Geoscience Reference
In-Depth Information
Figure 4.15
Distribution of rainfall (mm) over Exmoor, southwest England, during a twenty-four-hour period on 15 August 1952
which produced catastrophic local flooding at Lynmouth. The catchment is marked (by dashes). Seventy-five per cent of the rain fell
in just seven hours.
Source
: Dobbie and Wolf (1953).
In view of the complex controls involved, no brief
explanation of these precipitation distributions can be
very satisfactory. Various aspects of selected precip-
itation regimes are examined in Chapters 10 and 11,
after consideration of the fundamental ideas about
atmospheric 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:
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.
4 Regional variations in the altitudinal
maximum of precipitation
1
The limit imposed on the maximum moisture content
of the atmosphere by air temperature. This is impor-
tant in high latitudes and in winter in continental
interiors.
The increase of mean precipitation with height on
mountain slopes is a widespread characteristic in mid-
latitudes, where the vertical increase in wind speed
augments the moisture flux. An increase may be
observed up to at least 3000 to 4000 m in the Rocky
Mountains in Colorado and in the Alps. In western
Britain, with mountains of about 1000 m, the maximum
falls are recorded to leeward of the summits. This
probably reflects the general tendency of air to continue
rising for a while after it has crossed the crestline and the
time lag involved in the precipitation process after con-
densation. Over narrow uplands, the horizontal distance
may allow insufficient time for maximum cloud buildup
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).
3
The distribution of the landmasses. The southern
hemisphere lacks the vast, arid, mid-latitude conti-
nental interiors of the northern hemisphere. The
oceanic expanses of the southern hemisphere allow
the mid-latitude storms to increase the zonal
