Geoscience Reference
In-Depth Information
resulted in 48 deaths, destroyed 50,000 homes
and caused damage losses of $10 billion. In
September, renewed cyclonic activity associ-
ated with the seasonal southward shift of the
polar front, at a time when mT air from the
Gulf is still warm and moist, typically causes a
resumption of rainfall. Later in the year drier
westerly airstreams affect the continental
interior as the general airflow becomes more
zonal.
The diurnal occurrence of precipitation in
the central United States is rather unusual for
a continental interior. Sixty percent or more
of the summer precipitation falls during
nocturnal thunderstorms (20:00-08:00 LST)
in central Kansas, parts of Nebraska,
Oklahoma and Texas. Hypotheses suggest
that the nocturnal thunderstorm rainfall that
occurs, especially with extensive mesoscale
convective systems (see p. 255), may be linked
to a tendency for nocturnal convergence and
rising air over the plains east of the Rocky
Mountains. The terrain profile appears to play
a role here, as a large-scale inversion layer
forms at night over the mountains, setting up
a low-level jet (LLJ) east of the mountains just
above the boundary layer. LLJs are most
frequent in the night-time to early morning
hours and occur more than 50 percent of the
time in Texas in summer. This southerly flow,
at 500-1000m above the surface, can supply
the necessary low-level moisture influx and
convergence for the storms (cf.
Figure 5.27
).
MCSs account for 30-70 percent of the May to
September rainfall over much of the area east
of the Rocky Mountains to the Missouri River.
3
East of the upper Mississippi, in the Ohio valley
and south of the lower Great Lakes, there is
a transitional regime between that of the
interior and the east coast type. Precipitation
is reasonably abundant in all seasons, but the
summer maximum is still in evidence (e.g.
Dayton).
4
In eastern North America (New England, the
Maritimes, Quebec and southeast Ontario),
precipitation is fairly evenly distributed
throughout the year (e.g., Blue Hill). In Nova
Scotia and locally around Georgian Bay there
is a winter maximum, due in the latter case to
the influence of open water. In the Maritimes
it is related to winter (and also autumn) storm
tracks.
It is worth comparing the eastern regime with
the summer maximum that is found over East
Asia. There the Siberian anticyclone excludes
cyclonic precipitation in winter and monsoonal
influences are felt in the summer months.
The seasonal distribution of precipitation is of
vital interest for agricultural purposes. Rain falling
in summer, for instance, when evaporation losses
are high, is less effective than an equal amount
in the cool season.
Figure 10.22
illustrates the
effect of different regimes in terms of the moisture
balance, calculated according to Thornthwaite's
method (see Appendix 1B). At Halifax (Nova
Scotia), sufficient moisture is stored in the soil
to maintain evaporation at its maximum rate
(i.e., actual evaporation = potential evaporation),
whereas at Berkeley (California) there is a
computed moisture deficit of nearly 50mm in
August. This is a guide to the amount of irrigation
water that may be required by crops, although in
dry regimes the Thornthwaite method generally
underestimates the real moisture deficit.
Figure 10.23
shows the ratio of actual to
potential evaporation (AE/PE) for North America
calculated by the methods of Thornthwaite and
Mather from an equation relating PE to air
temperature. It is drawn to highlight variation in
the dry regions of the country. The boundary
separating the moist climates of the east, where the
ratio AE/PE exceeds about 8 percent or more,
from the dry climates of the west (excluding the
west coast), follows the 95th meridian. The major
humid areas are along the Appalachians, in the
northeast and along the Pacific coast, while the
most extensive arid areas are in the intermontane
basins, the High Plains, the southwest and parts of
northern Mexico. In the west and southwest the
