.Jay 1955 MONTHLYWEATHERREVIEW 147
THE WEATHER AND CIRCULATION OF JULY 1955'
A Prolonged Heat Wave Effected by a Sharp Reversal in Circulation
JAMES F. ANDREWS
Extended Forecast Section, U. S. Weather Bureau, Washington, D. C.
1. INTRODUCTION 2. THE HEAT WAVE
Over most of the United States, a generally pleasant and
cool June was followed by one of the most prolonged and
persistent heat waves ever observed during July. This
marked change in temperature was associated with an
equally marked reversal in the circulation pattern. The
almost complete disappearance of blocking action, which
had been so effective in North America and the Atlantic
Ocean during June [l], was in large measure responsible
for these changes.
1 See Charts followhg p. 153 for analyzed climatological data for the month.
The most newsworthy feature of the month's weather
was the heat wave which covered the United States from
the Continental Divide to the Atlantic Coast, except for
the Gulf States. I t was different from most heat waves
in several respects: (1) its prolonged and persistent nature,
(2) the fact that no absolute maximum temperature
records were broken, and (3) the absence of widespread
drought conditions.
The hot, humid weather set in rapidly during the last
few days of June and became well established by early
C
FIGURE 1.-Weekly departure of average temperature from normal, July 1955. (A) July 4-10, (B) July 11-17, (C) July 18-24, (D) July
25-31. Persistence of the temperature pattern is apparent, with below normal temperatures in the Far West and southern portions of
the country, and above normal temperatures elsewhere. (From [2].)
148 MONTHLY WEATHER REVIEW JULY 1 W
July. From then on through the remainder of the month
there was little change in the week-to-week temperature
pattern [2](fig. 1). Thus, the patterns shown in Chart
I-B and figure 2B were characteristic of the entire month.
Temporary relief, however, was afforded residents of the
Midwest near the middle of the month when a somewhat
cooler air mass pushed in from the Pacific (fig. 1B). The
hottest weather occurred during the last week and was
centmered in Iowa where temperatures as much as 12' F.
above normal were observed for the 7-day period (fig.
1D). Des Moines, Iowa reported maximum temperatures
of looo P. or more on the last 5 days of the month with a
high of105' F. on the 31st. Omaha, Nebr. also had its
hottest day of the month at the same time with 108' F.
A change to cooler weather in the Northeast, during the
last week, brought to most of New England its only
period of below normal temperatures for the month
(fig. 1D).
Surprisingly enough, no absolute maximum temperature
records for July were broken, although many large cities
reported their highest July monthly mean temperature
of record. These included Chicago, Ill., Cleveland, Ohio,
Detroit, Mich., Hartford, Conn., New York, N. Y.,
Philadelphia, Pa., Baltimore, Md., and Washington, D. C.
Another measure of persistence of the heat is the large
number of cities where the number of days with tempera-
ture 90' F. or above broke or tied July records. These
included, in addition to most of those listed above,
Minneapolis, Minn., Lansing, Mich., Indianapolis, Ind.,
Albany, N. Y., and Boston, Mass. Furthermore, several
cities in Indiana and Kentucky did not report a single day
on which the temperature was below normal. Statewide
temperature averages showed that New Jersey expe-
rienced its hottest July since 1887.
TABLE 1.-Number of Julys in the period 1893-1952 when United States mean temperature, weighted according to State area, was above or belowthe normal of 74.3" F . Julys with exactly normal means are omitted
1893-1912 1933-1952 1913-1932
-"
Above normal _____ _____ ____ __ - ______ - ___ ____
~
__________ 6 10 13 Below normal ______________._________________________--
14 10 6
-
To some extent the recent heat wave represents a con-
tinuation of a long-period climatic warming. Table 1
shows the number of Julys when the mean temperature
for the United States (weighted according to the area of
each State) was above or below normal during the three
20-year periods from 1893 to 1952. Those Julys with
exactly normal values (74.3' F., based on 62 years of
record) are omitted from the table. If we include July
1953, 1954, and 1955 in the last column, it then becomes:
Above normal 16 and Below normal 5. Thus, there
appears to have been a definite warming trend during July.
A glance at figure 2 shows how marked the temperature
reversal was from June to July. Namias [3] found that,
on the average, there is about 70 percent persistence
~-
~~ ~~
U. S. DEPARTMENT OF COMMERCE
WEATHER BUREAU
U. S DEPARTMENT OF COMMERCE
WEATHER BUREAU
FIQURE 2.-Monthly mean surface temperature anomalies for (A)
June 1955 and (B) July 1955. The classes above, below, and
near normal occur on the average one-fourth of the time; while
much above and much below each normally occur one-eighth of
the time. Note the marked reversal in pattern from the Rocky
Mountains eastward.
(0 or 1 classchange) in the temperature pattern from
June to July. This year there was only 41 percent per-
sistence. Moreover, 15 percent of reporting stations
observed the extreme change from much below to much
above normal temperature.
3. THE HEAT WAVE
RELATED TO THE MEAN CIRCULATION
As is usually the case, such rapid changes in the tempera-
ture pattern were produced by marked changes in circu-
lation. In the June article of this series, Hawkins [l]
described the effect of blocking action in North America
upon the weather and circulation in the United States.
At that time the main westerly jet stream was located far
south of its normal position in the eastern United States
(fig.3A), and wind speeds were nearly 7 m./sec. below
normal over the Great Lakes. A vast area of subnormal
wind speeds extended from the central Rockies eastward
to the Atlantic Coast and northward to Newfoundland
JULY 1955 MONTHLY WE-REVIEW 149
FIQURE 3.-Mean700-mb. wind speeds (isotachs drawn at inter-
vals of 4 meters/sec.) for (A) May 31-June 29, 1955 and (B) July
2-31, 1955. Solid arrows indicate the average position of the
principal jet stream at this level. Dashed arrows give the normal
position of the jet stream for each month. "F" refers to areas of
fast wind speeds; "S", to areas of light winds. Far northward posi-
tion of the westerlies in eastern North America in July was a
marked change from June.
and Hudson Bay. The monthly mean 700-mb. High cell
was over northeastern Mexico.
The change that followed in July is indicated by com-
paring t,he average monthly positions of t'he jet streams in
figure 3. Notice (fig. 3B) t.hat in eastern North America
the main westerly jet was displaced far north of both
its June and its normal July positions. The monthly
mean700-mb. High cell over the Tennessee Valley in
July appears to be traceable from its June position over
Mexico to the Louisiana coast from middune to midJuly
(fig. 4). In the southern United States, where the westerly
jet was strongest in June, July had a zone of relative calm
associated with the extensive upper-level High. From
Hudson Bay eastward to the Atlantic Coast, July wind
speeds were 5 to 6 m./sec. above the normal with observed
speeds of 12 m./sec. The contrast between June and July
is further highlighted by comparing their 700-mb. zonal
windspeedprofiles (Western Hemisphere) (fig. 5). The
latitude of maximum west wind speed (jet) moved north
about 10' from 42' N. in June to 52' N. in July.
The general northward displacement of the belt of
maximum winds (figs. 3 and 5) resulted in a subnormal
value of the monthly mean zonal index at 700mb. for
the Western Hemisphere (35' N.-55' N.). A value of
6.6 m./sec. ties with that of July 1952 for the lowest of
record (since 1943). At the same time the index of polar
westerlies (55' N.-70° N.) at 700 mb. was 4.4 m./sec.,
the highest of record (since 1945). In this connection it is
interesting to note that the lowest polar westerlies index
ever observed, 0.6 m./sec. in July 1950, was accompanied
by subnormal temperatures over nearly the entire country.
Thus, in July high values of the polar westerlies and low
values of the zonal westerlies have been associated with
mainly above normal temperatures in the United States,
and low polar westerlies and high zonal westerlieswith
subnormal temperatures, a long-suspected general rela-
tionship. To check this statistically, simple linear correla-
tion coefficients were computed relating the mean United
States weighted temperature averages for all Julys for
the period 1945-1954, with both the corresponding polar
westerly and zonal westerly indices. The correlation
between temperature and polar westerlies was "0.59, and
that between temperature and zonal westerlies -0.57.
These coefficientswould undoubtedly have beenhigher
if July 1955 could have been included, but the mean
temperature for the United States was not yet available.
%he shortness of the period of record does not allow any
strong inferences to be made regarding these correlations.
Establishment of the vast, warm anticyclonic cell in
the eastern United States early in the month exerted a
basic control upon the weather over nearly the entire
country, with the exception of the Far West. This cell
extended vertically to the upper reaches of the troposphere
(Chart XV), and was intimately related to the heat wave
previously discussed.
Changes that evolved in the general circulation were
related to the growth of this high pressure cell. During
June the eastern Pacifk semi-permanent anticyclone was
located about 12' of longitude northeast of its position
on the normal map [4] ; the ridge associated with this cell
had heights of 260 ft. above the normal. Continued
building and northward motion of this cell occurred in
July and heights remained considerably above the normal.
At the same time, blocking action, centered over Hud-
son Bay in June, began to relax over North America and
to spread its effects into Alaska and northwestern Can-
ada. In July this blocking surge was somewhat weaker
than in June and was associated with an anomaly of
4-80 ft. centered in the northern Yukon. It is conceiv-
able that this blockalso had someeffect in helping to
150 MONTHLY JULY 1955
build the eastern Pacific ridge. Note the zone of positive
anomaly extending from the Yukon through the Gulf
of Alaska to the +190-ft. center in the eastern Pacific
(fig. 4). The building of this ridge, combined with a
relaxation of blocking in most of North America, led to
major changes in the circulation pattern downstream.
In June the major trough-ridge systems in the Western
Hemisphere were relatively weak and had little amplitude.
The 700-mb. pattern for July in the same area shows
deeper-than-normal troughs and stronger-than-normal
ridges, and, in general, much greater amplitude. Strength-
ened northwesterly flow off the Pacific Coast, and its
associated transfer of momentum downstream, resulted in
a deep trough near its normal position on the Pacific Coast.
At the same time an upper-level ridge began to build
sharply into the eastern United States. Its location over
the Great Lakes represents a displacement of about 20'
east of its normal position [4]. One way of portray.ing the
magnitude of the changes just discussed is to compute the
change of monthly mean 700-mb. height anomaly from
June to July (fig. 6). Note in particular the areas of
strong positive anomalous change in Alaska and the
JULY 1955 M O N T H L Y W E A T H REVIEW 151
-6 -4 -2 0 2 4 6 8 IO I2
M. P. S.
FIGURE 5.-Mean 700-mb. zonal wind speed profile in the Western
Hemisphere for June 1955 (dashed line) and July 1955 (solid line).
The normal July profile (not shown) is approximately half way
between the two curves. Note the sharpness of the July west
wind maximum and its increase and northward displacement from
the June value.
FIQIJR~ 6.-Change in mean 700-mb. height departure from normal
from June to July 1955. The lines of equal anomalous height
changearedrawn at 50-ft. intervals with the centers labeled in
tens of feet. The sharp changes from North America eastward to
central Asia are most striking.
Middle Atlantic States, also the negative area in the
Pacific States. Increased anomalous southerly flow over
the central United States was conducive to the deployment
of warm, moist tropical air masses into the country.
Summing up then, we can say that the heat wave in the
eastern two-thirds of the United States was accompanied
by strong anticyclogenesis resulting largely from: (1)
transfer of momentum downstream, (2) relaxation of
blocking in North America, and (3) the rapid shift north-
wardof the jet stream (in part, a normal climatological
feature).
The circulation pattern of July 1955 was quite typical
of heat wave situations in the Midwest and eastern United
States. In 1952 Klein [5] prepared a composite 700-mb.
chart based on the 10 hottest 5-day mean periods at
Kansas City during the summer months from 1947 to
1951. This chart (not shown here) bears a closeresem-
blance to the observed map for July 1955. Features
common to both maps are: stronger-than-normal ridges
near 150° W. in the Pacific and over the Great Lakes,
a stronger-than-normal trough along the west coast, a
weak trough off the east coast, and a confluence zone in
southeastern Canada. The persistent southwesterly flow
thus created, from the Rocky Mountains to the Great
Lakes, effectively prevents the intrusion of cooler Cana-
aian air masses into the country.
4. OTHER ASPECTS OF THE WEATHER
IN THE UNITED STATES
While the eastern two-thirds of the country sweltered,
the region west of the Continental Divide continued its
pattern of below normal temperatures. Cool weather in
the West has prevailed, almost without exception, since
December 1954. In the interior valleys of California and
in portions of northern Nevada, temperatures were aa
much as 6 O F. below normal. Fresno, Calif. had its coldest
July of record; also, its 14th consecutive month of sub-
normal temperatures. The coolest weather occurred early
in the month when temperatures of 1 2 O F. below the
normal were reported (fig. 1A). Winnemucca, Nev.
recorded a minimum temperature of 29OF. on the 5th,
the lowest ever observed there in July. These subnormal
temperatures were caused by frequent intrusions of cool
Pacific air masses into a deeper-than-normal 700-mb.
trough (fig. 4). At sea level, no closed High centers were
tracked inland (Chart IX) with these air masses,which
were associated only with protrusions of the semi-
permanent Pacific High (Chart XI). Subnormal tem-
peratures also prevailed quite generally in the Southern
States from the Rockies to the Atlantic Coast. These
cool conditions were caused by the prevalence of cloudiness
and showers in moist tropical air and were related to
anomalous easterly flow components (fig. 4).
Precipitation during July was fairly well distributed
over the country and was generally heavier than in June.
The deep trough along the Pacific Coast resulted in
greater-than-normal rainfall over nearly all regions west
of the Continental Divide (Charts I1 and III-B). An ex-
ception was central and southern California wherenor-
mally no rain falls during the summer. Strong comer-
gence in the mean trough effected a release of moisture
both in Pacific air masses and in the southwestern moist
tongue.
Rainfall was also above normal from the northern
Rockies to Wisconsin, where surface fronts were present
70 percent of the time (fig. 7). It was in this region that
the main cyclonic activity in the United States developed
152 MONTHLY REVIEW JULY 1955
I
Q
FIQWE 7.-Number of days in July with surface fronts of any type
(within squares with sides of approximately 500 miles). Frontal
positions taken from Daily Weather Map, 1:30 p. m., EST. The
high frequency of fronts from Wisconsin to the Pacific Coast was
related to the heavier-than-normal precipitation in that region.
(Chart X). This activity, in the form of rather weak
frontal waves, fed on the strong thermal contrast (fig. 2B
and Chart I-B) and was steered northeastward by the
upper-levelflow.Also related to heavier-than-normal
precipitation in this area were tbe stronger-than-normal
anomalous southerly flow components in the monthly
mean 700-mb. circulation (fig. 4). Over most of Minnesota
and northwest Wisconsin more than twice the normal
amount of rain fell (Chart III-B). The combination of
high temperatures and heavy precipitation here was rather
unusual in view of the negative correlation that exists
between these two elements in summer. Early in the
month numerous hailstorms occurred in the northern
Great Plains and the upper Mississippi Valley. Several
tornadoes were reported in Minnesota.
Widespread and generally heavy precipitation also fell
in the Gulf States and Florida. Rainy, cloudy weather
here was the result of frequent easterly wave activity to
the south of the upper-level High center. The season’s
second tropical storm (Brenda) developed in the Gulf of
Mexico at the end of the month and moved into Louisiana
on August 1, accompanied by heavy rains. This storm,
however, never attained hurricane force.
Precipitation was generally deficient in the central and
southern Great Plains (Charts I1 and III-B). The ex-
planation for this is not too clear, in view of the anomalous
southerly flow (although weak) in this area. Factors in-
volved were probably: (1) the northward shift of the axis
of the mid-tropospheric High at 700-mb. from its normal
position, near latitude 27’ N. in the eastern Gulfof
Mexico, to latitude 35’ N.; and (2) a westward shift of
the southwestern moist tongue. Subnormal amounts of
rainfall were also observed in the Great Lakes region and
in the Northeast. There wa8 abundant sunshine (Chart
VII), associated with anticyclonic curvature and north-
westerly flow aloft, in these areas where only one cyclone,
and that a rather weak system, passed during the month
(Chart X). New York State reported its second driest
July on record, while Pennsylvania and New Jersey also
approached records for dryness. The Tennessee Valley,
directly beneath the Continental anticyclone, also had a
deficiency of precipitation.
5. CIRCULATION AND WEATHER
HIGHLIGHTS ELSEWHERE
When blocking reached its peak in Alsska-July 20 to
27-and700-mb. heights were 400 ft. above the normal,
the residents of Alaska had a heat wave of their own. The
temperature reached 93’ F. in Fairbanks and 80’ F. in
the Eskimo village of Wainwright on the Arctic Coast.
The heat brought with it the rare phenomena of lightning
and thunder to the Arctic regions.
In Ontario, Canada, the Combination of high tempera-
tures and subnormal rainfall, associated with the persistent
upper-level ridge (fig. 4 , brought a rash of forest fires to
the sun-baked woodlands. The situation wasdescribed
as the worst in the Province’s history.
The retrogression of the Canadian block to Alaska
helped to develop a deep trough in the Davis Strait, where
an anomaly of -350 f t . was observed (fig. 4). Strong
northwesterly flow to the rear of this trough and south-
westerly flow in western United States combined to pro-
duce the strong confluence zone in southeastern Canada.
The jet of maximum winds emanating from this zone, was
accompanied by wind speeds of 16 m./sec. in the North
Atlantic (fig. 3B)”some 11 m./sec. above the normal for
July. North of the jet, cyclonic activity was extremely
frequent, in both central Canada and the North Atlantic
(Chart X),
Blocking operated very strongly in Europe and Asia.
Notice (fig. 4) the strong height anomaly of $350 ft. over
Great Britain and the zonal band of positive anomaly
extending eastward to the +150-ft. center in Northern
Asia. South of this positive anomaly zone was an exten-
sive band of negative anomaly stretching from Portugal
eastward to China. A split jet, a warm High, and a
closed low-latitude Low, all characteristics of blocking,
were observed during the month. In Europe the block
occurred in two major surges, one in each half of the
month. The first, and strongest, brought with it an 8-day
heat wave in Western Europe and Scandinavia, with
temperatures of91’ F. reported in Norway. Persistent
anticyclonic conditions over the British Isles resulted in
the sunniest and driest July on record i n . many districts,
while a maximum temperature of 88’ E”. was observed in
the London area on the 18th.
Additional weather highlights included several typhoons
in the South Pacific and a reported temperature of122’ F.
in Ghardimaou, a village in northern Tunisia.
JULY 1955 MONTHLY WEATHER REVIEW 153
REFERENCES
1. H. F. Hawkins, Jr. “The Weather and Circulation of
June 1955“Illustrating a Circumpolar Blocking
Wave,” MonthlyWeatherReview, vol. 83,No. 6,
June 1955, pp. 125-131.
2. U. S. Weather Bureau, WeeklyWeather and Crop
Bulletin, National Summary, vol. XLII, Nos. 28, 29,
30, 31, July 11, 18, 25, and August 1, 1955.
3. J. Namias, “The Annual Course of Month-to-Month
Persistence in Climatic Anomalies,’’ Bulletin of the
American Meteorological Society, vol. 33, No. 7,
Sept. 1952, pp. 279-285.
4. U. S. Weather Bureau, “Normal Weather Charts for
the Northern Hemisphere,” Technical Paper No. 21,
Washington, D. C., 1952, 74 pp.
5. W. H. Klein, “The Weather and Circulation of June
1952-A Month with a Record Heat Wave,” Monthly
Weather Rewiew, vol. 80, No. 6, June 1952, pp. 99-104.
Blank page r e t a i n e d for p a g i n a t i o n
LXXXIII-87
B. Departure of Average Temperature from Normal (OF.), July 1955.
A. Based on reports from 800 Weather Bureau and cooperative stations. The monthly average is half the sum of the monthly
average maximum and monthly average minimum, which are the average of the daily maxima and daily minima, respectively.
B. Normal average monthly temperatures are computed for Weather Bureau stations having at least 10 years of record.
LXXXIII-87
B. Departure of Average Temperature from Normal (OF.), July 1955.
A. Based on reports from 800 Weather Bureau and cooperative stations. The monthly average is half the sum of the monthly
average maximum and monthly average minimum, which are the average of the daily maxima and daily minima, respectively.
B. Normal average monthly temperatures are computed for Weather Bureau stations having at least 10 years of record.
JULY 1955 M. W. R. LXXXIII-89
B. Percentaae of Normal Precipitation, July 1955.
Normal monthly precipitation amounts are computed for stations having at least 10 years of record.
B. Percentage of Normal Sky Cover Between Sunrise and Sunset, Julv 1955.
JULY 1955 M. W. R. . LXXXIII-91
Chart VII. A. Percentage of Possible Sunshine, July 1955.
LXXXIII-92 JULY 1955 M. W. R.
1
”I
0
LXXXIII-94 JULY 1955 M. W. R.
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JULY 1955 M. W. R. LXXXIII-95
JULY 1955 M. W. R. Lxxxin-97
WULIII-98 JULY 1955 M. W. R.
JULY 1955 M. W. R. W[XIII-99