upon the Pacific is unable to
Alaska Panhandle or the Bri

there is a wedge of very slowly moving air ahead of it.
In this case a low-pressure system develops along the
boundary between the two masses. This low-pressure
system is generated at first by the undercutting and lifting
of the warmer and moister air belonging to the most southerly
portion of the wedge lying on and off the British Columbia,
Washington, and Oregon Coasts. The forced ascent, sub-
sequent cooling and removal of the pressure of water-
vapour by condensation at high levels at a faster rate than
compensation by the influx of dry air at the lowest levels,
produces a fall of pressure. A cyclonic whirl is generated
over the north Pacific with strong north and northwest
winds carrying the cold air from the Asiatic Coast or the
Bering Sea region to more southerly latitudes. Eventually,
Say in two or three days, as the whole system moves east-
ward it approaches the coast, preceded by rain and low
cloud from the coastal air which is undergoing lift. Still
later the new cold air reaches the coast with
northwest winds and a sh
littoral region.

advance rapidly toward the
tish Columbia Coast because

strong north-
arp drop in temperature in the
There is now clearing and cooler weather
upon the coast, but occasional or intermittent showers
continue because of the instability of the new cold air.
This instability arises from the characteristics of the new
polar air which assures that the fall of temperature with
height is greater than the adiabatic rate, at least at medium
and higher levels. It follows that when the lower levels
are warmed by contact with warmer inland waters and
warmer soils, and therefore rise over patches which con-
tribute heat, the warmer air, cooling by rising at only the
adiabatic rate will find itself, at every level to which it
rises, surrounded by colder and denser air. At every level
it, therefore, retains buoyancy which will allow further
rise. Eventually it will reach a level where condensation
begins because the temperature has fallen to the dew-point,
clouds pile up and if a sufficient volume of air continues to
Tise, rain disposes of the condensed water-vapour. By
these processes is produced steady or intermittent rain
upon the coast from the wedge of coastal air and later
showers from the unstable, advancing cold air.

The process above described, finally reaches the stage
where the circulation is anticyclonic with cold air passing
southward along the Pacific Coast and also crossing the
mountainous interior as a northwesterly wind at moderate

‘elevations. The considerable flow of polar air to low lati-

tudes produces much weather south of Canada or in Eastern
Canada with which there is no present concern and need
not be considered. However, account must be taken of
the return flow from the south on the western side of the
coastal anticyclone. Regardless of the origin of this return
flow (that is, whether it is new polar air, warmed, moistened
and recurving northward, or whether it is air mixed from
various sources and driven northward in the anticyclonic
circulation), it is materially changed by sojourn over a
warmer expanse of ocean, Therefore, when the centre of

[ 146 }

high pressure has passed the coast moving eastward, a mild
spell affects the coastal region with increased heat and
Wwater-vapour-content aloft but increased stability. These
mild conditions may extend well to the northward and if
low-pressure exists over the western interior of the con-
tinent, the mild spell may spread over a great part of the
whole western region. Toward the low-pressure of th

interior the cold air superincumbent upon the Mackenzie

Slave, Athabaska, Peace, and north Saskatchewan Valleys
will be flowing eastward. This will afford opportunity)
for the mild Pacific air not only to cross the mountains)
but sometimes to descend rapidly to the prairie valleys in
the wake of the departing cold air. When the arrival of
the Pacific air on the surface of the plains is abrupt with
practically no weather associated, the change is called the
“chinook”. There may, however, have been some weather
in the interior valleys of British Columbia, prior to the
chinook on the plains. The coast mountains impose a
selective circulation on air with a southerly component
of motion, forcing much of the moisture in the denser low-
levels to flow northward along the range without much
orographical lifting. Such lifting, in general, requires a
layer of much colder air to be trapped on the seaward face
of the coast-range, providing a frontal or upglide slope
upon which the air from the sea may ascend. This may be
observed to occur on the coast on winter or spring mornings
when radiational, nocturnal cooling in fairly clear air and
a gentle westerly movement has provided a slip-surface a
few degrees cooler than air arriving from the sea. Early
morning low cloud or light rain will cease when the mountain-
slopes are warmed by the morning sun and when the denser
lower levels of the oncoming air are deflected along the coast
in the characteristically prevalent southeast winds of the
British Columbia winter. Forced ascent of low-level air
up a precipitous slope requires great power, ordinarily
not present under anticyclonic conditions.

Generally then, only the drier middle and higher layers }
of coastal air pass into the interior. Through the passes
and by by-passing the peaks, moister air, it is supposed, gains
some access to higher levels of valleys which discharge by
even circuitous routes to the sea. This, in part, accounts
for higher dew-points observed in such valleys, compared
with those to which maritime air has less direct access
during anticyclonic conditions. Yet in January, 1945,
during such conditions radiosonde flights above Coal
Harbour showed 3-5 to 4:0 grams of water-vapour per
kilogram of dry air at 3 km. (22 to 28 grains per pound at
10,000 feet). There is, therefore, a steady transport of
water-vapour vacross the interior valleys between 1 and
3 km. above sea which can produce cloud and rain if given
sufficient dynamical lift. This is provided by the barrier-
effect of interior mountain chains. Thus, in the case
quoted, it is found that although the temperature in the
free air at 2 km. was 31 to 33 degrees F., the temperature
on Old Glory peak was varying between 18 and 26 degrees F.
at a height of approximately 2 km. Lifting occurs by both