B Warm and cold spells

Two types of synoptic condition are of particular significance for temperatures in the interior of North

America. One is the cold wave caused by a northerly outbreak of cP air, which in winter regularly penetrates deep into the central and eastern United States and occasionally affects even Florida and the Gulf Coast, injuring frost-sensitive crops. Cold waves are arbitrarily defined as a temperature drop of at least 11°C in twenty-four hours over most of the United States, and at least 9°C in California, Florida and the Gulf Coast, to below a specified minimum depending on location and season. The winter criterion decreases from 0°C in California, Florida and the Gulf Coast to 18°C over the northern Great Plains and the northeastern states. Cold spells commonly occur with the buildup of a north-south anticyclone in the rear of a cold front. Polar air gives clear, dry weather with strong, cold winds, although if they follow snowfall, fine, powdery snow may be whipped up by the wind, creating blizzard conditions over the northern plains. These occur with winds >10 ms-1 with falling or blowing snow reducing visibility below 400 m. On average, a blizzard event affects an area of 150,000 km and over two million people.

Another type of temperature fluctuation is associated with the chinook winds in the lee of the Rockies (see Chapter 6C.3). The chinook is particularly warm and dry as air descends the eastern slopes and warms at the dry adiabatic lapse rate. The onset of the chinook produces temperatures well above the seasonal average so that snow is often thawed rapidly; in fact the Indian word 'chinook' means snow-eater. Temperature rises of up to 22°C have been observed in five minutes. The occurrence of such warm events is reflected in the high extreme maxima in winter months at Medicine Hat (Figure 10.18). In Canada, the chinook effect may be observed a considerable distance from the Rockies into southwestern Saskatchewan, but in Colorado its influence is rarely felt more than about 50 km from the foothills. In southeastern Alberta, the belt of strong westerly chinook winds and elevated temperatures extends 150 to 200 km east of the Rocky Mountains. Temperature anomalies average 5 to 9°C above winter normals, and a triangular sector southeast of Calgary, towards Medicine Hat, experiences maximum anomalies of up to 15 to 25°C, relative to mean daily maximum temperature values. Chinook events with westerly winds >35m s-1 occur on forty-five to fifty days between November and February in this area as a result of the relatively low and narrow ridge line of the Rocky Mountains between 49 and 50°N, compared with the mountains around Banff and further north.

Figure 10.18 Mean and extreme temperatures at Medicine Hat, Alberta.


Figure 10.18 Mean and extreme temperatures at Medicine Hat, Alberta.

Chinook conditions commonly develop in a Pacific airstream that is replacing a winter high-pressure cell over the western high plains. Sometimes the descending chinook does not dislodge the cold, stagnant cP air of the anticyclone and a marked inversion is formed. On other occasions the boundary between the two airmasses may reach ground level locally. Thus, for example, the western suburbs of Calgary may record temperatures above 0°C while those to the east of the city remain below -15°C.

The weather impact of very cold and very hot spells in the United States is costly, especially in terms of loss of life. In the 1990s, there were 292 and 282 deaths per year, respectively, attributed to extreme cold/hot conditions, more than for any other severe weather.

Continue reading here: C Precipitation and the moisture balance

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