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The orographic effect is also known as the Foehn effect and orographic precipitation or relief rainfall, although these two terms only comprise a part of the process. The orographic effect is the series of changes at a synoptic scale in temperature and humidity as a result of the flow of air masses over mountain ranges or oceanic islands. It may engender orographic precipitation when moisture is captured, cloudiness, and differences in air temperature and humidity between the windward and leeward sides of the range. A large local gradient, or spatial variation, of precipitation and temperature results.
When a moving mass of stratified air containing abundant moisture hits a topographic barrier, the air is forced up. In the upward movement, the air cools adiabatically and becomes increasingly saturated. When temperature falls below the dew or condensation point, orographic clouds develop, water vapor condenses, and precipitation occurs as rain or snow, so that some moisture is removed. Rainfall increases as air raises the barrier. The air keeps rising until it reaches the same temperature as the surrounding air. In the condensation process latent heat is released to the air and this is warmed.
During the descent down the lee side of the barrier, the drier air mass warms adiabatically by increasing its pressure. Precipitation drops radically and cloud coverage dissipates. The heat gained in condensation adds to compressional heating. This dry air mass increases its temperature-about 6 degrees per kilometer-more quickly than leeward moist air decreases when ascending-about 10 degrees per kilometer-so that it reaches a higher temperature at an equal elevation. The warmer dry area to the lee side conforms to a precipitation or rain shadow, although sometimes-depending on wind speed-precipitation is projected leeward, a phenomenon called spillover.
Different types of vegetation respond to the contrasting environmental conditions; while the windward side of mid-latitudes mountain ranges are covered by forests, the leeward side frequently exhibits arid, stepparian vegetation-the grassland in the lee of the Rocky Mountains-or, in certain cases, deserts. Death Valley in California and the Great Basin both illustrate the latter.
Examples of significant descending winds produced by a steady manifestation of the orographic effect over a short time are commonly designated by local names, such as Foehn in the European Alps, Chinook in the Front Range of the Rocky Mountains, or Zonda in Western Argentina. Eventually, surface winds flow very strongly and reach disruptive speeds that are considered to have some effects on human behavior. The Santa Ana wind in Southern California reveals some differences. The dry air mass originating from the north and east high deserts over the Great Basin, or northernmost areas, advances over the San Gabriel Range, where no cloud formation occurs, and descends to the coast. The lower altitude of the Los Angeles area allows the increase of temperature combined with a lower relative humidity. Chinook winds are originated by oceanic air moving over the Cascade Mountains and Sierra Nevada Range, where moisture is drained; then the air mass descends to the lee side, creating the dry Great Basin, and the hot and dry wind progresses to the Rocky Mountains, where it may melt the snow cover.
Other less common types of orographic effects include the feeder-feeder effect and the Puget Sound Convergence Zone. The feeder-feeder effect starts with precipitation from an upper-level cloud over a lower-level orographic cloud that feeds precipitation of the lower, increasing the total precipitation. In the second case, moist air masses from the Pacific Ocean, instead of rising, are blocked, split into two air streams, and deflected by the Olympic Mountains. The two branches converge over Puget Sound (on the leeward side of the mountains), and are forced to uplift-leading to convection-and causing precipitation in the Puget Sound Convergence Zone. Although an orographic process, katabatic wind or mountain breeze originates from differential cooling of high elevation and valley air masses. As soon as solar radiation ceases, top mountain air cools faster than in lower areas, becomes denser, and flows downslope, creating a local high pressure.
- W. Brinkmann, “What Is a Foehn?” Weather (v.26, 1971);
- Kerry Emanuel, Atmospheric Convection (Oxford University Press, 1994);
- Lawrence Nkemdirim, “Canada’s Chinook Belt,” National Journal of Climatology (v.16, 1996);
- Marilyn N. Raphael, “The Santa Ana Winds of California,” Earth Interactions (v.7, 2003);
- Gerard H. Roe, “Orographic Precipitation,” Annual Review of Earth and Planetary Sciences (v.33, 2005).