In several mountainous regions of the world, residents sometimes wake to find that the temperature has risen dramatically overnight. A village in the Swiss Alps that was at -10°C the previous evening might be at +15°C by morning — a 25-degree warming in a few hours. The wind is blowing strongly and steadily down from the mountains, dry and warm. People with respiratory conditions feel uncomfortable. Headaches are common. Sometimes the wind is so strong it knocks down trees and damages structures. Avalanches become more likely. Snow disappears rapidly. This phenomenon is the foehn — a warm, dry wind that descends the leeward side of mountain ranges. It is one of the more dramatic local-scale wind systems in the world, and it has been recognised and named in many mountainous regions, where it is known by various local terms: chinook in the Rocky Mountains of North America, zonda in Argentina, halny in the Tatra Mountains, sundowner in California, berg wind in southern Africa, helm wind in northern England.
The Name and Its Reach
"Foehn" comes from the German "Föhn" (also spelled Phön), which is itself derived from the Latin "favonius" — a Roman name for the west wind. The Alpine foehn is the original named instance, recognised since at least medieval times. The same physical phenomenon occurs in mountainous regions worldwide and has produced a rich local vocabulary. The chinook in the lee of the Canadian and US Rocky Mountains is a foehn-type wind. The zonda east of the Andes in Argentina is another. In all cases, the underlying physics is similar even if the local manifestations differ in their details.
The Physics
The foehn effect arises from the asymmetry between rising air on the windward side of a mountain and descending air on the leeward side, complicated by phase changes of water. When moist air rises on the windward side, it cools at the "moist adiabatic rate" of approximately 5°C per kilometre — slower than the "dry adiabatic rate" of approximately 10°C per kilometre because the condensation of water vapour releases latent heat that partially offsets the cooling. As the air rises, its water vapour condenses to form clouds and precipitation, which falls out on the windward slope. By the time the air reaches the summit, it has lost much of its water content. The dry air then descends on the leeward side. With less water vapour to evaporate (which would otherwise cool the air), the descending air warms at the dry adiabatic rate of 10°C per kilometre. The asymmetry between slow cooling on the way up and fast warming on the way down means the air arriving at low altitude on the leeward side is much warmer than the air at the same altitude on the windward side — and it is also much drier.
The Numbers
The temperature difference produced by the foehn effect depends on the height of the mountain range and the amount of water that was condensed out on the windward side. For a 2,000-metre mountain range with substantial precipitation on the windward side, the air arriving in the lee might be 10-15°C warmer than the air that started on the windward side. For a 3,000+ metre range like the higher Alps or Rockies, the temperature increase can be 20-25°C or more. The largest recorded chinook in the US lifted the temperature of Spearfish, South Dakota by 27°C in two minutes on 22 January 1943 — possibly the most extreme temperature change ever recorded over such a short period. The descending air is also extraordinarily dry — relative humidities can drop to 10-20 percent within hours.
The Alpine Foehn
The original foehn winds are those of the Alps, blowing northward across the range from Italy into Switzerland, southern Germany, and Austria, or sometimes southward across the range from Switzerland into Italy. The Alpine foehn produces dramatic warming and dramatic snowmelt in the affected regions. Foehn winds can melt as much snow in 24 hours as would normally melt in weeks of normal weather. The accelerated melting can trigger avalanches and flooding. The foehn winds also affect local communities in subtler ways — they have been associated with increased rates of migraines, headaches, irritability, and various health complaints in foehn-affected regions, though the exact mechanisms are debated. The term "foehn illness" or "foehn sickness" is used in some Alpine communities to describe these effects.
The Chinook
The chinook is the foehn wind of the lee side of the Rocky Mountains in Canada and the US. The name "chinook" comes from the Chinook indigenous people of the Pacific Northwest, applied to the warm winds by 19th-century settlers. The chinook is particularly noted in southern Alberta and Montana, where it can produce dramatic warmings in winter — temperatures can rise from -20°C to +15°C in a few hours. The chinook also produces a distinctive "chinook arch" cloud pattern — a curved band of clouds on the leeward side of the Rockies, sometimes accompanied by a clearing in the western sky and the warm wind blowing down the mountain slopes. Calgary, Alberta is one of the most famous chinook-affected cities, with the chinook producing as many as 30-35 chinook events per year and often producing the warmest winter weather of any inland city at the same latitude.
The North American chinook is the most extensively documented member of the family, and it holds the world records for both the fastest and the largest temperature rise ever recorded. Those events, the chinook arch cloud that announces them, and the belt along the Rocky Mountain Front where they occur are covered in detail in the guide to chinook winds.
The Zonda
The zonda is the foehn-type wind of the Argentine Andes, blowing eastward across the Andes from Chile into the Argentine pampas region. The zonda is most active in spring and can produce temperatures of 35-40°C in regions that would normally be much cooler. The zonda has been associated with substantial impacts on agriculture — accelerated snowmelt in the Andes, increased fire danger in the pampas, water stress on crops. The zonda has also been associated with social effects similar to those reported with the Alpine foehn — increased headaches, sleeplessness, and emotional disturbance in affected populations.
Because the Andes at that latitude are the highest mountains outside Asia, the zonda descends further than almost any other foehn wind and heats correspondingly. Its season, its distinctive split between a version that reaches the ground and one that stays aloft, and its effect on the Mendoza vineyards are set out in the guide to the zonda.
Foehn Wave Clouds
One of the most characteristic visual features of foehn conditions is the formation of "foehn wall" or "foehn cap" clouds along the mountain ridge, and "lenticular" wave clouds in the air downstream from the mountains. The mountain forces the airflow to wave up and down as it passes over, and these gravity waves can produce stationary clouds that appear lens-shaped and remain fixed in position while the wind blows through them. The cloud forms where the wave motion lifts air into cooler regions and condensation occurs; it dissipates where the wave brings the air back down. The lenticular clouds are particularly impressive over high mountains — they can stack vertically and reach altitudes of 8-10 kilometres.
Foehn and Snow
Foehn winds are particularly hard on snow cover. The combination of warm temperatures, low humidity, and strong winds produces rapid melting and sublimation. A 24-hour foehn event can melt 5-10 centimetres of snow cover. The accelerated melt can trigger avalanches as the snow pack becomes destabilised. In some Alpine and Rocky Mountain communities, the arrival of a foehn or chinook in mid-winter is greeted with both relief (warmer temperatures, clear roads) and apprehension (avalanche danger, flooding from melt runoff). Backcountry skiers and mountaineers monitor foehn conditions carefully — the increased avalanche danger during and after a foehn event has produced numerous accidents and fatalities over the years.
Foehn-Like Effects in Larger Systems
The foehn effect is not limited to the immediate vicinity of mountains. A similar effect occurs whenever air descends over a long distance, including in large-scale weather systems. The descending air of high-pressure systems undergoes a similar warming and drying as it sinks. The "Santa Ana" winds of southern California are a foehn-type wind, with dry continental air descending from the high deserts toward the coast, warming as it descends and producing the dry, hot, dangerous fire-weather conditions that have caused some of California's most devastating fires. The Sundowner winds along the central California coast are another foehn-type phenomenon. Foehn-type effects can be detected even in mid-latitude cyclones, where the warm sector air descending on the leeward side of cold fronts can produce localised warmings.
Climate Change and Foehn
Climate change may be affecting the frequency and intensity of foehn winds, though the connections are complex. Warmer baseline temperatures mean that foehn events occur in a generally warmer climate, with implications for snow cover, glacier mass balance, and ecosystem response. Changes in the position and intensity of major pressure systems may be altering the frequency of conditions favourable for foehn formation. The increased moisture content of warmer air may be increasing the latent heat release on the windward side, producing more pronounced warming on the leeward side. The exact response of foehn winds to climate change is an active research topic, but the underlying physics ensures that foehn events will continue to occur and to produce dramatic local weather effects in the mountainous regions where they are most prominent.
The foehn is only one of the ways terrain makes a wind. Where air is funnelled between ranges rather than lifted over them it stays cold, which is what produces the mistral of the Rhone valley — the same principle of geography shaping air, arriving at the opposite result.