When air flows downhill from a high eleva-tion, its temperature is raised by adiabatic compression. Foehn winds are katabatic winds caused by adiabatic heating of air as it descends on the lee sides of mountains.
Foehn winds occur frequently in our western mountain states and in Europe in the late fall and winter. In Montana and Wyoming, the chinook is a well-known phenomenon; in southern Califor-nia, the Santa Ana is known particularly for its high-speed winds that easily exceed 50 knots. For the purpose of illustrating a Foehn wind, the Santa Ana is used.
The condition producing the Foehn wind is a high-pressure area with a strong pressure gradient situated near Salt Lake City, Utah. This gradient directs the wind flow into a valley leading to the town of Santa Ana near the coast of California. As the wind enters the valley, its flow is sharply restricted by the funneling effect of the mountain sides. This restriction causes the wind speed to in-crease, bringing about a drop in pressure in and near the valley. This pressure drop in and near a valley is caused by the Bernoulli effect. Generally speaking, when the Santa Ana blows through the Santa Ana Canyon, a similar wind simultaneously affects the entire southern California area. Thus, when meteorological con-ditions are favorable, this dry northeast wind blows through the many passes and canyons, over all the mountainous area, including the highest peaks, and quite often at exposed places along the entire coast from Santa Barbara to San Diego. Therefore, the term Santa Ana refers to the general condition of a dry northeast wind over southern California.
In the Rocky Mountain states, the onset of Foehn winds have accounted for temperature rises of 50°F or more in only a few minutes. In southern California, the temperature, though less dramatically, also rises rapidly and is accom-panied by a rapid decrease in humidity (to 20 percent or less) and a strong shift and increase in wind speeds.
Although these winds may on occasion reach destructive velocities, one beneficial aspect is that these winds quickly disperse the severe air pollutants that plague the Los Angeles Basin.
PRACTICAL TRAINING EXERCISE
Now that you have some background infor-mationabout atmospheric circulation, it is time for some practical application of what you have learned so far.
This practical exercise involves you and eitheryour supervisor, the duty forecaster, or your chief. It is designed to help answer questions that you may have and give you a better understanding of atmospheric circulation.
Talk to your supervisor, duty forecaster, orchief and inform him of the requirements of the training exercise. Arrange a suitable time when both of you have about an hour of uninterrupted time to complete the exercise. During this prac-tical training session it is recommended that the following be discussed and performed:
1. Select a current surface weather analysisand identify all the high and low pressure centers and their central pressures.
2. Discuss the circulations associated withthese centers. Discuss wind flow patterns which are not immediately evident as being associated with a high or low center. Normally these areas are near the edges of the chart.
3. Discuss the relationship between isobarspacing, friction caused by terrain differences, wind speed, and direction. Pay special attention to the differences in wind speed and cross isobar flow due to equal isobar spacing at various latitudes and over various terrains and ocean regions.
4. Identify cold core, warm core, anddynamic highs and lows. Have your supervisor, forecaster, or chief show you the vertical struc-ture of these systems on upper air charts.
5. Discuss any tertiary circulations that affectthe weather at your ship or station.
This practical training exercise should be in-formal.Ask questions when in doubt or curious, and try to relate atmospheric circulation to your previous experience as an observer. The more you put into this and the forthcoming exercises, the more you will ultimately learn and understand.
Aerographer’s Mate 1 and C,NAVEDTRA 10362-B, Naval Education and Training Pro-gram Development Center, Pensacola, FL, 1974.
Aerographer’s Mate 3 and 2,NAVEDTRA 10363-E1, Naval Education and Training Pro-gram Development Center, Pensacola, FL, 1976.
Byers, Horace Robert, General Meteorology, Fourth Edition, NAVAIR 50-1B-515, McGraw-Hill Book Company, NY, 1974.
Elementary Meteorology, NAVAIR 50-110R-7, McGraw-Hill Book Company, Inc., NY, 1942.
Forecasting For The Mid-Latitudes,NAVEDTRA 40502, Naval Education and Training Support Center, Pacific, 1978.
Glossary of Meteorology,American Meteoro-logical Society, Boston, MA, 1959.
Haurwitz, Bernhard and Austin, James, M.,Climatology, NAVAIR 50-1B-529, McGraw-Hill Book Company, Inc., NY, 1944.
Trewartha, Glenn T. and Horn, Lyle H., An In-troduction To Climate, McGraw-Hill Book Company, NY, 1980.
The Use of the Skew T, Log P Diagram In Analysis and Forecasting, Department of the Air force, 1979.
Willett, Hurd C., Descriptive Meteorology, NAVAIR 50-1B-502, Academic Press, Inc., Publishers, NY, 1952.