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01. Somewhat related to Elevation is the climatic control
Mountain Barriers. You will remember that "weather" does not
extend very far above the Earth's surface. The Boundary Layer,
where most of our weather is found, only extends upward from the
surface some 4000 to 5000 feet.
- For this reason, one can easily see that a range of mountains,
with a local relief of only 3000 or 4000 feet, will have a huge
effect on the weather. Such mountains can block or channel various
air masses or, in the event that air masses do in fact cross the
mountains, it is very likely that the weather such an air mass
would create would be very different in terms of its temperature
and humidity characteristics.
- Whether a mountain barrier results in favorable or unfavorable
weather/climate conditions will in large measure depend upon three
factors: (1) where are you and what is it that you need (more
moisture, less moisture, warmer/colder temperatures and the like);
(2) where are the mountains; and (3) in which prevailing wind belt
are you located? For instance, if you are in Nevada (and in the
belt of the Prevailing Westerlies) and need rain, you are only
likely to receive the most minimal amounts because the mountains
are to your west between you and the Pacific Ocean (the source of
moisture). On the other hand, the presence of mountains is not
always bad. The east-west trending Alps of Europe provide
protection to the Mediterranean region by blocking most of the
cold air masses of northern Europe from entering the region.
- Again, where are you and what is it you need (or don't need),
where are the mountains and what prevailing wind belt are you
02. In addition to their barrier nature, mountains are associated
with some unique winds. We earlier discussed the land-sea breezes
so common along most coasts. Mountains also have a similar wind.
During the day as the Sun heats the mountain surface; low pressure
tends to develop and air from the relatively higher pressures of
the valley floors is often drawn upslope to create a valley wind
(again, winds are generally named for the direction from which
03. Mountain and Valley
Breezes. If you have spent much time in mountains, you
have probably seen these winds at work. While surrounding mountain
peaks may be clearly visible from the valley in the early morning
hours, frequently clouds begin to form on the lower slopes by
mid-morning and one will often encounter heavy cloud cover, rain
or even snow in the higher elevations by early afternoon. Valley
breezes are most common in the summer season when solar radiation
is most intense. Again, what we have here is air ascending into
the atmosphere and cooling.
04. If air is drawn up the mountain during the day in response to
heating of the slopes, then we will most likely find the reverse
at night when the "thin" air permits the heat to rapidly leave the
surface. At such times we often find plunging temperatures and air
beginning to move down the slopes into the valleys under the force
of gravity. While such winds can be found the year round in upland
locations, they are most common in winter when cold air is most
05. Chinook Winds. Perhaps a
better-known mountain wind would be the Chinook (eastern slopes of
the Rocky Mountains) or Foehn winds (associated with the Alps of
Europe). These winds have their origin in air masses found within
the mountain ranges themselves. As is so often the case, these
winds can be drawn out of the mountains and down the slopes onto
adjacent plains by passing frontal systems (low pressure systems).
As these wind move down the mountain slopes, they are compressed
and as a result temperatures rise. Temperature increases of 30 to
40 degrees F are not uncommon in Chinooks (the word Chinook
actually comes from the Indian and roughly translates "snow
eater"). The results created by Chinooks can be dramatic.
06. The graphic below depicts a couple of the more dramatic
examples of a Chinook at work. Compare this with the temperature
drops we typically experience with the passage of a strong winter
cold front. But of course here we are experiencing a temperature
rise instead of a temperature decrease.
- And consider the consequences of such a dramatic temperature
rise in a mountainous area. In Houston, a 30 minute thunderstorm
that leaves in its wake an inch of rain will paralyze many parts
of the city. The effect of rapidly rising temperatures on a snow
pack are much more dramatic. As a general proposition, 10 inches
of snow contains approximately one inch of water. Of course this
is only a general rule of thumb as we have wet snow, dry snow,
snow that has been on the ground a while and as a result has been
somewhat compacted. But if we can use this for the purposes of
example, consider the results when a portion of the snow pack is
melted quickly. Unlike Houston, where the land is relatively flat
and the water spread out over a large area, in mountains snow
melts quickly and finds its way into area creeks and streams that
rise quickly -- often with devastating results. But like most
things in weather, Chinooks aren't all bad. Such warming winds are
important in that they often evaporate snow cover off the high
pastures permitting livestock and wild animals to graze on the
tender vegetation below.
07. Santa Ana Winds. A variant
of the Chinook is the well-known Santa Ana -- a hot, dry east wind
associated with southern California. Like the Chinook, the source
of this wind is the air masses found in the high elevations of the
intermountain west -- the area between the Rocky Mountains to the
east and the Sierra Nevada and Cascade Mountains to the west. As
with the Chinook, these air masses are drawn out of these upland
areas by pressure differences found between the source region
(high pressure) and the lower-lying coast (low pressure).
08. The graphic on the left depicts conditions favorable for the
development of a Santa Ana wind. Note not only the pressure
gradient, but the extreme difference in temperatures found in the
Los Angeles area compared to surrounding locations. These hot, dry
winds (often having relative humidities of less than 20 percent)
further dry out vegetation in its path and acts as an added
encouragement to fires. And as most of us have noted over the
years, once fires begin in the canyons around Los Angeles, these
winds often act to further fan the already difficult to control
- In fact, we occasionally find that area fires are so
widespread and intense that the fires themselves have the effect
of creating their own "low pressure" areas thus drawing the Santa
Ana winds to themselves thereby increasing the difficulty of
getting the fires under control. The graphic on the left depicts
an instance where fires, fanned by a Santa Ana, have created their
own low pressure system. Note the wind carrying the smoke westward
-- against the prevailing westerly winds.
09. Katabatic Winds.
Antarctica and Greenland, both ice-covered, flat-surface plateaus,
are well known for the presence of gravity or katabatic winds.
These high velocity, often destructive, winds have their origins
in the very cold temperatures found over the ice of these two high
latitude landmasses. Similar winds are found in Europe and carry
such names as the Mistral (Rhone River valley in France) and the
Bora (Adriatic Coast).
10. As the name implies, these very cold winds are carried by
gravity (they are said to "drain") to the surrounding lower
You have now completed Unit 6: Mountain
Barriers. You might wish to check your knowledge of the
material presented in this section by working through the Multiple
Choice, and True-False Quiz Questions as well as the essay-style
Review Questions available through The
Course dropdown located in the header of this page.
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