ARCTIC METEOROLOGY 
currents normally flowing toward and across northern 
arctic America. Seasonal variations in arctic circulation 
patterns at the gradient wind level will be described 
in terms of average sea-level pressure charts for the 
midseason months: July, October, January, and April. 
These charts represent a personal synthesis of what are 
conceived to be the most realistic features appearing 
in the previously discussed charts of Baur, Sverdrup, 
and Dzerdzeevskii. In describing the seasonal trends in 
arctic circulation patterns, it was thought more logical 
to start with the minimum stage of air flow, represented 
by July conditions, than to break in near the annual 
maximum of atmospheric motion. 
July. The chart of average sea-level pressure for the 
month of July (Fig. 1) shows the relative weakness of 
Fie. 1.—Average sea-level pressure (mb) during July. 
the circulation patterns in this midsummer period of 
minimum thermal contrast between the polar and tem- 
perate zones. The circumpolar belt of low pressure is 
at its highest latitude in this month, with a definite 
extension of cyclonic activity toward the North Pole 
itself, although a feeble anticyclonic circulation persists 
over the congested pack-ice area northwest of the 
Canadian Archipelago and over the northeast Green- 
land—Fridtjof Nansen Land area. Along the Laptev Sea 
coast line, the contrast between warm air from the in- 
terior of Siberia and cool air from the pack ice is 
sufficient to maintain persistent cyclonic activity. Weak 
disturbances, originally associated with poleward 
thrusts of North Atlantic maritime air masses, may 
here be reintensified to resume their eastward pro- 
gression toward the similar but milder thermal gradients 
along the arctic coast of Canada. 
It has been mentioned that the July chart retains the 
best features derived by prior research of other arctic 
specialists [1, 6, 19]. The more widely circulated pub- 
lication, ‘““Normal Weather Maps, Northern Hemi- 
sphere, Sea-Level Pressure,” [20] has presented a differ- 
ent pattern for July, especially over northern arctic 
America. With a 1017-mb isobar encircling the well- 
substantiated col area (average pressure less than 1013 
mb) of the North Pole, and with little hint of a semi- 
945 
permanent cyclonic system over northern Baffin Bay, 
the “40-yr normal July” could be very misleading. 
However “abnormal” the recent Julys may have ap- 
peared to be in this area, expedition reports indicate 
that midsummers over a hundred years ago were more 
dominated by cyclonic than by anticyclonic conditions. 
Because of the simple thermal aspects of the July 
circulation scheme and the annual return to relatively 
the same percentages of ice-covered or ice-free surfaces 
and total solar heating, radical departures from the 
flow patterns described by this critique are not ex- 
pected. Cyclonic activity develops and travels along 
the major frontal zones indicated by continuous thin 
lines on Fig. 1, except in some Julys when the Okhotsk- 
Bering Sea frontal zone becomes more active. The 
secondary cyclonic centers may be occupied by indi- 
vidual low-pressure cells, depending on the relative 
strength of the zonal circulation in the observed July. 
Frontogenesis in the North Greenland—Ellesmere Island 
topographic trough is associated with the reintensifica- 
tion of northern Baffin Bay disturbances and their sub- 
sequent rapid movement toward the North Pole. The 
arctic anticyclone usually centers on one or the other 
of the lobes shown, but not on both. 
October. Comparison of the average October pressure 
patterns (Fig. 2) with those of the Weather Bureau 
Fig. 2.—Average sea-level pressure (mb) during October. 
“normal” map reveals errors of greater importance than 
for July, considering the increased zonal circulation and 
the onset of autumn storms. The many years of few 
data and extrapolated analyses apparently resulted in 
too great an over-all smoothing and in a loss of knowl- 
edge regarding the terrain-imposed location of cyclonic 
centers associated with variations in the general circula- 
tion. Again the arctic high is overemphasized, although 
shown at a lower central pressure than July. The im- 
portant eastward extension of North Atlantic cyclonic 
activity to the Kara Sea is not definitely shown, nor 
is there sufficient indication that two cyclonic cells are 
to be expected in the Icelandic system. The arctic high 
tends to be centered over North Greenland or west 
