Geography Reference
In-Depth Information
The simplest system is that of coastal mountains with moisture-laden winds ap-
proaching from the ocean. As the air is lifted from sea level, the resulting precipitation
is clearly due to the landforms (Mass 2008). Exceptions may occur where the mountains
are oriented parallel to the prevailing winds and/or where the frontal systems resist lift-
ing. In southern California, for example, precipitation is often heavier in the Los Angeles
coastal lowlands than in the Santa Inez and San Gabriel Mountains, due to the block-
ing of storms. The orographic component of precipitation increases only when the ap-
proaching air mass is unstable. Under stable conditions, the wind will flow around the
mountains (which are oriented east-west), so there is no significant orographic lifting
and the precipitation is related entirely to frontal lifting. The mountains apparently re-
ceive less rainfall than the lowlands under these conditions, because the shallow cloud
development does not allow as much depth for falling precipitation particles to grow by
collision and coalescence with cloud droplets before reaching the elevated land.
The situation becomes more complex in interior high-elevation areas where there is
more than one source region and storms enter the area at various levels in the atmo-
sphere. For instance, in the Wasatch Mountains of Utah, precipitation is highly variable;
the valleys may receive greater amounts than the mountains during any given storm or
season (Sassen and Zhao 1993). The average over a period of years, however, shows
an increase with elevation. The greater precipitation in valleys is apparently associ-
ated with certain synoptic situations, particularly when a “cold low” is observed on the
upper-air charts. These occur as closed lows on the 500-millibar pressure chart (i.e.,
at a height of about 5,500 m [18,000 ft]), and are associated with large-scale upward
(vertical) movement of air, which is not displayed in normal cold- or warm-front precip-
itation (Schultz et al. 2002).
BLOCKING OF STORMS
By hindering the free movement of storm systems, mountains can cause increased pre-
cipitation. Storms often linger for several days or weeks as they slowly move up and
over mountains, producing a steady downpour (Mass 2008). This is best displayed in the
middle latitudes with high barrier mountains. Winter storms linger with amazing per-
sistence in the Cascades and the Gulf of Alaska before they pass across the mountains,
or are replaced by another storm. Storms of similar character in the Great Plains travel
much more rapidly, since there are no restrictions to their movement. In northern Italy,
between the Alps and the Apennines, heavy and persistent rains are associated with the
“lee depressions” caused by the interception of polar air by the Alps (Hoggarth et al.
2006).
THE TRIGGERING EFFECT
Another important variable influencing the amount of precipitation is the stability of the
air, that is, its resistance to vertical displacement. This is controlled primarily by tem-
perature. When there is a low environmental lapse rate, that is, less than 1.4°C per 300
m (2.5°F per 1,000 ft), as there often is at night in mountains, the air is stable. Dur-
ing the day, when the sun warms the slopes and the surface air is heated, the environ-
mental lapse rate increases and the air will tend to rise, frequently producing afternoon
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