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region of 75 to 85 grains per pound in July. Nevertheless, there is a noticeable increase in Precipitation in the northern regions that begins in May and in the Mackenzie region reaches its maximum in August or

September. This is accomplished generally as follows. . Warm air moves north- eastward to the Hudson Bay. or

northern Quebec, rising over the colder air to the north: The cyclonic system sets

in motion the surface air lying over the Mackenzie Valley. There are then two major possibilities according to the synoptic situation obtaining at the time. First, the very cold, dry air from the Beaufort Sea may flow over the valley in the rear of the cyclone, producing low temperatures for the season. Alternatively, air from the north Pacific may cross the mountains and descend to the Mackenzie Valley in the wake of surface air moving southeastward into the cyclonic system. An anticyclonic system extending northward of a line from Point Barrow to the Anadyr Peninsula in Asia may then feed dense air directed eastward toward the Aleutians; and along the quasi-stationary front thus created between Arctic and Pacific air a series of waves with secondary fronts trailing southward may move eastward, producing light rains in the Yukon and the Mackenzie Valley and occasionally moderate rains in north- ern British Columbia. The rainy period ends when the air from the polar archipelago flows southward into the north- western regions with sufficient momentum to dislodge the quasi-stationary front. Very cool weather then follows.

A similar situation, but causing heavier rains from Fort Smith northward, requires very hot but rather humid air to move across the prairies from the mid-continent without encountering lift along an extensive front. This is seen in extreme fashion during extensive droughts on the prairies when, although there is little or no rain, the water-vapour- content of the air is high. It may approach 100 grains per pound as an average for a dry month. Southerly winds continuing for several days may move considerable vapour toward the far north, but east of the Rockies. Wherever @ quasistationary boundary between this air and polar air exists, showers and intermittent rains may persist for some days. In such a fashion there may be in one year an unusu ally wet summer month at Fort Smith, but very little at Simpson, Norman, or Aklavik. In another year Fort Smith may be south of the quasistationary front, getting little rain, while Norman and Good Hope are well watered in com- parison with the normal precipitation of those latitudes. Therefore, even in wet summers the rainfall may be very spotty. Toward the end of the summer season, the first outbreaks of polar air which have near-winter intensity

‘encounter, in the Mackenzie Valley, air which generally

carries considerably more moisture than in the winter season. The contrast across a cold front is ordinarily much greater than in winter or even in spring so that precipitation

holds up well into September or even October. A con

siderable proportion of the late summer or early autumn precipitation will, however, be snowfall.

[ 150 }

Generally, it may be said that a source of moisture from

which precipitation may readily be abstracted in north- western Canada is not available, once the coastal mountains are crossed. Along the coast the precipitation is 60 to 180 inches per annum, but when the summit of the Cascades is reached it is found that even in valleys draining to the sea precipitation falls off sharply. Thus at Aiyansh, surrounded by the peaks of the Cascades, although itself only 500 feet above sea, the precipitation has fallen to 43 inches. The percentage due to the water-content of snowfall is 27 per cent, which is much increased over the 5 per cent of Prince Rupert. Even on the Portland Canal, Premier and Stewart at moderate elevations, although having heavy precipitation, show a large proportion of snow and quite low temperatures (inconsistent in fact with elevation, perhaps affected by the great glaciers nearby). Past the summit of the Cascades, the precipitation at New Hazelton falls to 19 inches, of which 23 per cent is snowfall, and to 14 inches at Telkwa, with 36 per cent snowfall. Past the divide, Vanderhoof, Wistaria, Fort St. James, Babine Lake, and Prince George, average 13 to 20 inches, of which snow- fall contributes 32 to 43 per cent. Conditions change very rapidly from the very wet maritime weather of the coastal lowlands, with a ‘marked peak of precipitation in winter, to dry uplands with poorly marked winter and summer maxima. Aiyansh is the last station to retain a marked winter maximum. Along the north-flowing Fraser and McBride Rivers, and Dome Creek average 20 and 27 inches respectively, 35 per cent of which is snow. On the high plateau at Barkerville annual precipitation is 37 inches, nearly half snow. Further north the precipitation may be computed roughly from the recent observations at Dease Lake, Takla Landing, McDame Creek, as well as from eight years’ record at Watson Lake (about 9 miles north of

the British Columbia-Yukon boundary). There is also a long record from Atlin.

=e Precip- |Per Cent

xis Precip- |Per Cent itation | Snow

itation | Snow

JENS. 5 Res 6 11 47 v@arcross. . ... 9 48 Watson Lake... 18 48 »Whitehorse.. . 11 35 McDame Creek . 16 70 Swede Creek, . 10 43 Dease Lake..... 12 33 MEW Ons chess 11 36 Takla Wake). 13 23 Dawson...... 13 45

During the snowless season, therefore, there is only 4 to 6 inches of rain available for crops, east of the crest of the Cascades. Since there are no figures of run-off or storage, an estimate of the percentage of snow-water that may be retained by the soil or stored for various periods at high levels cannot be made. The experience in the south, in the Okanagan and Similkameen Valleys, is that with so little rainfall, irrigation, drawing upon the lakes at levels above the land, must be used to concentrate the run-off from a comparatively large slope-area upon a relatively

very small area of agricultural land. In these southern