Geoscience Reference
In-Depth Information
as snow (see Figure 10.16) and some of the greatest
snow depths in the world are reported from British
Columbia, Washington and Oregon. A US national
record seasonal total of 28.96m was observed at the Mt
Baker ski area (1280 m) in 1998 to 1999. Generally, 10
to 15 m of snow falls annually on the Cascade Range at
heights of about 1500 m, and even as far inland as the
Selkirk Mountains snowfall totals are considerable. The
mean snowfall is 9.9 m at Glacier, British Columbia
(elevation 1250 m), and this accounts for almost 70
per cent of the annual precipitation (see Figure 10.16).
Near sea-level on the outer coast, in contrast, very little
precipitation falls as snow (for example, Estevan Point).
It is estimated that the climatic snowline rises from
about 1600 m on the west side of Vancouver Island
to 2900 m in the eastern Coast Range. Inland, its eleva-
tion increases from 2300 m on the west slopes of the
Columbia Mountains to 3100 m on the east side of
the Rockies. This trend reflects the precipitation pattern
referred to above.
Large diurnal variations affect the Cordilleran val-
leys. Strong diurnal rhythms of temperature (especially
in summer) and wind direction are a feature of mountain
climates and their effect is superimposed upon the
general climatic characteristics of the area. Cold air
drainage can produce remarkably low minima in
the mountain valleys and basins. At Princeton, British
Columbia (elevation 695 m), where the mean daily
minimum in January is -14°C, there is on record an
absolute low of -45°C, for example. This leads in some
cases to reversal of the normal lapse rate. Golden in the
Rocky Mountain Trench has a January mean of -12°C,
whereas 460 m higher at Glacier (1250 m) it is -10°C.
summer are illustrated in Figure 10.18, showing the
frequency with which hourly temperature readings
exceed or fall below certain limits. The two chief
features of all four maps are (1) the dominance of the
meridional temperature gradient, away from coasts, and
(2) the continentality of the interior and east compared
with the 'maritime' nature of the west coast. On the July
maps, additional influences are evident and these are
referred to below.
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 inci-
dence 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
3 Interior and eastern North America
Central North America has the typical climate of a con-
tinental interior in mid-latitudes, with hot summers and
cold winters (Figure 10.17), yet the weather in winter is
subject to marked variability. This is determined by the
steep temperature gradient between the Gulf of Mexico
and the snow-covered northern plains; also by shifts
of the upper wave patterns and jet stream. Cyclonic
activity in winter is much more pronounced over central
and eastern North America than in Asia, which is domi-
nated by the Siberian anticyclone (see Figure 7.9A).
Consequently there is no climatic type with a winter
minimum of precipitation in eastern North America.
The general temperature conditions in winter and
