Environmental Engineering Reference
In-Depth Information
only 3 cm before evaporating; drops of 100 µm and 1 mm would fall 150 m and 40 km
respectively. Despite the relatively small size of the raindrops, the areas affected by rising
air are vast. For a particular rain belt, it may take several hours of steady rain before the
system has passed, giving a total fall of perhaps between 5 mm and 10 mm. An example
of rainfall from a cyclone is shown in Figure 7.9. In the deep and widespread clouds
associated with low-pressure systems, it is quite common for ice-crystal clouds at higher
levels to act as a source of supply to the mixed clouds of ice and water droplets at lower
levels - a process known as
seeding
. The addition of extra ice crystals speeds up the
precipitation process and leads to more intense rainfall. Convectional systems may be
embedded within the cyclonic circulation to produce more complex patterns of surface
precipitation.
OROGRAPHIC PRECIPITATION
Almost all mountain areas are wetter than the surrounding lowlands. To take two
examples, Hokitika on the west coast of New Zealand receives an average of 2950 mm
per year. At Arthur's Pass, 740 m higher in the New Zealand Alps, the annual average
has risen to 3980 mm, compared with less than 670 mm for Christchurch, on the more
sheltered lowlands to the east. Even the Ahaggar and Tibesti mountains in the centre of
the Sahara receive more rain than do the surrounding lowlands - Asekrem, at 2700 m,
has an annual average of about 125 mm, compared with only 13 mm at Silet, 720 m
above sea level. Why should this be so?
Where air meets an extensive barrier it is forced to rise. Rising, as we know, leads to
cooling of the air, and cooling encourages condensation. On the mountain slopes and
above the mountain summits the clouds start to pile up, reflecting the forced ascent of air.
Often they reach thicknesses sufficient to give drizzle and rain. From a distance
Plate 4.7
Orographic cloud near Glencoe, Scotland. The
thickest areas of cloud are where uplift is greatest.
Photo: Peter Smithson.
