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Foehn Winds. By Elena, Sarah, Stacey and Stormy. Characteristics. A foehn wind is when a deep layer of prevailing wind is forced over a mountain range. The wind is created when air flows up the windward side of the mountain and cools and is warmed when the air flows down the leeward side.
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Foehn Winds By Elena, Sarah, Stacey and Stormy
Characteristics • A foehn wind is when a deep layer of prevailing wind is forced over a mountain range. • The wind is created when air flows up the windward side of the mountain and cools and is warmed when the air flows down the leeward side. • As the wind descends on the leeward side of the mountain the air heats up because it is under greater atmospheric pressure which creates strong gusty warm and dry winds. • Foehn winds can raise temperatures as high as 30’C in only a few hours. • This wind only occurs on the leeward side of a mountain range and when the air is stable it flows over the range by a large-scale pressure gradient. • The air is dry and warm at the foot of the mountain and undergoes further heating and drying as it flows down the slopes.
Fohn winds have been called “snow eaters” along the eastern coast of the Rocky Mountains because of their hot and dry nature which tends to melt snow on the leeward side. • When the airflow descends from a high ridge of a previous mountain barrier, (eg. The Rockies onto Alberta’s high plains) it warms through the compression of the air. • This can be compared to how air in a bicycle pump heats up when the plunger is pushed down • The air mass can warm up by a further 9,8 degrees Celsius per 1000m of descent • This wind can reach speeds exceeding 160km/h . When blowing at these speeds, it can tip railcars off their tracks!
Where is the Fohn wind found? • The Fohn winds are famous in the Alps, but are not totally restricted to that area alone. • The term is also used for all similar winds around the world • The Fohn often rises as the first front of a low, moving from the South of France to the Mediterranean. • The wind will be south there, blowing warm and moist air to the Alps. Along the southern slopes the air is forced to ascend. The air cools down and condenses, causing clouds to form. If rain falls, it will ruin the day for many French and Italians!
Some poetic names for Fohn winds around the world • France – Autan wind • Sumatra – Bohorok • North American Rockies – Chinook • Alps – Fohn (go figure) • Libya – Ghibli • Java – Koembang • Southern California – Santa Ana • Japan - Yama Oroshi
How do the Fohn winds influence us? • Visibility around the top of the mountain will be poor. • Downdraughts could endanger a Paraglider or Light aircraft by forcing it down into high terrain. Therefore this is a dangerous condition and pilots should be aware of this. • Sometimes strong winds blowing up the mountains cause air to be deflected upwards high into the air. • This can cause severe turbulence to aircraft passing through the smooth air that was above it. The aircraft is suddenly hit by a massive updraught which may be rolling through the air. • Sometimes you can see this by the presence of a rolling Cumulus type cloud normally of small size above the mountains. This can also be dangerous. These updraughts are called standing waves and the cloud, which is famous in Cumbria, is known as a helm cloud.
Some other influences • The Föhn winds can cause trouble to many people. • They get sick, have sleepless nights or a lack of concentration. Of course there's been a lot of research into these symptoms. It seems that there's a relation to vibrations caused by the waving airstreams. • The vibrations prickle the auditory organs, irritating the nerve systems.A nasty and problematic side effect of a wonderous phenomenon of nature.
