Back to basics: Fohn and chinook winds. Emma L. Quaile University of Reading. Weather Vol. 56 April 200 1

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1 Weather Vol. 56 April Back to basics: Fohn and chinook winds Emma L. Quaile University of Reading Fohn and chinook winds flow down slopes such as mountains and have similar characteristics, but occur in different geographical locations. Both are warm and dry but, whereas the fohn wind occurs in the European Alps where the name originated, the chinook occurs on the eastern flanks of the Rocky Mountains of North America. There are many other winds similar to the fahn and chinook but known by a variety of local names. These include the koschava and ljuka of Yugoslavia, the afganet and ibe of central Asia, the germich of the south-western Caspian Sea, the Santa ha of southern California, and the zonda and puelche of the Andes (Brinkmann 197 1). Fohn and chinook winds appear as unusually warm, dry and strong winds blowing down the lee slopes of their respective mountains, and continue for as much as lookm in the lee of the range. Temperatures rise, by amounts ranging from a few degrees Celsius to over 20degC in some Chinooks. Relative humidities usually fall below 50% and have gone below 10% on occasion (McIlveen 1997). For example, the influence of fdhn winds in the Polish Carpathians is an average increase of about 1 degc on the mean annual temperature and a decrease in relative humidity of about 10% (Ustrnul 1992). Winds can often exceed gale force and local topography can magnify their strength. Fohn winds also have a considerable influence over the South Island of New Zealand. They create highly favourable meteorological conditions for wind erosion in the Southern Alps and over the adjacent lowlands. The thermal and dynamical characteristics of fahn windstorms regularly initiate duststorms by drying exposed surface sediments. These storms have been found to occur when mean near-surface wind speeds exceed 7-8 m s- over a 20-minute period in the absence of precipitation. Wind speed maxima of m s- have been observed during severe fohn windstorms (McGowan 1997). The chinook has the same characteristics as the fahn and continually attracts attention because of its occasional great violence and gustiness. Riehl (1 97 1) reported an interesting case in the Rocky Mountains when surface pressure fell abruptly by 3.5 mbar at the onset of the extreme winds, then rose almost 7 mbar in 3 hours. Slope winds differ essentially in their structure depending on whether the slope surface is heated or cooled, due to different stability conditions in the boundary layer. Fohn winds Characteristics, causes and definition A fohn is a warm, dry wind that descends in the lee of a mountain range. Although the name is applied to any warm downslope wind, fohn is the regional name for this type of wind in the European Alps. More specifically there is the south fohn north of the Alps, and the north fohn south of the Alps. The terms south fohn and north fohn are used when the synoptic winds cross the mountains from the south or from the north, respectively (Hoinka 1985b). The main characteristics of these descending winds are considered to be high temperatures accompanied by low relative humidities. Both of these variables can change quite rapidly at the beginning and end of a spell of fahn winds. Winds of this type are also noted for their violence and gustiness with perhaps the most violent ones being experienced on the eastern slopes of the Rocky Mountains at Boulder, Colorado. Maximum wind speeds have reached in excess of 56m s-. Typical changes in speed have been recorded, from less than 5 m s-l to over 45 m s-l in just a few seconds (Julian and Julian 1969). General wellknown characteristics of airflow over mountains include interesting cloud formations such as the fohn wall of the Alps or the tablecloth of Table Mountain to name but two. However, temperature, humidity, wind, and clouds are not always conclusive evidence of the occurrence of fohn winds. 141

2 ~~ Weather Vol. 56 April What is the cause of such winds? In 1866 Hann, an Austrian meteorologist, showed that moisture is condensed and removed from air rising and cooling on windward slopes. The latent heat liberated during the process causes the temperature of this air to be higher in the lee of the mountain range than on its windward side. He later added, in 1885, that precipitation does not always have to occur on the windward side since adiabatic compression of the descending air will result in warmth and dryness (Brinkmann 1971). It was later concluded that there is no essential difference between the fohn and air descending in an anticyclone since both are characterised by dynamic warming. Even with these characteristics it is difficult to arrive at a generally acceptable definition of a fohn wind. Variations in the importance of surface characteristics depend on location and season. Wind speed has sometimes been ignored on the grounds that the wind may be of any strength (Brinkmann 197 1). Fohn events have a considerable influence on weather and climate in areas close to the Alps with there being approximately 50 days a year when these strong orographic winds occur in the northern lee. Hoinka (1985a) presented a detailed analysis of the large-scale, mesoscale and local features of a south fohn event in the Alps on 8 November He found a blocking of cold air in northern Italy on the nearsurface upstream side and peak gusts were observed on the lee side of the Alps. He also observed light to moderate turbulence immediately to the lee in a low-level turbulence zone over the region of strong gusty surface winds. Fohn clearance The violence of fohn winds decreases rapidly with increasing lee-side distance from the barrier. At about 50 km the fohn is noticeable only by increased temperature and decreased relative humidity. Temperature rise can even be observed 100 km away from the barrier. It is this type of weather, where there are temperature and humidity characteristics similar to a fohn but no winds, which is termed fohn clearance. Hoinka (1985b) reported on fohn clearance events in the Alps. He found that in most cases gusts were reduced in magnitude below 60% of their maximum, at a distance of 70km from the barrier. No gusts were experienced beyond lookm. Mostly, the south fohn does not extend very far into the flat terrain in southern Bavaria. Some fohn clearance can be clearly identified from satellite observations by broad gaps in cloud cover in the lee of the Alps, and this is also true for the Rocky Mountains. Fohn wind events in New Zealand Lake Tekapo is an area commonly used for the study of fohn winds in New Zealand. It is situated in the immediate lee of the Southern Alps (Fig. l), and classic fohn conditions such as high ambient air temperatures, low relative humidities and gusty winds can be monitored there. In the South Island of New Zealand, the local fohn is referred to as the nor wester due to its usual direction of origin being the northwest. Unlike the European fohn, only limited studies of the nor wester have been conducted. The Southern Alps act as a barrier to the mid-latitude westerlies with a mean height of between 2500 and 3000m above sea-level. Onshore west to north-westerly airflow is forced to ascend the Southern Alps and consequently can produce extensive orographic rain along the western slopes. Thus in some regions of the South Island, west coast annual precipitation may regularly exceed 10 m. In contrast to this, the annual precipitation in the semi-arid environment of the lee of the alpine divide is often less than 600 mm. McGowan and Sturman (1 996) presented two case-studies which examined the development of fohn conditions over southern New Zealand, in the Lake Tekapo catchment, during two different synoptic situations. In both cases observations from automatic weather stations were made in the immediate lee of the alpine divide. They found that the onset of fohn conditions occurs first at the start of large alpine river valleys and may rapidly proceed to down-valley sites only a few kilometres away. Here, local circulations such as the lake-breeze may dominate. As the flow intensifies ahead of an approaching frontal system, displacement of the fohn wall usually occurs. Extremely high wind speeds were 142

3 Weather Vol. 56 April 2001 These downslope breezes develop when relatively mild air aloft is compressed adiabatically as it descends in the leeward slopes of mountains. The chinook does not flow downslope under the influence of gravity due to the fact that the air is relatively warm, which is also the case for fohn winds. Typical development is when strong winds force a layer of stable air in the lower troposphere to ascend the windward slopes of a mountain range. The air's stability when it reaches the leeward slopes causes the air to descend to its original altitude and further descent is caused by the larger-scale circulation. Figure 2 shows an example of how chinook winds are drawn down leeward slopes of the Rocky Mountain Front Range by strong west winds associated with cyclones and anti- Fig. 1 Location map showing a common area of study cyclones located east of the mountains. offihn winds (McGowan 1997) Compressional heating is the main source of warmth for a chinook as the descending air is compressed and warmed at the adiabatic lapse rate (10 degc km- '). Sometimes, surface air temperature changes can be rather dramatic at Chinook winds the onset of a chinook. Temperatures can often The chinook climb abruptly by tens of degrees in response to this compressional warming. For example, on 6 January 1966, at Pincher Creek, Alberta, a chinook caused a temperature rise of 21 degc in only 4 minutes. Even more dramatic was the temperature surge recorded at Spearfish, South Dakota, on 22 January 1943, which was recorded within the Godley River Valley, sometimes in excess of 50 to 60 m s- '. Chinook winds are fundamentally the same as fdhn winds in terms of their characteristics, but 'chinook' is the name given to the warm, dry, descending winds in the Rocky Mountains. The region of the chinook is rather narrow (only several hundred kilometres wide) and extends from north-eastern New Mexico into Canada. 27degC in only 2 minutes Morgan 1994). (Moran and Fig. 2 A diagram showing that chinook winds develop when the circulation about an anticyclone or cyclone situated well w the ke of a mountain range pulk air down the leeward slopes ofthe range (Moran and Morgan 1994) 143

4 Weather Vol. 56 Large temperature rises are not the only dominant characteristics of the chinook. Winds can reach destructive velocities. At Boulder, Colorado, violent downslope winds can sometimes gust to 160 km h or higher. Buildings are unroofed, electricity poles fall, and, on average, the community experiences US $1 million worth of property damage each year. A chinook may be enhanced when clouds and precipitation occur on the mountain s windward side (Fig. 3). As cloud forms on the windward side, the release of latent heat supplements the compressional heating on the leeward side. Thus the descending air at the base of the mountain on the leeward side is warmer than it was before it started its upward journey on the windward side. Since much of the moisture was removed as precipitation on the windward side, the air is also drier. The chinook has many similarities to the fohn but one important difference is that the fohn usually occurs as a result of the impact of orography on a pre-frontal atmosphere, which is not so often the case with the chinook. This suggests that there could and should be differences in the weather between the two phenomena (Moran and Morgan 1994). Snow-eating effects According to native American Indian language, the word chinook actually means snow eater. The wind has a catastrophic effect on snow- April cover because of the compressional warming that takes place and the dramatic drop in relative humidity. Due to the chinook s warmth and dryness, snow-cover melts and vaporises rapidly. For example, it is not unusual for half a metre of snow to disappear in this way in just a few hours. It follows that there are several important economic aspects to the chinook. Not only does its warmth bring relief from the winter cold, but also these snow-eating effects uncover prairie grass that livestock can graze on in the open range. Also, railway tracks are kept clear of snow so that trains can continue to run. On the other hand, the dramatic drying effect can create an extreme fire hazard and also a buildup of static electricity (Ahrens 1994). Summary Fohn winds occur in the European Alps and tend to be associated with certain synoptic conditions, such as a ridge of high pressure at the surface on the windward side and a trough in the lee. A steep pressure gradient exists across the mountains accompanied by strong acrossmountain flow at upper levels. However, it seems to be difficult to obtain a generally acceptable definition of these winds because of the variations in the importance of their surface characteristics (Brinkmann 197 1). In some definitions the wind may be of any strength, while other definitions require the fohn to be a I l l 144 Fig. 3 A diagram co show the conditions char may enhance a chinook (Ahrens 1994)

5 Weather Vol. 56 April strong wind. As discussed earlier, the main characteristics are high temperatures accompanied by low relative humidities which can change quite rapidly. Brinkmann (1971) concluded that a unique criterion for defining a fohn has not yet been found. The cause appears to be moisture condensing and being removed from the rising air and then warming by adiabatic compression on the leeward slopes. In New Zealand, the fohn is known as the nor wester and Lake Tekapo is a common study area for this event. Extremely high wind speeds can be observed here within the Godley River Valley, i.e. 50 to 60 m s-i. Wind erosion consequently results. Topographic channelling of the airstream down the Godley River Valley enhances wind speeds by compressing the near-surface streamlines (McGowan and Sturman 1997). Chinook winds are basically the same as fohns but occur in the Rocky Mountains. They are drawn down leeward slopes by the largescale circulation and are warm and dry. Temperature changes can be dramatic and winds can reach destructive speeds. Snow-cover can melt due to the chinook s warmth; hence the name snow eater. Acknowledgements This article was produced as a student essay as part of the degree in meteorology at the Department of Meteorology, University of Reading. I would like to thank my personal tutor, Ross Reynolds, for his help and guidance. References Ahrens, C. D. (1994) Metemlogy maby. West, St. Paul, USA Brinkmann, W. A. R (1971) What is a foehn? Weather, 26, pp. 23C239 Hoinka, K. P. (1985a) Observation of the airflow over the Alps during a foehn event. Q.3 R. Meteorol. SOC., 111, pp (1985b) What is a foehn clearance? Bull. Am. Meteorol. SOC., 66, pp Julian, L. T. and Julian, P. R. (1969) Boulder s winds. Weatherwise, 22, pp. 108,112, 126 McGowan, H. A. (1997) Meteorological controls on wind erosion during foehn wind events in the eastern Southern Alps, New Zealand. Can. 3 Earth Sky 34, pp McGowan, H. A. and Sturman, A. P. (1996) Regional and local scale characteristics of foehn wind events over the South Island of New Zealand. Meteorol. Amos. Phys., 58, pp (1997) Characteristics of aeolian grain transport over a fluvio-glacial lacustrine braid delta, Lake Tekapo, New Zealand. Earth Surjiue R.ocess. Land., 22, pp McIlveen, R (1997) Fundamentals of weather and climate. Chapman & Hall, London Moran, J. M. and Morgan, M. D. (1994) Meteorology. Maxwell Macmillan, New York Riehl, H. (1971) An unusual chinook case. Weather, 26, pp Ustrnul, Z. (1992) Influence of foehn winds on air temperature and humidity in the Polish Carpathians. Theor. AppL Climatol., 45, pp Correspondence to: Miss E. Quaile, Department of Meteorology, University of Reading, Earley Gate, PO Box 243, Reading, Berkshire RG6 6BB. Video review Coral reef: The silent sentinels. ABC International, London. Price A$llO (available from This 60-minute video presents an overview of known and inferred effects of changes in climate on coral reefs. It comes with five pages of notes. Coral reefs are complex biological systems. What we think of as coral is the hard external limestone skeleton secreted by colonial animals, coral polyps, living in association with microscopic algae, dinoflagellates called zooxanthellae. Coral reefs build up over time due to the continued deposition of this exoskeleton. In evolutionary terms, corals are ancient organisms. They first appeared in the fossil record just under 250 million years ago. Lke trees, corals secrete growth rings. The rings may be related to yearly growth seasons and even to tidal fluctuations. They can also be used in a similar way to tree rings as indicators of environmental stress and change. Coral reefs are highly susceptible to sea temperature 145