pure orographical ascent near the crest of the ridge and also
on the slip-surface created by the colder air which is trapped
against the mountain side, and which may extend a very
considerable distance downslope on the windward side,
especially in the early morning. On the lee side of ridges
there is generally a slight subsidence of the eastward-
moving current which does not favour rain or snow. Hence,
during the period quoted, light precipitation was reported
across the interior of British Columbia on windward slopes.
Beyond the Stikine Mountains, by the radiosonde flights
above Fort Nelson airport (lat. 58°50'N., long. 122°38’,
height above sea 1,230 ft.) the arrival of this current from
the Pacific was easily recognized from the following figures:

Vapour- Elevation
Elevation Temperature Content (Approximate,
Above Sea Degrees F. (Grains in
per Pound) Feet)
Ground ssa ns oa 5 below zero 5 Ground
Osoakmyre fonts. ch. . 5 above zero 7 1,640
de Oskinesses ee Se 31 above zero 25 3,300
Mek Seah se 28 above zero 24 4,900
DAO) See Se. 21 above zero 16 6,500
BOORKMEn SS Sele 17 above zero 14 8,200
SC Oakm epee iss el eo 16 above zero 14 9,800
40) ent ante eee 8 above zero 6 13,100
SONI sare neta oes 5 below zero 5 16,400

Here is shown an increase of 10 degrees in the first 410 feet
above the ground (at the time of flight, approximately

20" Pacific time), and a rise of 36 degrees in the first 2,000
feet, while an altitude of 15,000 feet above ground was
reached before again encountering air as cold or dry as that
on the ground. Note also, that there is fivefold water-
vapour in the layers more than 2,000 feet above ground.

On this occasion described above (which is a fairly
typical case for the region just east of the Stikine Range),
there was stratus and altostratus cloud in the warm Pacific
air, but only very light snowfall resulted from the over-
riding of this warm sheet above the surface air. In the days
immediately following, 3 or 4 inches of snow fell in the
northern districts of British Columbia and in the middle
and upper reaches of the Mackenzie River system, including
the Peace and Athabaska Valleys. Since this event is
also a typical weather situation, it will be helpful to describe
the mechanism producing the snowfall. The warmer air,
having reached as far north and east as Great Bear Lake
and having raised the temperatures on the ground in the
Tegion immediately east of the mountains by 10 to 15 degrees,
was in less than 100 hours attacked by a fresh flow of polar
air from the Alaskan region through the Yukon Territory.
This invasion undercut the warmer air giving it sufficient
lifting and dynamical cooling to produce snowfall. As the
new cold air from the northwest pushed southward and
replaced the warmer air, there was a return to severe
winter conditions as shown by the following figures:

Temperature, Water—Vapour
Degrees F. (Grains per Pound)
Elevation Above Sea Elevation Above Sea
Fort Norman 5 Fort Norman ,
Nelson Wells Aklavik Nelson Wells Aklavik
(Grounds et nd ete ey eyee ot — 5 —23 —45 Ground haa nies ren Bees 4 2 0
OBO Bembpee S080 eet re an et — 7 —16 —15 O40 sfeetiea ar Renee were teem 4 3 1
AC OMC Mapas erie ne ees ie = 7 — 8 —10 3300 Mee tnvee ahaetine cee eae et nat 3) 3 2
LOG Title Seah are ea es 0) — 8 —12 49000 feetien pate saan tt =n aes a 2 2
KM Ma sg, Seer seers SS —2 — 8 —10 Gi500 feetst Sei via wate ene 7 2 2
De ISIN scsi Se Gea Nee ee eee = 8 — 9 12 S200 feet i liea te ee ee eset tare Gm ik Ee eae 2
SUOMkmsyeuea aires sneer ne ee —10 —12 —14 O:800 fection ras vane. eon ce ee Sopeloulegtenes res 2
Gs ONKIn epee ce as nase Coan ey) —18 —18 UB MOO KES ccs ee haevanduesaeue. Digan oP 1
Oe UKM anne eae mete ote te TAS, — 28 —25 —26 LOM OOfe ets ie ecct einen ante 1 1 1

It will be seen that between 3,000 and 10,000 feet there is a
slight latitudinal effect, the more southerly region having
slightly warmer air aloft, probably because of slight mixing.
At the ground the radiational loss is apparently much
more efficient in producing a shallow layer with very
low temperature in the more northerly region.

To recapitulate the discussion of winter weather in far
northwestern Canada, it may be stated that the controls are:

(1) very cold and dry polar airmasses moving from the

Beaufort Sea or the polar archipelago;

(2) similar airmasses moving across the far northern
reaches of the Pacific, but tempered by the ocean
route in respect to the lower levels;

(3) polar airmasses, considerably maritimized by longer
sojourn on the Pacific and finally moving inland,
but mostly as upper sheets. To these may be
added for the Mackenzie Valley

(4) polar air which after movement eastward to the |
Hudson Bay and Missouri Valley regions is return
ing northward on the western side of a polar
anticyclone which is itself moving very slowly
southeastward;

(5) very rare cases where an extensive Pacific anti
cyclone has reached the southwestern states and

feeds very mild air into the whole British Columbia,/
Alberta, and Mackenzie region. |

{ 147 ]