Chinooks-Warm West Winds

Along the eastern slopes of the Rockies, the Chinook wind provides a welcome respite from the long winter chill. Few people spend very much time along the eastern slopes without experiencing these wonderful warm winds. The change can be dramatic. On Jan. 11, 1983, the temperature in Calgary rose 30?C (from -17?C to 13?C) in 4 hours, and on February 7, 1964, the temperature rose 28?C (51?F?), and the humidity dropped by 43 percent.

In some of our earliest written records, the Chinook stands out. Alexander Mackenzie referred to the Chinook as a 'perfect hurricane', and in 1877, David Thompson stated that the temperature rose as much by heading west as it did by traveling south.

At the turn of the century, the Calgary Herald wrote.

'Those who have not the warm, invigorating Chinook winds of this country, cannot well comprehend what a blessing they are. The icy clutch of winter is lessened, the earth throws off its winding sheet of snow. Humanity ventures forth to inhale the balmy spring like air. Animated nature rejoices.' (1900-Calgary weekly Herald)

The winds are caused by moist weather patterns, originating off the Pacific coast, cooling as they climb the western slopes, and then rapidly warming as they drop down the eastern side of the mountains. The Chinook usually begins with a sudden change in wind direction towards the west or southwest, and a rapid increase in wind speed.

As moist weather patterns blow ashore on the coast, they run into a barrier of mountains. As they are forced to climb upwards to crest the mountains, they cool down at a specific rate. Weather patterns cool at rates of .54?C/100 m for moist systems, and 1?C/100 m for dry systems. For example, a coastal weather pattern beginning at -1?C near Vancouver will cool at the dry rate until it becomes saturated with moisture. From that point on, it will cool at the slower moist rate. If the saturation level is reached at 1,000 m, it will cool to -11?C to this point, and then slow to .54?C/100 m. When it crests the summit of a 3,050 m (10,000 ft) peak, it will have dropped to-22?C. During this process of rising and cooling, it will release most of its moisture in the form of snow or rain. This results in rainforest conditions on the western side of the mountains, while the eastern side of the divide remains quite dry.

Once the now dry weather system crests the summit, it begins to move downhill. Dry weather patterns warm up with drops in elevation at almost twice the rate of moisture laden patterns. (1? C/100 m). This means that the above example, in dropping from 3,050 m (10,000 ft) to the valley bottom at 1,370 m (4,500 ft) will rise to -5.2? C. If the ambient temperature prior to the Chinook was -25? C, the site would see a rise of 19.8? C over a very short period.

Red Belt

Sometimes, looking upwards, the mountains may show a belt of evergreens with dead foliage. The dried out needles take on a reddish tinge and from a distance the damage is clearly visible. This is known as red belt and is often incorrectly referred to as red belt disease. It is not a disease, but a dehydration caused by the warmth of the Chinook wind.

As these warm winds make their way over the mountains, trees feel the sudden change in temperature. In severe cases, the change may be so dramatic that the trees lose their winter preparedness and begin to wake up. As the tree begins to photosynthesize, turning sunlight into food energy, the needles will need moisture to replace that which is lost through the foliage. Since the ground is still frozen, there is no fresh water with which to replace that which is lost, and so the needles dry out and die. As they turn red, the distinctive appearance is gained.

Red Belt is most common in areas that have irregular Chinooks. In areas that are consistently Chinook blasted, trees like the lodgepole pine disappear. They, like the virtually absent white birch, are very intolerant of the rapid temperature changes wrought by Chinooks.