Geoscience Reference
In-Depth Information
Large diurnal variations affect the Cordilleran
valleys. 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 charac-
teristics of the area. Cold air drainage can produce
remarkably low minima in the mountain valleys
and basins. At Princeton, British Columbia (eleva-
tion 695m), where the mean daily minimum in
January is -14
Continental and oceanic influences
The large annual temperature range in the interior
of the continent shown in
Figure 3.24
demon-
strates 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 10.18C
). 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 1km) at some times of 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 moderating effect of minimum
temperatures at coastal stations this is scarcely
evident on generalized maps such as
Figure 10.17
.
°
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 460m higher at Glacier
(1250m) it is -10
°
°
C.
3 Interior and eastern North
America
Central North America has the typical climate of
a continental 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 dominated by the Siberian anti-
cyclone (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 summer are illustrated in
Figure 10.17
,
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 tempera-
ture 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.
