Geoscience Reference
In-Depth Information
2
Heated air rises over
warmer land exposed to sun,
causing lower pressure
1
Heat lost quickly
from mountain slope
Sun's rays
strike mountain
1
3
Cooler air in valley flows
toward lower pressure
2
Cooler air sinks
downward into valley
(a) Daytime valley breeze
(b) Nighttime mountain breeze
Figure 6.29 Local wind flow caused by differences in elevation.
(a) During the daytime, air flows upslope in a valley breeze.
(b) At night, the flow reverses in a mountain breeze.
relatively higher pressure over land and low pressure over
the water. When this adjustment occurs, the air reverses di-
rection and moves from land toward sea in a land breeze
(Figure 6.28b).
Greenland and Antarctica. From the Greek katabatikos , which
means “descending,” this process is most common during the
winter months, when extremely cold air accumulates over
higher-altitude regions covered by ice sheets. This extremely
cold, dense air then flows downhill under the force of gravity.
These winds sometimes flow at great speeds that can be quite
destructive, especially where the air is funneled down a narrow
valley. Although the air associated with katabatic winds typi-
cally warms when descending, it usually remains colder than
the air that it replaced.
The last kind of topographically related wind process is
the Chinook winds (also called foehn winds ). These winds
also form in mountainous regions, but occur only when a steep
pressure gradient develops along the range (Figure 6.30). For
this gradient to develop, a high-pressure system must be on
the side of the range that faces the direction of oncoming
winds; this side is called the windward side . On the other side
of the range, or leeward side , a low-pressure system must be
in place. When this combination of systems occurs, air flows
along the pressure gradient—that is, from the windward to
leeward side of the mountain range. The resulting airflow
moves downslope on the leeward side, resulting in dry air that
warms as it descends.
Topographic Winds
In addition to the winds that develop along coastlines, sev-
eral kinds of winds form due to variations in topography
(Figure  6.29). These winds generally form because large
temperature differences can occur with elevation in hilly or
mountainous landscapes. For example, a valley breeze is
a wind system that develops when mountain slopes heat up
due to re-radiation and conduction over the course of the day.
When this occurs, a zone of relatively low pressure devel-
ops on the mountain slopes, whereas high pressure is found
in the lowlands below. As a result, air can flow upslope to
fill the pressure void created by the upward movement of air
over the heated slopes. Like the coastline breezes, the situa-
tion in mountainous regions reverses at night, resulting in a
downslope mountain breeze .
A more extreme example of mountain breezes is called ka-
tabatic winds , which form in particularly cold places such as
Land breeze Nighttime circulatory system along coasts
where winds from a zone of high pressure over land flow to a
zone of relatively low pressure over water.
Katabatic winds Downslope airflow that evolves when pools
of cold air develop over ice caps and subsequently descend into
valleys.
Chinook winds Downslope airflow that results when a zone
of high air pressure exists on one side of a mountain range and
a zone of low pressure exists on the other.
Valley breeze Upslope airflow that develops when mountain
slopes heat up due to re-radiation and conduction over the
course of the day.
The side of a mountain range that faces on-
Windward side
Mountain breeze Downslope airflow that develops when
mountain slopes cool off at night and relatively low pressure
exists in valleys.
coming winds.
Leeward side The side of a mountain range that faces away
from prevailing winds.
 
Search WWH ::




Custom Search