Foehn Winds

Foehn winds represent a special type of local wind associated with mountain systems, particularly in the Western United States. In most mountainous areas, local winds are observed that blow over the mountain ranges and descend the slopes on the leeward side. If the downflowing wind is warm and dry, it is called a foehn wind. The wind is called a bora or fall wind if the air is originally so cold that even after it is warmed adiabatically in flowing down the mountain slopes it is still colder than the air it is replacing on the leeward side. The bora rarely occurs in North America and is not important in this discussion, because of its cold temperatures and the fact that the ground is often snow-covered when it occurs. We are concerned more with the warmer foehn, which creates a most critical fire-weather situation.

The development of a foehn wind requires a strong high-pressure system on one side of a mountain range and a corresponding Low or trough on the other side. Such pressure patterns are most common to the cool months; therefore, foehn winds are more frequent in the period from September through April than in the summer months.

Two types of foehn winds are common in mountains of the Western U.S.

Foehn winds of the first type result when a deep layer of moist air is forced upward and across a mountain range. As the air ascends the windward side, it is cooled dry-adiabatically until the condensation level is reached. Further lifting produces clouds and precipitation, and cooling at the lesser moist-adiabatic rate. The water vapor that has condensed and fallen out as precipitation is lost to the air mass. Upon descending the leeward slopes, the air mass warms first at the moist-adiabatic rate until its clouds are evaporated. Then it warms at the dry-adiabatic rate and arrives at lower elevations both warmer and drier than it was at corresponding levels on the windward side. In descending to the lowlands on the leeward side of the range, the air arrives as a strong, gusty, desiccating wind.

Moist Pacific air forced across the Sierra Cascade range loses some of its moisture and exhibits mild foehn characteristics on the eastern slopes. Forced across the Rocky Mountain range, the same air loses additional moisture and may produce a well-developed foehn on the eastern slopes in that region. The Plains east of the Rockies are often under the influence of a cold air mass of Canadian origin in the cooler months. If this air mass is then moved eastward by a favorable pressure gradient and replaced by a warm descending foehn, abrupt local temperature rises are experienced.

The second type of foehn is related to a cold, dry, usually stagnated high-pressure air mass restricted by mountain barriers. If a low-pressure center or trough is located on the opposite side of the barrier, the strong pressure gradient will cause air to flow across the mountains. Since the mountains block the flow of surface air, the airflow must come from aloft. The air above the surface high-pressure system is subsiding air and is therefore dry and potentially quite warm. On the leeward side of the mountains, surface air is forced away by the strong pressure gradient, and it is replaced by the air flowing from aloft on the windward side and descending to the lowland on the leeward side. Surface wind speeds of 40 to 60 miles per hour are common in foehn flow of this type, and speeds up to 90 miles per hour have been reported. The wind often lasts for 3 days or more, with gradual weakening after the first day or two. Sometimes, it stops very abruptly.

High-pressure areas composed of cool air masses frequently stagnate in the Great Basin of the Western United States during the fall, winter, and spring months. Depending on its location, and the location of related Lows or troughs, a Great Basin High may create foehn winds which move eastward across the northern and central Rockies, westward across the Oregon and Washington Cascades and the northern and central Sierra Nevada, or southwestward across the Coast Ranges in southern California. A combination of high pressure over the State of Washington and low pressure in the Sacramento Valley causes north winds in northern California. Brief foehn wind periods, lasting 1 or 2 days, may result from migrating Highs passing through the Great Basin.

The course of the foehn may be either on a front many miles wide or a relatively narrow, sharply defined belt cutting through the leeside air, depending on the pressure pattern and on the topography.

A foehn, even though it may be warm, often replaces cooler air on the lee side of the mountains. Counterforces sometimes prevent this, however, and cause the foehn to override the cooler air and thus not be felt at the surface at lower elevations. At other times the foehn may reach the surface only intermittently, or at scattered points, causing short-period fluctuations in local weather.

Two mechanisms come into play. One is a favorable pressure gradient acting on the lee-side air in such a way as to move it away from the mountains so that the warm foehn can replace it.

A second mechanism is the mountain wave phenomenon. The wavelength and wave amplitude depend upon the strength of the flow bearing against the mountains and the stability of the layers in which the wave may be embedded . When these factors are favorable for producing waves which correspond to the shape of the mountain range, the foehn flow will follow the surface and produce strong surface winds on the lee slopes. There is evidence that strong downslope winds of the warm foehn on lee slopes are always caused by mountain waves. The change in wavelength and amplitude can account for the observed periodic surfacing and lifting of foehn flow. Surfacing often develops shortly after dark as cooling stabilizes the air crossing the ridge.

Foehn winds in the Western United states

The Chinook

The Chinook, a foehn wind on the eastern slopes of the Rocky Mountains, often replaces cold continental air in Alberta and the Great Plains. Quick wintertime thawing and rapid snow evaporation are characteristic. If the cold air is held in place by the local pressure and circulation system, the foehn will override it; or if the cold air stays in the bottoms because of its greater density, the Chinook may reach the surface only in the higher spots. Relative humidities dropping to 5 percent or less and temperature changes of 30° F. to 40° F. within a few minutes are common in Chinooks.

East winds

Along the Pacific coast a weak foehn may be kept aloft by cool marine air flowing onshore. On the other hand, a strong, well-developed foehn may cut through all local influences and affect all slope and valley surfaces from the highest crest to the sea. East winds in the Pacific Northwest, for example, sometimes flow only part way down the lee slopes of the Cascades, and then level off above the lowlands and strike only the higher peaks and ridges of the coastal mountains. At other times virtually all areas are affected.

North and Mono winds

North and Mono winds in northern and central California develop as a High moves into the Great Basin. North winds develop if a High passes through Washington and Oregon while a trough is located in the Sacramento Valley. Mono winds occur after the High has reached the Great Basin, providing there is a trough near the coast. Both North and Mono are foehn winds bringing warm, dry air to lower elevations. At times they will affect only the western slopes of the Sierra Nevada, and at other times they push across the coastal mountains and proceed out to sea. This depends upon the location of the low-pressure trough. These winds are most common in late summer and fall.

Santa Ana winds

The Santa Anas of southern California also develop with a High in the Great Basin. The low-pressure trough is located along the southern California coast, and a strong pressure gradient is found across the southern California mountains.

In the coastal mountains, and the valleys, slopes, and basins on the ocean side, the Santa Ana varies widely. It is strongly channeled by the major passes, and, at times, bands of clear air can be seen cutting through a region of limited visibility. The flow coming over the tops of the ranges may remain aloft on the lee side or drop down to the surface, depending upon whether the Santa Ana is "strong" or "weak" and upon its mountain-wave characteristics. If the foehn flow is weak and remains aloft, only the higher elevations in the mountains are affected by the strong, dry winds. Local circulations, such as the sea breeze and slope winds, are predominant at lower elevations, particularly in areas away from the major passes.

Typically in southern California during the Santa Ana season, there is a daytime onshore breeze along the coast and gentle to weak upslope and upcanyon winds in the adjacent mountain areas. With nighttime cooling, these winds reverse in direction to produce downcanyon and offshore winds, usually of lesser magnitude than the daytime breeze. A strong Santa Ana wind wipes out these patterns. It flows over the ridges and down along the surface of leeward slopes and valleys and on to the sea. The strong winds, along with warm temperatures and humidities sometimes lower than 5 percent, produce very serious fire weather in a region of flashy fuels. The strong flow crossing the mountains creates mechanical turbulence, and many eddies of various sizes are produced by topographic features.

A strong Santa Ana, sweeping out the air ahead of it, often shows little or no difference in day and night behavior in its initial stages. But, after its initial surge, the Santa Ana begins to show a diurnal behavior. During the daytime, a light sea breeze may be observed along the coast and light upvalley winds in the coastal valleys. The Santa Ana flow is held aloft, and the mountain waves are not of proper dimensions to reach the surface. The air in the sea breeze may be returning Santa Ana air, which has had only a short trajectory over the water and is not as moist as marine air. After sunset, the surface winds reverse and become offshore and downslope. Increasing air stability may allow the shape of the mountain waves to change so that the lower portions of waves can strike the surface and produce very strong winds down the lee slopes. As the Santa Ana continues to weaken, the local circulations become relatively stronger and finally the normal daily cycle is resumed.