180 M O N T H L Y W E A T H E R R E V I E W SEPTEMBER 1960
THE WIDESPREAD SMOKE LAYER FROM CANADIAN FOREST FIRES
DURING LATE SEPTEMBER 1950
CLARENCE D. SMITH, JR.
WBAN Analysis Center, U. S. Weather Bureau, Washington, D. C.
INTRODUCTION
During the latter part of Septembcr 1950, an extensive
layer of smoke originated from forest' fires in t'he Canadian
Provinces of British Columbia and Alberta. Subse-
quently it spread over large areas of Canada and the
eastern United States. The resulting unusual appearance
of the sky and sun and the diminution of normal daylight
caused widespread interest among meteorologists and the
public alike. This report describes some of the informa-
tion about this phenomenon which has been collected and
analyzed during the first three weeks in October.
THE SOURCE AND SPREAD OF THE SMOKE
The weather in British Columbia and Alberta was un-
seasonably warm and dry during the first half of Septem-
ber, contributing materially to tlhc onset, of extensive forest
fires. According to conservat'ive estimates furnished by
the Canadian Embassy a t Washington, D. C., approxi-
mately 60 fires burned in British Columbia, and 37 were
count,ed by forestry officials in Alberta. The fires, begin-
ning about mid-September, extended west from Grande
Prairie, Alberta, through Fort St. John, British Columbia,
to the foothills of the Rockies and north to Fort Nelson,
British Columbia, with others raging near Wanham,
Alberta, 340 miles northwest of Edmonton, Alberta, and
Newbrook, Alberta, 70 miles north of Edmonton. The
dashed hatching in figure I indicates the principal fire
area. Undoubtedly other scattered forest or tundra fires
in western Canada contributed smoke to the atmosphere
since the fall season is usually fraught with such fires.
During the last week of September, light snow and rain
helped extinguish most of the fires.
The smoke from the fires spread eastward passing over
central Canada, then southward over the Great Lakes
region and the eastern United States as far south as
Georgia and Tennessee. The denseness and unusual
optical effects of the smoke first became very noticeable
over the nor'theastern United States on September 24.
The weather observer a t Idlewild, N. Y., reported that
the sun appeared pink; a t Allentown, Pa., and Buffalo,
N. Y., it looked purple; a t Findlay, Ohio, and Parkcrs-
burg, W. Va., blue; and a t Washington, D. C., lavender.
The sky was usually described as yellowish or greyish-tan.
Williamsport, Pa.; Dun.kirk, N. Y.; Martinsburg, W. Va.;
Sault Ste. Marie, Xlich.; and Nakina, Ontario, were some
of the stations reporting unusually dark conditions during
the day with a few experiencing reduction of light to
nighttime darkness. By noon September 26, the smoke
had been carried across t'he Atlantic as indicated by the
observation of a blue sun from the Isle of Man inthe
British Isles. Later other reports of unusual optical
effects came from European stations.
The smoke aloft persisted over the northeastern United
States at least until the afternoon of September 29when
generally cloudy conditions prevented further observations
of it from the surface. It was certainly absent from the
Washington area by 1630 EST September 30 when clear
skies were again reported.
Such Widespread smoke from forest fires has been ob-
served in past years. I n 1918 forest fires in Minnesota
produced smoke which was observed as far away as Texas
and South Carolina [I]. The record of dark days inthe
United States resulting from smoke pollution extends
back to 1706 according to Plummer [a]. The effects of
smoke pollution have also been observed for centuries as
dry fogs and colored rains. Forest fires are the chief
cause of widespread smoke aloft, but many notable ex-
FIGURE 1.-Surface weather map for 0030 GMT, September 20,1950. Shading indicates
areas of active precipitation. Dashed hatching indicates principal area of forest fires.
SEPTEMBER 1960 MONTHLY WEATHER REVIEW 181
amples of smoke and dust from volcanic eruptions are
recorded as well [3 ]. The eruption of Kra,katoa in 1883
is well known as the cause of very extcnsivc smoke and
dust aloft as well as of brilliant opt,ical effccts (41.
THE SYNOPTIC SITUATION IN THE FIRE AREA
During the period September 5 to 23, that part of
British Columbia and Alberta shown in figure 1 as the fire
area experienced a drought principally due to a ridge of
high pressure a t t h e surface and aloft which occupied
the region most of that period. The ridge acted as an
efficient blocking obstacle to the passage of the cust'omary
train of maritime fronts. The fronts that moved up to
the ridge weakened markedly or frontolized; consequently
almost no precipitation occurred in the region. Figure
1 illustrates one such weak maritime Pacific front that
passed through the area. The region between the mari-
time occlusion and the stationary polar front experienced
temperatures in the mid-seventies. Some rain fell west
of the Rockies, but the east side remained dry due to the
combined effect of topography and the semi-permanent ridge
which weakened the front as it approached. Thcsc
conditions are representative of the drought period
preceding the forest fires.
As would be expected in a persistent ridge, the surface
windswere generally light and variable. These fact,or.s
contributed not only to the production of the forest fires,
but also to the accumulation of a large amount of smoke
in the fire area. An examination of the surface obser-
vations in the area for September 17-24 clearly shows the
large, dense supply of smoke available for transport by
the winds to other regions. The observations of Sep-
tember 22, 23, and 24, the period of maximum fire activity,
contain evidence of the smoke in the form of reduced
FIGURE 2.-700-mb. chart for 0300 GMT, Septembcr 23, 1950. Contours (solid lines) at
100-foot intervals are labeled in hundreds of feet. Isotherms (dashed lines) are in
knots, half barb=:, knots, and pennant=50 knots.
intervals of 5' C . Barbs on wind shafts are for wind speeds in knots; full barb =10
visibility (less than 1 mile at many stations), obscured
skies, remarks of smoke aloft, or pilot reports of smoke
and forest fires in thc vicinity.
UPPER AIR CONDITIONS SEPTEMBER 22-27, 1950
Since the smoke produced a t t h e surface in British
Columbia and Alberta was observed several days later as
a thick layer aloft far to the east it was obviously neces-
sary to examine the upper air conditions to establish a
sequence of events in the movement of the smoke. The
700-mb. charts of 0300 GMT September 22, 24, and 27
were selected to illustrate the upper air flow (figs. 2,3, and
4). The pattern of flow a t 500 mb. was similar to that at
700 mb.
FIGURE 3.-700-mb. chart for 0300 GMT, September 24, 1950.
FIGURE 4.-700-mb. chart for 0300 GMT, September 27,1950.
182 MONTHLY REVIEW SEPTEMBER 1950
The period September 22 through 24 was charactcrizd
by a ridge over the western United States and Canada
and a Low in eastern Canada with a well defined north-
westerly or northerly current over cent,ral Camda and the
eastern United States. Figures 2 and 3 are sufFicicnt to
give the general impression that smoke added to the atmos-
phere over the fire area was swept over central Canada
and southward over eastern United States. An attempt
was made to demonstrate this by calculating trajectories
both up and down stream on the 700-mb. and 500-mb.
surfaces. It was found that such trajectories did not'
correspond to the observed location of the smoke layer
or to the proper time sequences. In order t80 track t:he
smoke more precisely the following procedure was uscd.
All available reports of t,he height of the base and tap of
the smoke layer were sludied. These consisted mainly of
pilot reports over the eastern United States. I t was
found, as might be expeckd, t,lmt the smoke layer was
not obscrved at, the same height from place to place and
time to time. Also multiple layers were reported by
pilots. On September 24 the base was reported variously
12,000 to 14,000 feet over Pennsylvania, and a piIot
rcported that he was in dense smoke a t 17,000 feet over
Sault Stc. Marie. On September 25, the basc was re-
ported variously 11,000 to 14,000 feet and the top 14,500
feet to indefinitely higher. By September 26, the base
was 9,000 to 11,000 feet and top around 14,500 feet.
Then by September 27 and 28, the base was 6,000 to
FIGWEE 5.-Trajectories of air parcels eslculated on isentropic charts of 8=312O A. Dashed hatching indicates firearea. Threedigit numbers by trajectory points are heights
hundred3 of feet. Large dots near Great Lakes Illrvk stations reporting greatly dimirlished daylight on September 24. Large dots in Newfoundland mark the Same condition on
Septembcr 25. Dashed parts of smoke boundary are problematicsl. Thin solid lines across the Qreat Lakcs and eastern United States are isochrones at 5-hour intervals on
Septembcr 24 showing southeastward progress of the smoke layer.
SEPTEMBER 1950 MONTHLY WEATHER REVIEW 183
9,000 feet and top 10,000 to 15,000 feet. The appropriate
adiabatic diagrams were examined in order to coordinate
the reported heights with significant lapse rate changes.
It was found that the reported heights did not correspond
to a constant pressure surface; however, the potential
temperature of 312' A was common to a majority of the
heights. This suggested that a series of isentropic charts
of potential temperature 312' would be superior to con-
stant pressure charts for reconstructing trajectories of air
parcels from the fire area. A series of 7 such isentropic
charts spaced a t 12-hour intervals was prepared for the
period 0300 GMT September 22 through 0300 GMT
September 25 and trajectories were calculated in 6-hour
steps using the isentropic stream functions and a set of
geostrophic wind scales designed to give velocities in
knots. Sample trajectories calculated on these charts
are shown in figure 5.
The examination of the adiabatic charts revealed the
atmosphere over western Canada to be conditionally
unstable to 6,000 feet and a t times as high as 18,000 feet.
The convection caused by the heat from the fires carried
the smoke several thousand feet aloft initially, and the
instability of the air assisted farther rise to heights corre-
sponding to those of the 312' potential temperature
surface. In figure 5 the heights (accurate to A300 feet)
along the trajectory on the 312' surface are shown to vary
between 9,200 and 15,600 feet in the fire area.
The trajectories beginning 1000 EST September 22
(dashed) and 2200 EST September 22 (dotted) in the
fire area show the approximate path of the main stream
of smoke. Note how the dotted trajectory runs through
the middle of the group of marked stations near the
Great Lakes. The marked stations are some that re-
ported very dark sky conditions or reduction of light to
nighttime conditions on September 24 indicating a thick,
dense, smoke layer aloft. The marked stations in New-
foundland experienced the same condition on Septem-
ber 25.
Trajectories were also calculated upstream from points
in the northeastern United States. One is shown in
figure 5 (solid line). It also corresponds well with the
main stream of smoke. In the calculations of trajectories
it was found the resulting particle path is very sensitive
to the choice of the starting place and time. By varying
each initial condition it can be shown that air passing
over the fire area moved over approximately the area
designated in figure 5 as that covered by the smoke.
The three isochrones in figure 5 were drawn after
summarizing the hourly surface reports. They agree
reasonably well with the times on the trajectory points
and illustrate the southward progress of the smoke.
While the smoke proceeded eastward, a polar outbreak
developed in the Hudson Bay region as a cyclone on the
polar front deepened over Hudson Bay. The low contour
center aloft associated with the surface cyclone deepened
rapidly forming a strong northerly current just west of
Hudson Bay as illustrated by figures 2 and 3. The smoke
layer was caught up by this rapid current above the cold
dome and accelerated to speeds greater than 50 knots.
If this development had not occurred it is improbable that
the smoke layer would have covered such large areas of
the United States.
The cold dome of continental polar air that moved
southward over the northeastern United States on Sep-
tember 23 and 24 extended to approximately 14,000 feet.
Comparing this height with the height of the smoke layer
as reported by pilots it is established that the smoke layer
was in the maritime Pacific air above the polar dome.
The Pittsburgh sounding reproduced in figure 6 shows the
top of the continental polar air near 600 mb. (approxi-
mately 14,000 feet) with a potential temperature of ap-
proximately 310' A. The Washington sounding of 3 days
later, also illustrated in figure 6, shows the continental
polar dome had subsided to 740 mb. (approximately 8,500
feet) with a potential temperature of approximately 308'
A. This height corresponds well with the reported height
of the smoke layer on September 27.
"7100
-200
- 300
cd
3 M
E
-400
-500
- 600
700
- 800
900
-lo00
FIQURE 6,"Radiosonde observations plotted on a pseudo-adiabatic diagram: Pitts-
burgh, Pa., 1200 EST, September 24, 1950; Washington, D. C ., 1000 EST, September
27, 1950.
184 MONTHIAI~ WEATHER REVIEJV SEPTEMBER 1950
One of the interesting aspects of the smoke layer was its
persist>ence over the northeastern United States during
most of t,he week beginning September 24. An examina-
tion of the upper air situation revealed one explanation
for the persistance. On September 24 and 25, a ridgc
located to the west of this area contributed to the produc-
tion of the northwesterly current over the area as shown
in figures 2 and 3. This current provided a continuous
supply of smoke on those days. By September 26, the
ridge had moved over the area producing a large region of
light, variable winds aloft. Figure 4 illustrates this con-
dition. Apparently a large quantity of the smoke was
cut off from the main stream of the westerlies and re-
mained tra,ppcd over the northeastern United States.
EFFECTS O F THE SMOKE LAYER
The smoke layer reduced the amount of solar radiation
received at the ground, and consequently the tempera-.
tures recorded by stations in the smoky area were affected.
Mr. Sigmund Fritz of the U. S. Weather Bureau, investi-
gating the effect on the temperahre at Washington on
September 26 and 27, has made preliminary estimates
that the maximum temperature was lower than it other-
wise would have been by about IOo F., and the minimum
temperature higher by a smaller amount.
This paper has dealt with the synoptic aspects of the
smoke layer. AdditJional information about the insola-
tional and optical aspects can be found in a paper by Dr.
H. Wexler published in Weatherwise 151.
ACKNOWLEDGMENTS
The author acknowledges the valuable assistance of Mr.
D. Lee Harris in calculating isentropic stream functions
and Mr. Kenneth M. Nagler in calculating trajectories.
REFERENCES
1. H. Lyman, Smoke from Minnesota Forest Fires,
Monthly Weather Review, vol. 46, No. 11, November
2. F. G. Plummer, Forest Fires, U. S. Dept. of Agricul-
ture Forest Service Bulletin 117, 1912, p. 15-22.
3. E. E. Free, The Movement of Soil Material by the
Wind, U. S. Dept. of Agriculture Bureau of Soils
Bulletin, 68, 1911, p. 116-119, 146-151.
4. S. C. Blacktin, Dust, The Sherwood Press, Cleveland,
Ohio, 1934, p. 188-189.
5. H. Wexler, The Great Smoke Pall of September 24-30,
1950, Vieatherwise, vol. 3, No. 6, December 1950.
1919, p. 506-509.