Geoscience Reference
In-Depth Information
Figure 10.14
A synoptic example of depressions associated with three frontal zones on 29 May 1963 over North America.
Source
: Based on charts of the Edmonton Analysis Office and the Daily Weather Report.
of northern Keewatin and Hudson Strait. One study of
airmass temperatures and airstream confluence regions
suggests that an Arctic frontal zone occurs further south
over Keewatin in July and that its mean position (Figure
10.15) is related closely to the boreal forest-tundra
boundary. This relationship reflects the importance of
Arctic airmass dominance for summer temperatures and
consequently for tree growth, yet energy budget differ-
ences due to land cover type appear insufficient to
determine the frontal location.
Several circulation singularities have been recog-
nized in North America, as in Europe (see A.4, this
chapter). Three that have received attention in view
of their prominence are (1) the advent of spring in
late March; (2) the midsummer high-pressure jump at
the end of June; and (3) the Indian summer in late
September (and late October).
The arrival of spring is marked by different climatic
responses in different parts of the continent. For
example, there is a sharp decrease in March to April
precipitation in California, due to the extension of the
Pacific high. In the Midwest, precipitation intensity
increases as a result of more frequent cyclogenesis
in Alberta and Colorado, and northward extension of
maritime tropical air from the Gulf of Mexico. These
changes are part of a hemispheric readjustment of the
circulation; in early April, the Aleutian low-pressure
cell, which from September to March is located about
