................
Pllilarlr~lphia, Pa. ....................
I'horniu, S r i z ........................
Pittsburgh, Pa-. .....................
Pocatello, I d a h o .~~~.~~~~~~~~~~~~~~~~.
Port Arthur, '1.e. .....................
Portland, Maine"~. ..............
Portland, reg..^.^.^.^.^^^.^^..^^^..
Provirlrncc, R .1 .~~~~~~~~~~~~~~~~~~~~~
Purhlo, Colo ........................
Ralrigh, ... C .........................
Rapirl City, S. Dah- ..................
Richmond, Va. ......................
Observations at 0300 and 1500 GXT Observations a t 0000 and 1200 GMT
03
Sashvillr, 'I'enn., June 1957-May 1959 51 24 35 51
60 4 8 5ti
iLi 1 R 73
71 16 38 64
Nashville, Tom., June 1955-LMay
195i.
~~~~~~~~~~~~~~
21 i 1X '24
36 3 36 39
45 1 22 21 48 1 24 30
28 28 40 48 31 8 15 50
29 '21 30 50
68 (63 3(i 74
55 18 3 69 74 9 4 i 6
i 9 51 20 67
66 83 10 i 7
"3 35 14 4.5
a 13 x4 21
58 62 15 6'
ti3 23 1 73
77 66 17 79
ti2 28 2 94
84 69 3 84
63 38 33 59 70 15 6 t i l
88 5 3 8fi
65 27 10 7'2
61 S i 43 i2
58 23 8 M I
i o 15 Y 80
71 42 22 64
31 3 , 9 45
H~mpstead Air Force Base, W.Y., Sq)tcmber 1954-August 1956.
Sorfolk, Va., Fleet Wwther Central, Sc[ltr3nlbrr 1953-August 1955.
North I'lattc, Sc,br., .June 1955-May
1937.
International Airport, N.Y., June 1957-
May 1959.
Norfolk, Va., hlurlicipal Airport, June 1957-May 1959.
North Platte, Sebr., June 1957-May 1959.
Oakland, Calif., June 1957-May 1959.
Salt Lake City, Utah, June 1957-May
1959.
Oklahoma City, Okla., dune 1957-
May 1959.
Omaha, Nehr., June 1957-May 1959.
Oakland, Calif., .June 1955-hfay 1957-
Oklahoma City, O k h ., June 1955- May 19,ii.
Omaha, Nehr., .June 1955-May 1957..
Phoeniy, A r k , June 1955"ay 1957".
Pittsburgh, Pa., June 1955"ay 1957..
Yuma, Ariz., June 1957-May 1959.
Pittsburgh, Pa., June 1957-May 1959.
fortland, Mairw, June 1955-May 1957.
l ' (~r t h n d , Oreg., .June 19.54-May 1'356..
l'ortkud, Maine, Juue 1957-May 1959.
Salem, Ort'g., June 1957-May 1959.
BO (i 48 53
78 4 41 5.5
i o I6 li5 71
36 41 8 SI
lY 20 1 63
14 3 0 i 4
19 43 " il
Rapid City, S. Ilak., Junc 1955May
lY5i.
Rapid City, S. Dak., June 1957-May 1959.
......................... I 54
338 PIIOSTIILY WJL~TI11~3R RI3VIElV
APPENDIX-Continued
SEPTFXBEK 1961
-~
Station and period of record usptl
-1 -
Rrasm
-I
i
32
3;
46
li4
fi3
40
39
56
36
58
38
50
53
30
19
6 2
47
36
41
tilJ
57
44
4.5
(io
fil
Rlunicipal Airport, Shrcvcport, La., June 1957-May 1Y59.
34
A2
45
55
5 i
IiA
liX
27
18
33
55
17
Spokane, Wash., June 1957-May 1959. 53 59 16 .54
23 41 I1 63
3 T
57 57 4 73
16 26 1 83
St. Cloud, Llinn., June 1955-.\Iay 195i
1947-51.
St. Cloud, Minn., June 1957-JZay 1959,
Tampa, Fla., June 1957-hlay 1959.
SEPTEXBER 1961 339
APPENDIX-Continued
Station
-I -