Chinooks/Foehns

The chinook is a warm, dry, and sometimes gusty downslope wind that blows leeward of the Rocky Mountains. The term chinook is an aboriginal word meaning “snow eater,” because of the rapid melt of snow that almost invariably accompanies its occurrence in winter. It is a type of wind that occurs in many parts of the world where large orographic barriers are oriented more or less perpendicular to the prevailing direction of atmospheric circulation. Alternative names used to identify this phenomenon include the foehn in Europe, the ibe in Central Asia, the Zonda in Argentina, the Berg in South Africa, and the Nor'wester in New Zealand. The environmental consequences of chinooks include increased wind erosion, reduced air quality, vegetation mortality, and adverse health-related effects.

The onset of chinook conditions is typified by a rapid increase of temperature and wind speed and a corresponding decrease of humidity (Figure 1). Additional distinguishing characteristics include wind flow oriented perpendicular to the orographic barrier, a dry adiabatic lapse rate, and leewave clouds.

Synoptic conditions commonly associated with chinooks involve a surface ridge of high pressure on the windward side of an orographic barrier and a trough of low pressure on the lee side, which produces a steep pressure gradient and strong cross-barrier flow. Changes in air temperature and humidity during a chinook are generally ascribed to the forced ascent of warm, moist air over the barrier. The rising air cools and expands at the dry adiabatic lapse rate (≅9.8 °C/km [per kilometer]) until the dewpoint is reached, after which the air continues to cool at the saturated adiabatic lapse rate (≅6 °C/km). Air descends and warms at the dry adiabatic lapse rate on lee slopes, arriving warmer than it was at equivalent elevations on the windward side. In some cases, this change in air temperature can be dramatic. At Pincher Creek, located in southwestern Alberta, Canada, R. W. Turner reported an increase of 25.5 °C in 1 hour.

The explanation for increased wind speed down the lee side is not straightforward and is a topic of ongoing investigation. One of the leading hypotheses involves hydrodynamic theories of fluid [Page 395]mechanics, analogous to the behavior of water flowing over obstacles in river channels. As air ascends an orographic barrier, it accelerates toward the crest, where it transitions to supercritical flow and continues to accelerate down the leeslope. At some distance leeward of the barrier, the flow decelerates and produces a hydraulic jump. The latter typically corresponds to the position of the chinook arch, which is a narrow leewave cloud band oriented parallel to the orographic barrier.

Figure 1 Chinook conditions in Saskatchewan, Canada (50°11 ‘ N, 109°12’ W), on April 1, 2005 Source: Author.