Geoscience Reference
In-Depth Information
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
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.
Chinook conditions commonly develop in a
Pacific airstream that is replacing a winter high-
pressure cell over the western high plains. Some-
times the descending chinook does not dislodge
the cold, stagnant cP air of the anticyclone and a
marked inversion is formed. On other occasions
the boundary between the two air masses may
reach ground level locally. Thus, for example, the
western suburbs of Calgary may record tempera-
tures above 0°C while those to the east of the city
remain below -15
°
°
C
in California, Florida and the Gulf Coast to -18
C
over the northern Great Plains and the north-
eastern 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 >10m s
-1
with falling or blowing snow
reducing visibility below 400m. On average, a
blizzard event affects an area of 150,000km and
over 2 million people.
Another type of temperature fluctuation is
associated with the
chinook
(föhn) winds in the lee
of the Rockies (see Chapter 5C.2). 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 normal so
that snow often thaws rapidly; in fact the Salish
word 'Chinook' means snow-eater. Temperature
rises of up to 22
°
C.
The weather impact of very cold and very hot
spells in the United States is costly, especially
in terms of loss of life. In the 1990s, there were
292/282 deaths/year, respectively, attributed to
extreme cold/hot conditions, more than for any
other severe weather.
°
Precipitation and the moisture
balance
Longitudinal influences are apparent in the
distribution of annual precipitation, although this
is in large measure a reflection of the topography.
The 600mm annual isohyet in the United States
approximately follows the 100
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 at a
considerable distance from the Rockies into
southwestern Saskatchewan, but in Colorado
its influence is rarely felt more than about 50km
from the foothills. In southeastern Alberta, the
belt of strong westerly chinook winds and elevated
temperatures extends 150-200km east of the
Rocky Mountains. Temperature anomalies
average 5-9°C above winter normals, and a
triangular sector southeast of Calgary, towards
Medicine Hat, experiences maximum anomalies
of up to 15-25
°
W meridian
(
Figure 10.19
), and westward to the Rockies is an
extensive dry belt in the rain shadow of the
western mountain ranges. In the southeast, totals
exceed 1250mm, and 1000mm or more is received
along the Atlantic coast as far north as New
Brunswick and Newfoundland.
The major sources of moisture for precipita-
tion over North America are the Pacific Ocean
and the Gulf of Mexico. The former need not
concern us here, since comparatively little of the
precipitation falling over the interior appears to
derive from that source. The Gulf source is
extremely important in providing moisture for
precipitation over central and eastern North
America, but the predominance of southwesterly
airflow means that little precipitation falls over the
°
C, relative to mean daily maxi-
mum temperature values. Chinook events with
westerly winds >35m s
-1
occur on 45-50 days
between November and February in this area as a
°
