Geoscience Reference
In-Depth Information
moderating effect of minimum temperatures at coastal
stations this is scarcely evident on generalized maps
such as shown in Figure 10.17. More significant climatic
effects are in fact found in the neighbourhood of Hudson
Bay and the Great Lakes. Hudson Bay remains very cool
in summer, with water temperatures of about 7 to 9°C,
and this depresses temperatures along its shore,
especially in the east (see Figure 10.17C and D). Mean
July temperatures are 12°C at Churchill (59°N) and
8°C at Inukjuak (58°N), on the west and east shores
respectively. This compares, for instance, with 13°C at
Aklavik (68°N) on the Mackenzie delta. The influence
of Hudson Bay is even more striking in early winter,
when the land is snow-covered. Westerly airstreams
crossing the open water are warmed by 11°C on average
in November, and moisture added to the air leads to
considerable snowfall in western Ungava (see the graph
for Inukjuak, Figure 10.20). By early January, Hudson
Bay is frozen over almost entirely and no effects are
evident. The Great Lakes influence their surroundings
in much the same way. Heavy winter snowfalls are a
notable feature of the southern and eastern shores of the
Great Lakes. In addition to contributing moisture to
northwesterly streams of cold cA and cP air, the heat
source of the open water in early winter produces a
low-pressure trough, which increases the snowfall as
a result of convergence. Yet a further factor is frictional
convergence and orographic uplift at the shoreline.
Mean annual snowfall exceeds 2.5 m along much of
the eastern shore of Lake Huron and Georgian Bay, the
southeastern shore of Lake Ontario, the northeastern
shore of Lake Superior and its southern shore east of
about 90.5°W. Extremes include 1.14 m in one day at
Watertown, New York, and 8.94 m during the winter of
1946 to 1947 at nearby Bennetts Bridge, both of which
are close to the eastern end of Lake Ontario.
Transport in cities in these snow belts is disrupted
quite frequently during winter snowstorms. The Great
Lakes also provide an important tempering influence
during winter months by raising average daily minimum
temperatures at lakeshore stations by some 2 to 4°C
above those at inland locations. In mid-December,
the upper 60 m of Lake Erie has a uniform temperature
of 5°C.
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 anom-
alies 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.
b Warm and cold spells
Two types of synoptic condition are of particular
significance for temperatures in the interior of North
