Geoscience Reference
In-Depth Information
western Great Plains (see
Figure 10.19
). Over the
southern United States, there is considerable
evapotranspiration and this helps to maintain
moderate annual totals northward and eastward
from the Gulf by providing additional water vapor
for the atmosphere. Along the east coast, the
Atlantic Ocean is an additional significant source
of moisture for winter precipitation.
There are at least eight major types of seasonal
precipitation regime in North America (
Figure
10.20
); the winter maximum of the west coast and
the transition type of the intermontane region
in mid-latitudes have already been mentioned;
the subtropical types are discussed in the next
section. Four primarily mid-latitude regimes are
distinguished east of the Rocky Mountains:
1
A warm season maximum is found over much
of the continental interior (e.g., Rapid City). In
an extensive belt from New Mexico to the
Prairie provinces more than 40 percent of
the annual precipitation falls in summer. In
New Mexico, the rain occurs mainly with late
summer thunderstorms, but May to June is
the wettest time over the central and northern
Great Plains due to more frequent cyclonic
activity. Winters are quite dry over the plains,
but the mechanism of the occasional heavy
snowfalls is of interest. They occur over the
northwestern plains during easterly upslope
flow, usually in a ridge of high pressure.
Further north, in Canada, the maximum is
commonly in late summer or autumn, when
I
in
mm
8
200
in
mm
150
6
4
100
100
4
2
50
2
50
INTERIOR
0
J J
Indukjuak (410mm)
0
0
0
D
J J
Portland (940mm)
D
P
BH
in
mm
in
mm
RC
4
100
4
100
2
50
2
50
0
J J
Rapid City (380mm)
0
0
0
D
D
J J
Blue Hill (1220mm)
D
C
in
mm
100
4
2
50
in
mm
LR
4
100
0
0
J J
Dayton (910mm)
D
2
50
Ab
SD
0
0
J J
San Diego (250mm)
D
J
mm
in
in
mm
100
4
100
4
2
50
2
50
0
0
in
mm
0
J J
Jacksonville (1350mm)
0
J J
Abilene (580mm)
D
6
150
D
4
100
in
mm
2
50
4
100
0
J J
Little Rock (1240mm)
0
2
50
0
miles
500
D
Jet Stream
0
0
J J
Columbia (940mm)
D
0
km
500
Figure 10.20
North American rainfall regime regions and histograms showing mean monthly precipitations for
each region (January, June and December are indicated). Note that the jet stream is anchored by the Rockies in more
or less the same position at all seasons.
Source: Mostly after Trewartha (1981); additions by Henderson-Sellers and Robinson (1986).
