The Foehn effect is a weather phenomenon that causes air to become warmer and drier on the downwind (or leeward) side of a mountain range. It plays a significant role in shaping local climates and can have both beneficial and hazardous consequences.
What is the foehn effect?
In simple terms, the foehn effect describes a shift from cool, moist conditions on one side of a mountain to warm, dry conditions on the other. As moist air is forced up the windward side of a mountain, it cools and condenses, often producing rain or snow. Once the air passes over the peak and descends the leeward side, it warms rapidly and dries out, creating the foehn effect.
Foehn winds, also spelled “Föhn”, are common in mountainous regions around the world. They are known by different names depending on the location: the Chinook or “snow eater” in the Rocky Mountains, the Zonda in the Andes, and the Helm wind in the English Pennines.
One of the most dramatic examples of the foehn effect occurred in Montana, USA, in January 1972. During a Chinook event, temperatures rose by 57 °C in just 24 hours, from -48 °C to 9 °C, marking the largest temperature change over a single day ever recorded in the United States.
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In the UK, foehn events are most commonly observed in the Scottish Highlands. Moist westerly winds from the Atlantic are forced over the mountains, bringing wet weather to the west while the east enjoys warmer, sunnier conditions.
Foehn events are often accompanied by striking cloud formations, including lenticular clouds and rotor clouds, which signal turbulence and overturning in the atmosphere.
Impacts of the foehn effect
The foehn effect can bring benefits such as a longer growing season and milder conditions in affected regions. However, it is often the negative impacts that draw attention.
The sudden warmth associated with foehn winds can increase the risk of avalanches in mountainous areas, accelerate glacial melt, and contribute to flooding downstream. In polar regions, foehn winds have been linked to the disintegration of ice shelves.
Foehn windstorms can also cause damage to buildings and infrastructure and pose serious risks to climbers. The north face of the Eiger in the Swiss Alps is particularly notorious for dangerous foehn conditions.
Another major concern is the increased risk of wildfires. The combination of warm, dry air and strong winds creates ideal conditions for fires to ignite and spread rapidly. In California, the Santa Ana winds, another type of foehn wind, have been responsible for many major wildfires. In October 2003, 12 such fires burned over 300,000 hectares and caused more than $1 billion in damage.
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There is also anecdotal evidence linking foehn winds to effects on mental health. In Alpine regions, the term “Foehnkrankheit” (foehn sickness) is used to describe symptoms such as headaches, sleeplessness, and mood changes. While scientific evidence is limited, some studies have found a correlation between foehn winds and increased migraine occurrences.
How does the foehn effect work?
There are four main mechanisms that contribute to the foehn effect, often working together depending on the mountain shape and weather conditions:
- Condensation and precipitation: As air rises over mountains, it cools and loses moisture through rain or snow. The release of heat during condensation and the loss of moisture result in warmer, drier air descending on the leeward side.
- Draw-down of air from aloft: If low-level air is blocked by the mountain, only higher-level air passes over. This air warms as it descends, contributing to foehn conditions.
- Turbulent mixing: As air flows over mountains, turbulence mixes the atmosphere vertically. This brings warmer, drier air down into valleys.
- Radiative warming: Clear skies in the lee of mountains allow more sunlight to reach the surface, further warming the air.
Together, these processes explain the dramatic and sometimes dangerous changes in weather associated with the foehn effect.
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