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Weather Almanac for August 1998
Weather in the Rockies
Less than an hour ago, the peak of Mt. Robson snagged a passing cloud as it tried to cross this section of the Canadian Rockies. The cloud struggled to loosen the grasp much as a fly caught in the spider's web. The sweat from the cloud's exertion fell on the snow-capped summit and upper glacial cirque. The changeling cloud transformed to a rainy sheet that obscured first the western face and then the eastern as the grand summit cleaved the cloud asunder. A gusting wind rolled down slope, an exhilaration of satisfaction from the great cloud-catcher.
Half an hour later, we had returned to our traveling home over the 40-km road from the foot of Robson, but that cloud, which had finally torn itself free, had cut across the mountains -- a downhill journey all the way -- and intercepted us. As I looked to the northeast, slate grey virga streaked the sky, the shower pushing ever closer. Now, the iced notches of Canoe Mountain were fading from view, buried by the towering cumulus. No thunder or lightning were apparent to me, but, had I seen this cloud rolling across the prairie of Illinois, I would have called it a thundershower, or maybe even a thunderstorm if the wind was strong enough, without hesitation.
Unlike the prairie thunderstorm whose gust front wind is cool, the gusting winds announcing this cloud felt pleasant to my bare chest. The first drops of precipitation began hitting me, but I postponed seeking shelter, for it has been many years since I have felt the caress of a small thundershower on my body and mind. The aspens waved to the dust blowing down the lane as the cloudy grey wash muted the face of Canoe Mountain, its snowy patches shining in the afternoon sun.
For one born a flat-lander, the many facets of mountain weather never ceases to amaze me. I currently live close to sea-level, but I am surrounded by mountains on all sides. Victoria, British Columbia and the Georgian Basin are situated within the rain shadow of these ranges, particularly the Olympic Mountains to the south and the Coastal Range to the east.
The rain shadow results when moisture-laden air is forced to ascend the encircling mountains and thus drops most of that moisture on the windward side of the range summits. As the air descends from the ridge, it is drier and warmer. Not only does the compression of descent warm the air, it also evaporates much of the liquid water, causing clouds to disappear. The downward motion of the air masse further inhibits cloud formation on the lee of these ridges.
The effect of the rain shadow in Victoria is a wide variation in annual rainfall across the region. In the extreme southeast of the city, less than 700 mm (27 in) fall on average during a year. But 20 to 30 km (about 12 to 20 miles) west in the Sooke Hills, the annual total is greater than 3000 mm (118 in), and at the airport 25 km (15 miles) north, the typical annual precipitation is around 1,000 to 900 mm (35 to 39 in).
Mountains catch the air low as well as high, and they affect more than just the moisture content of the air. Temperature and air quality are two other properties of the atmosphere influenced by mountains.
In the summer, semi-stagnant air masses can be trapped in the valleys between the mountain ranges and heat under the unrelenting summer sun. With no place to go but up, the trapped air is heated further with each passing day. Such a situation has just relented in the past week in the Okanagan Valley of British Columbia. There the stagnant air mass warmed to daytime highs in excess of 40C (104F).
If the heating during such times was not oppressive enough on its own, the mountains also trap air pollutants released by industry, agriculture, and transportation, particularly automobiles and trucks in the urban centres. In the Fraser River Valley just up-river from the Vancouver, BC metropolitan area, the released pollutants congregate to form the ingredients of a toxic chemical soup which simmers under clear skies and the strong summer sun. The result is photochemical smog, a melange of ozone, sulphate and nitrate particles, and a host of organic compounds. Not only is this air unhealthy to breathe, the particles reduce the visibility to the extent that the scenic mountain walls rimming the valley bowl are hidden from view.
Mountain weather is a very complex local situation, for each peak insists on having its say in the state of the atmosphere. No wonder some majestic peaks are called Storm King, they are indeed storm-catchers and cloud-catchers. Thus, forecasting in the mountains requires much more knowledge of local conditions than any other area, for nowhere is the variability of weather so great. Though born and raised on the prairies, I find mountain weather a great treat for weather watching.
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