Environmental Engineering Reference
In-Depth Information
Figure 4.7
Destabilization of the lower atmosphere through
uplift of a layer of air 100 hPa in thickness. Between 1000
hPa and 900 hPa the air is initially stable. It is moist near the
base and drier aloft. As a result of uplift, the new
environmental lapse rate indicates instability.
we can sometimes see these dense clouds enveloping the mountains (Plate 4.7).
Orographic rain is also produced in another way, due to changes in the stability of the
air as it rises. If the air is very moist near the ground surface but much drier above, as it
rises the rates of cooling between the top and bottom of the layer will be different (Figure
4.7). The upper part will cool more quickly and so become colder, leading to less stable
air. The cloud development associated with instability will increase and rain may fall
over the mountains. This situation is known as convective or potential instability.
Hills as well as mountains act as favourable areas for convectional showers. The
slopes facing the sun will be warmed more rapidly than flatter areas, because the slopes
act as thermal sources. The resulting cloud may produce rainfall which is restricted to the
upland area.
The orographic effect is most pronounced when it is already raining upwind of the
hills or mountains. Where air is rising - associated with a depression, for example - the
rate of uplift is increased by the extra ascent forced by the hills. This leads to a greater
rate of condensation on the windward side, larger drops of rain being formed, and so a
higher rainfall at the surface. Coupled with the slowing down of the rain belt as it passes,
owing to increased friction, the net effect is considerably greater rainfall (Figure 4.8).
On the leeward side of the hills,
subsidence
or descending air begins to dominate, so
that the cloud sheet thins or even dissipates and rainfall declines. As the air descends it
gets warmer, owing to compression, to give us the rain-shadow effect on the leeward
slope of the mountains. Here rainfall is far less than on the upslope side and sunshine
amounts and daytime temperatures are normally higher.
As much of the precipitation in mountains is due to an intensification of existing rain,
it would be wrong to think of orographic precipitation as a truly separate category. It can
occur as drizzle or by convective instability, but much more frequently it will depend
upon convergent or convection processes already operating. Even these two types can
occur together in depressions, so perhaps we should identify convectional, convergent
