ARCTIC METEOROLOGY 
4-km isobars [14] and to the new series of Historical 
Weather Maps, for additional support of the height 
increases required over the Okhotsk and Bering Seas. 
The air flow over northern arctic America, and over 
the Siberian Arctic, is seen to be predominantly cy- 
clonie at the 700-mb level in July, closely corresponding 
to the gradient circulation derived from average sea- 
level pressure. Weak anticyclonic flow is frequently ob- 
served over the northeast Greenland—Fridtjof Nansen 
Land area and western sections of the American Arctic, 
but is rare and of brief duration over northern Baffin 
Bay. 
January. The major revisions appearing in the new 
700-mb chart for January (Fig. 6) are directly related 
oe! 90° 45° 
“Ee Vy 9000 
LA be 
135° TPaTR 90° 45° 
Fic. 6.—Average 700-mb contours (ft) and isotherms 
(°C) during January. 
to the areas of excessive sea-level pressure, as previously 
noted over northern Baffin Bay and over the Kara Sea, 
on the Weather Bureau “normal” map. With the in- 
troduction of more realistic perturbations in the circum- 
polar westerlies over these areas came an equally logical 
intensification and southward displacement of the east 
Asiatic cyclonic cell. It is suggested that the intensity 
of the American cell will be exceeded by that of the 
Asiatic cell only during the anomalous absence, or com- 
plete enclosure, of the Barents-Kara perturbation. 
The cold-season cyclonic circulation over northern 
arctic America, being mainly a dynamic system of cold 
dry air, is relatively cloud free except during periods of 
reintensification. 
SEASONAL VARIATIONS IN ARCTIC AIR 
Variations over Northern Arctic America. The fore- 
going summary of average circulation patterns, and the 
geographical location of the American Arctic, indicate 
that all the large-scale air currents will have been sub- 
jected to arctic influences before their arrival over this 
region. The region itself provides the cold source area 
of arctic air masses affecting central and eastern North 
America. Variations in the observed properties of arctic 
air are directly related to the seasonal variations of the 
underlying surfaces and the air flow over them. The 
947 
vertical structure of arctic air over the Canadian Arch- 
ipelago during the different seasons of the year is shown 
by the average soundings for July, October, January, 
and April, which are plotted in Fig. 7. 
<< — 10 
AVERAGE SOUNDINGS 
NORTHERN ARCTIC 
AMERICA 
200 
300 
400 
l 1, 2, | 
61-48-47}. “341000L 
tule JAN, APR. OCT.|_ | wucy| me 9 
20 =40",=30°. -20°  -10° orc 
Fie. 7.—Average soundings over northern arctic America. 
July. The average sounding for July, based on 28 
ascents to the 250-mb level obtained at the Eureka 
station during July 1947, shows the midsummer struc- 
ture of arctic air over ice-free interior areas. Because of 
the predominantly cool and moist underlying surface 
conditions, the regional air mass is maritime arctic in 
summer. There is a characteristic surface inversion over 
the pack ice and cold water, but convective instability 
in the lower levels is readily produced by land heating 
after a short period of onshore flow. A moisture content 
of about 4 g ke near the surface is fairly representa- 
tive of the regional air masses. The cyclonic circulation 
around northern Baffin Bay and the associated middle- 
cloud system create a characteristic moisture inversion 
near the 700-mb level of the average July sounding. 
During this individual month, the average height of 
the tropopause was about 9000 m (29,500 ft). 
October. In sharp contrast to the relative warmth 
caused by 24-hr insolation in summer, the autumn 
conditions of almost no solar heating and predom- 
inantly frozen surfaces are relatively ideal for the rapid 
modification of maritime polar air. The average sound- 
ing for October, derived from 27 Eureka ascents to the 
250-mb level in October 1947, shows well-developed 
continental characteristics at this early stage in the 
transition to the deep, cold, continental arctic air of 
winter. The average October circulation pattern was 
closely approximated in 1947. Moderate anticyclonic 
conditions dominated northern arctic America, and the 
sounding has no obvious traces of Baffin Bay cyclonic 
activity. The tropopause was about 8600 m (28,000 ft) 
high, 400 m lower than in July. Continental arctic air 
of this type begins to form over the pack ice during 
early autumn, but it becomes unstable and produces 
heavy snow showers as it flows southward over un- 
frozen waterways and warmer land masses in the rear 
quadrants of autumn storms. 
January. The average sounding for January, based 
on 21 ascents to the 350-mb level above the Resolute 
