Geoscience Reference
In-Depth Information
More significant climatic effects are in fact found
in the neighborhood of Hudson Bay and the Great
Lakes. Hudson Bay remains very cool in summer,
with water temperatures of about 7-9
60m of Lake Erie has a uniform temperature
of 5
°
C.
C, and this
depresses temperatures along its shore, especially
in the east (see
Figure 10.17C
and
D
). Mean July
temperatures are 12
°
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
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 250cm 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 114cm in one day at Watertown, New
York, and 894cm during the winter of 1946-1947
at nearby Bennetts Bridge, both of which are close
to the eastern end of Lake Ontario.
Transport in cities in these snow belts is quite
frequently disrupted 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 2-4
°
C at Churchill (59
°
°
C in 24 hours over
Medicine Hat (50°N 111°W)
°F
°C
40
100
30
75
20
50
10
0
25
-10
0
-20
-30
-25
-40
-50
J
FM
A
M
J
J
A S O N D
C above those at
inland locations. In mid-December, the upper
°
Figure 10.18
Mean and extreme temperatures
at Medicine Hat, Alberta.
