Geoscience Reference
In-Depth Information
Even quite low hills such as the Chilterns and
South Downs cause a rise in rainfall, receiving
about 120-130mm per year more than the
surrounding lowlands. In South Wales, mean
annual precipitation increases from 1200mm at the
coast to 2500mm on the 500m high Glamorgan
Hills, 20km inland. Studies using radar and a dense
network of rain gauges indicate that orographic
intensification is pronounced during strong low-
level southwesterly airflow in frontal situations.
Most of the enhancement of precipitation rate
occurs in the lowest 1500m.
Figure 10.12
shows the
mean enhancement according to wind direction
over England and Wales, averaged for several days
with fairly constant wind velocities of about 20m
s
-1
and nearly saturated low-level flow, attributable
to a single frontal system on each day. Differences
are apparent in Wales and southern England
between winds from the SSW and from the WSW,
whereas for SSE airflow the mountains of North
WSW
SSW
0
100
0
100
km
km
SSE
0
100
Mean enhancement
in hr
-1
km
mm hr
-1
0.16
0.08
0.04
4
2
1
0.5
+
+
0.02
0.004
0.004
0.1
0.1
0.5
0
0
-
-
0.02
Figure 10.12
Mean orographic enhancement of precipitation over England and Wales, averaged for
several days of fairly constant wind direction of about 20m s
-1
and nearly saturated low-level airflow.
Source: After Browning and Hill (1981). By permission of the Royal Meteorological Society.
