Geoscience Reference
In-Depth Information
facing away from the predominant weather patterns
will receive less rainfall than their opposites.
Altitude
It has already been explained that temperature is a
critical factor in controlling the amount of water
vapour that can be held by air. The cooler the air is,
the less water vapour can be held. As temperature
decreases with altitude it is reasonable to assume
that as an air parcel gains altitude it is more likely
to release the water vapour and cause higher rainfall.
This is exactly what does happen and there is a
strong correlation between altitude and rainfall:
so-called
orographic precipitation
.
Slope
The influence of slope is only relevant at a very small
scale. Unfortunately the measurement of rainfall
occurs at a very small scale (i.e. a rain gauge). The
difference between a level rain gauge on a hillslope,
compared to one parallel to the slope, may be sig-
nificant. It is possible to calculate this difference if
it is assumed that rain falls vertically - but of course
rain does not always fall vertically. Consequently the
effect of slope on rainfall measurements is normally
ignored.
Aspect
The influence of aspect is less important than
altitude but it may still play an important part in
the distribution of precipitation throughout a catch-
ment. In the humid mid-latitudes (35° to 65° north
or south of the equator) the predominant source
of rainfall is through cyclonic weather systems
arriving from the west. Slopes within a catchment
that face eastwards will naturally be more sheltered
from the rain than those facing westwards. The same
principle applies everywhere: slopes with aspects
Rain shadow effect
Where there is a large and high land mass it is
common to find the rainfall considerably higher on
one side than the other. This is through a com-
bination of altitude, slope, aspect and dynamic
weather direction influences and can occur at many
different scales (see Case Study below).
Case study
THE RAIN SHADOW EFFECT
The predominant weather pattern for the South
Island of New Zealand is a series of rain-bearing
depressions sweeping up from the Southern
Ocean, interrupted by drier blocking anticyclones.
The Southern Alps form a major barrier to the
fast-moving depressions and have a huge influence
on the rainfall distribution within the South
Island. Formed as part of tectonic uplift along
the Pacific/Indian plate boundary, the Southern
Alps stretch the full length of the South Island
(approximately 700 km) and at their highest point
are over 3,000 m above mean sea level.
The predominant weather pattern has a westerly
airflow, bringing moist air from the Tasman Sea
onto the South Island. As this air is forced up over
the Southern Alps it cools down and releases much
of its moisture as rain and snow. As the air
descends on the eastern side of the mountains it
warms up and becomes a föhn wind, referred to
locally as a 'nor-wester'. The annual rainfall
patterns for selected stations in the South Island
are shown in Figure 2.2. The rain shadow effect
can be clearly seen with the west coast rainfall
being at least four times that of the east. Table 2.2
