Geoscience Reference
In-Depth Information
In the tropics and subtropics, maximum
precipitation occurs below the higher mountain
summits, from which level it decreases upward
towards the crest. Observations are generally
sparse in the tropics, but numerous records from
Java show that the average elevation of greatest
precipitation is approximately 1200m. Above
about 2000m, the decrease in amounts becomes
quite marked. Similar features are reported from
Hawaii and, at a rather higher elevation, on
mountains in East Africa (see Chapter 11H.2).
Figure 4.19A
shows that, despite the wide range of
records for individual stations, this effect is clearly
apparent along the Pacific flank of the Guatemalan
highlands. Further north along the coast, the
occurrence of a precipitation maximum below
the mountain crest is observed in the Sierra
Nevada, despite some complication introduced
by the shielding effect of the Coast Ranges
(
Figure 4.18B
), but in the Olympic Mountains of
Washington precipitation increases right up to
the summits. Precipitation gauges on mountain
crests may underestimate the actual precipitation
due to the effect of eddies, and this is particularly
true where much of the precipitation falls in the
form of snow, which is very susceptible to blowing
by the wind.
One explanation of the orographic difference
between tropical and temperate rainfall is based
on the concentration of moisture in a fairly
shallow layer of air near the surface in the tropics
(see Chapter 11). Much of the orographic
precipitation seems to derive from warm clouds
(particularly cumulus congestus), composed of
water droplets, which commonly have an upper
limit at about 3000m. It is probable that the height
of the maximum precipitation zone is close to the
mean cloud base, since the maximum size and
number of falling drops will occur at that level.
Thus, stations located above the level of mean
cloud base will receive only a proportion of the
orographic increment. In temperate latitudes,
much of the precipitation, especially in winter,
falls from stratiform cloud, which commonly
extends through a considerable depth of the
troposphere. In this case, there tends to be a
smaller fraction of the total cloud depth below the
station level. These differences according to cloud
type and depth are apparent even on a day-to-day
basis in mid-latitudes. Seasonal variations in the
Mean annaul precipitation (inches)
0
100
200 0
10
20
30
40
50
60
100 110 120 130 140 1
50
2500
8000
A
Guatemalan Highlands
14-15°N
B
Sierra Nevada
California
38-39°N
C
Olympic Range
California
38-39°N
?
7000
2000
6000
5000
1500
4000
1000
3000
2000
500
1000
0
0
0
1000
2000
3000
4000
5000
0
500
1000
1500
2500
3000
3500
Mean annaul precipitation (mm)
Figure 4.18
Generalized curves showing the relationship between elevation and mean annual precipitation for
west-facing mountain slopes in Central and North America. The dots give some indication of the wide scatter of
individual precipitation readings.
Source: Adapted from Hastenrath (1967) and Armstrong and Stidd (1967).
