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Mountain Barriers 

Reading Assignment
Gillespie, Netoff and Tiller, eWeather & Climate, as applicable (also note the search function on the CD).
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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 in?


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 they come).

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 prevalent.

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 flames.
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 elevations.

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. To return to the top of the page.

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