ANTICYCLONES 
ticularly true if a thick fog reflects much of the incident 
solar radiation. In industrial areas located in river or 
valley locations, pollution products help to create 
dense, peristent fogs which not only protect the in- 
version from “burning-off” but serve as a carrier for 
pollution products confined to the surface layer by 
the inversion. This in essence is the meteorological 
background of the’ disastrous Donora, Pennsylvania, 
“smog” of October 25-31, 1948 [32] and the similar case 
in Liége, Belgium, in 1930. Two earlier cases of Donora 
smog occurred in 1923 and 1938, each time in October. 
A suspected case occurred in April 1945. Wexler [32] 
analyzed the necessary conditions for a deep persistent 
anticyclone to occur over the eastern United States, 
and found that autumn (in particular October) fulfilled 
these conditions best, with winter next. 
“Back-Door”’ Cold Fronts. In late spring and summer 
along the North Atlantic Coast of the United States 
some of the most outstanding forecast failures occur 
When a seemingly persistent heat wave is broken un- 
expectedly by a sudden push of a shallow layer of polar 
Atlantic air southwestward along the coast. It is no 
exaggeration to say that this type of weather process 
brings the greatest relief to the heat-harassed populace 
and the bitterest self-recrimination to the forecasters 
involved. Overnight drops in temperature from 98F to 
65F and in dew-point temperature from 82F to 62F 
are by no means uncommon in Washington, D. C.; it 
would seem offhand that such spectacular weather 
changes must be accompanied by equally spectacular 
and well-marked patterns on the weather map, but 
such unfortunately is not the case. The preceding con- 
ditions are usually those in which a cold front has 
pushed into northeastern United States from Canada, 
stalled in the vicinity of Boston or New York and then, 
showing every sign of retreating northeastward as a 
warm front, gathers sufficient energy to push south- 
ward along the coast in a narrow tongue. The cause is 
usually a sudden but slight anticyclogenesis in the cold 
air north of the front, which ‘‘energizes” the surface 
cold air into motion. The anticyclogenesis itself seems 
to have its origin in the warm air above the front and 
not in any sudden accumulation of cold air below. 
This tantalizing and important problem has not had 
the study it deserves. 
EXAMPLE OF AN UNUSUAL ANTICYCLONIC 
THERMAL STRUCTURE 
In the first section some ‘typical’ values of the 
principal upper-air thermal features of anticyclones 
were presented. It is altogether probable, however, 
that other more bizarre combinations of upper-air struc- 
ture may contribute to the excess of pressure at the 
surface, known as the anticyclone. For example, the 
writer, in attempting to find illustrations of “typical” 
soundings of polar and warm anticyclones, studied the 
anticyclone of December 23-26, 1949, which established 
some record high pressures in New England. In this 
case a polar anticyclone left its source region in the 
Canadian Northwest on December 23 with a sea-level 
pressure of 2083 mio and typically cold air in the 
627 
troposphere; at the same time, an older polar anti- 
cyclone, which had previously pushed southward to 
the southwestern United States, proceeded to move 
northeastward with an initial sea-level pressure of 1028 
mb. The anticyclones were steered by their respective 
currents at 500 mb and approached each other on a 
collision course, intensifying slightly as they did so, 
and finally amalgamating with an explosively sudden 
increase of pressure of 17 mb in 24 hours from the 
24th to the 25th, the maximum pressure of 1054 mb 
being reached in northern Maine. In Fig. 1 it is seen 
that the sounding at Fort Smith, N.W.T., (665 ft 
elevation), taken at 0300Z on December 23 when the 
polar anticyclone of 1033 mb was nearly centered at 
the station, exhibits the strong surface inversion and 
isothermal layer above, characteristic of polar con- 
tinental air, but possesses a high (8.7 km), moderately 
cold (—60C) tropopause; above the tropopause inver- 
sion a steep lapse rate is found to the top of the sound- 
ing, apparently indicating the probable presence of 
another higher, and perhaps colder, tropopause above. 
In Fig. 1 is also shown a sounding taken sixty hours 
———FT. SMITH, CANADA 22 
12-23-1949 03Z 1033 MB. 50 
i 20 
CARIBOU, MAINE 
12-25-1949 15Z 1054 MB. i 
PORTLAND, MAINE 
12-25-1949 15Z 1052 MB. 
100 16 
—-—— WASHINGTON, D.C. 
12-25-1949 I15Z 1046 MB. a 
= 14 
a 
200 3!4 
U1 
10 
300 9 
8 
400 3 7 
6 
500 
5 
600 3 4 
700 43 
800 2 
900 1 
1000 ° 
=70 -60 -50 -40 -30 -20 -I0 0 10 20 30 KM, 
a die) 
Fie. 1.—Soundings taken near the center of the anticyclone 
of December 23-26, 1949. Tabulated pressures are reduced to 
sea level. 
later at Caribou, Maine (628 ft elevation), when the 
center of the amalgamated anticyclone of central pres- 
sure of 1054 mb was close to the station. This sounding 
shows a typically modified polar continental air mass 
in low levels; that is, a steep lapse rate in a thin surface 
layer and above this, to 2.6 km, marked stability and 
dryness characteristic of subsidence. Above, a steep 
