Geoscience Reference
In-Depth Information
ft
m
0
0
5
8
miles
km
1500
457
1000
305
Lynmouth
0
0
Porlock
229
Simonsbath
Exford
Figure 4.15
Distribution of rainfall (mm) over Exmoor, southwest England during a 24-hour period on 15 August
1952 which produced catastrophic local flooding at Lynmouth. The catchment is marked (dashed); 75 percent of the
rain fell in just seven hours.
Souce: Dobbie and Wolf (1953). Courtesy of the Institute of Civil Engineers.
In view of the complex controls involved,
no brief explanation of these precipitation
distributions can be very satisfactory. Various
aspects of selected precipitation regimes are
examined in chapters 10 and 11, after consid-
eration of the fundamental ideas about atmos-
pheric motion and weather disturbances. Here
we simply point out four factors that have to
be taken into account in studying
figures 4.16
and
4.17
:
3
The distribution of the land masses. The
Southern Hemisphere lacks the vast, arid, mid-
latitude continental interiors of the Northern.
The oceanic expanses of the Southern Hemi-
sphere allow the mid-latitude storms to
increase the zonal precipitation average for
45
S by about one-third compared with that
of the Northern Hemisphere for 50°N.
Longitudinal irregularities are also created by
the monsoon regimes, especially in Asia.
4
The orientation of mountain ranges with
respect to the prevailing winds.
°
1
The limit imposed on the maximum moisture
content of the atmosphere by air temperature.
This is important in high latitudes and in
winter in continental interiors.
2
The major latitudinal zones of moisture influx
due to atmospheric advection. This in itself is
a reflection of the global wind systems and
their disturbances (i.e., the converging Trade
Wind systems and the cyclonic westerlies, in
particular).
4 Regional variations in the
altitudinal maximum of
precipitation
The increase of mean precipitation with height
on mountain slopes is a widespread characteristic
in mid-latitudes, although actual profiles of
precipitation differ regionally and seasonally. An
