A Continental and oceanic influences

The large annual temperature range in the interior of the continent shown in Figure 3.24 demonstrates the pattern of continentality of North America. The figure illustrates the key role of the distance from the ocean in the direction of the prevailing (westerly) winds. The topographic barriers of the western Cordilleras limit the inland penetration of maritime airstreams. On a more local scale, inland water bodies such as Hudson Bay and the Great Lakes have a small moderating influence -cooling in summer and warming in the early winter before they freeze over.

The Labrador coast is fringed by the waters of a cold current, analogous to the Oyashio off East Asia, but in both cases the prevailing westerlies greatly limit their climatic significance. The Labrador current maintains drift ice off Labrador and Newfoundland until June and gives very low summer temperatures along the Labrador coast (see Figure 10.17C). The lower incidence of freezing temperatures in this area in January is related to the movement of some depressions into the Davis Strait, carrying Atlantic air northward. A major role of the Labrador current is in the formation of fog. Advection fog is very frequent between May and August off Newfoundland, where the Gulf Stream and Labrador current meet. Warm, moist southerly airstreams are cooled rapidly over the cold waters of the Labrador current and with steady, light winds such fogs may persist for several days, creating hazardous conditions for shipping. Southward-facing coasts are particularly affected and at Cape Race (Newfoundland), for example, there are on average 158 days per year with fog (visibility less than 1 km) at some time of the day. The summer concentration is shown by the figures for Cape Race: May - 18 (days), June - 18, July - 24, August - 21 and September - 18.

Oceanic influence along the Atlantic coasts of the United States is very limited, and although there is some

Fog Effect
Figure 10.17 The percentage frequency of hourly temperatures above or below certain limits for North America. (A) January temperatures <0°C. (B) January temperatures >I0°C. (C) July temperatures <I0°C. (D) July temperatures >2I°C.

Source: After Rayner (1961).

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.

Continue reading here: B Warm and cold spells

Was this article helpful?

0 0