Geoscience Reference
In-Depth Information
Berkeley (California)
Halifax (Nova Scotia)
in
in
mm
mm
Soil
moisture
recharge
(33mm)
6
6
Soil
moisture
used
(33mm)
140
140
5
5
120
120
Soil
moisture
recharge
(224mm)
Surplus
(107mm)
4
100
4
100
Surplus
(81mm)
Surplus
(81mm)
80
80
3
3
Deficit
(183mm)
60
60
2
2
40
40
Soil
moisture
Soil
moisture
used
(224mm)
Soil
moisture
used
1
1
20
20
used
(224mm)
(224mm)
0
0
0
0
J
FM
A
M
JJ
A
S
O
N
D
J
FM
A
M
J
J
A
S
O
N
D
Potential evapotranspiration
Actual evapotranspiration
Precipitation
Figure 10.22
The moisture balances at Berkeley, California, and Halifax, Nova Scotia.
Source: After Thornthwaite and Mather (1955).
ratio is small due to lack of precipitation whereas
in northwest Canada actual evaporation is limited
by available energy.
climatic conditions in the southwestern United
States serve to exemplify this climatic type, based
on the more reliable data for the semi-arid
margins of the subtropical cells.
Observations at Tucson (730m), Arizona,
between 1895 and 1957 showed a mean annual
precipitation of 277mm falling on an average of
about 45 days per year, with extreme annual
figures of 614mm and 145mm. Two moister
periods in late November to March (receiving 30
percent of the mean annual precipitation) and
late June to September (50 percent) are separated
by more arid seasons from April to June (8
percent) and October to November (12 percent).
The winter rains are generally prolonged and of
low intensity (more than half the falls have an
intensity of less than 5mm per hour), falling from
altostratus clouds associated with the cold fronts
of depressions that are forced to take southerly
routes by strong blocking to the north. This occurs
during phases of equatorial displacement of the
C THE SUBTROPICAL
MARGINS
1 The semi-arid southwestern
United States
Both the mechanisms and patterns of the climate
in areas dominated by the subtropical high
pressure cells are not well documented. The
inhospitable nature of these arid regions inhibits
data collection, and yet the study of infrequent
meteorological events requires a close network of
stations maintaining continuous records over
long periods. This difficulty is especially apparent
in the interpretation of desert precipitation data,
because much of the rain falls in local storms
irregularly scattered in both space and time. The
