The Favonius.Encyclopedic Definition.
Föhn s. m., Ger. [ant. High-ted. phonno, which is from Lat. Favonius: v. Favonio].
A local wind that blows from the south (sometimes from the north) for a barometric gradient between the two sides of the Alps, and that lost its moisture in the climb, the descent is very hot; brings clear skies and good visibility, but sometimes causes the premature fusion of snow and ice resulting in avalanches and floods.
Attending a summer sunset, with clear sky full of clouds over the Western Alps, is always a wonderful show; but if in the late afternoon, to the colors of the setting sun on the clouds, you add the wind Favonius, then...the whole sky takes on a fantastic Show!
It's like a perfect painting, done of steam and light pastel colors, were suddenly upset by a flamboyant and crazy artist.
This amazing painter is the heaven of the Favonius wind (foehn), a warm wind, hot, strong, his volleys shake the branches of trees with power and seem to go through our bodies, giving warmth to our bones after the harsh winter.
Description of the Phenomenon.The foehn wind is a bank of dry and mild in nature that is manifested by irregular and turbulent gusts, is defined as an orographic wind is just the terrestrial morphology that attaches particular thermodynamic characteristics. Also known as the foehn foehn south of the Alps, comes to life when we have a pressure gradient (pressure difference) between the two sides of the Alps at least a dozen hPa and may occur with opposite
circulatory situations, on both sides of the mountain range.
The synergy between the two huge "gear", causes a rapid current from the North Atlantic-maritime polar air towards the Alps.The cold and humid air impacting against the mountains, an effect STAU (dam), is forced to enter them for an uprising forced the slope of the same, at this stage, the air mass loses about 1 ° every hundred meters, but only up to a certain height; given that the dimension X (variable), reaches the point of condensation forming clouds and consequent precipitation.Through the phenomenon of condensation is released latent heat (heat produced by condensation in fact), this energy causes the temperature does not drop more than 1 °, but 05 ° / 06 ° C per 100m. share.
If we assume a simplified start 0mt share. above sea level. with a temperature of 30 °, we arrive at 1500 meters. with a temperature of 15 °; assuming that at this altitude phase begins condensing and we climb over the mountains at 3000mt., the temperature lost to achieve this fee will not be more than 15 °, but only 7.5 °, at this point there remains "a 'excess heat' of 7.5 °. In the descending phase (on the leeward side) the air mass in question has a gradual warming of 1 ° Celsius per 100 meters. up at low altitudes, with the addition of 7.5 ° of the phase of condensation on the windward side.
In the end, the more the air mass origin is wet, the more important the STAU the windward side, with considerable condensation and rainfall = major release of latent heat = favonico strong effect in the leeward side.
Dry air mass origin STAU = weak = weak then condensation latent heat release almost no effect favonico = absent or weak.The significant drop in humidity in the descending phase of the wind is due to the increase in
temperature, he often can be reached values around or below 10%.
Foehn & clouds.
The foehn is a warm, dry wind, often impetuous and irregular bursts, falling from a mountain range. It is not the only example of orographic wind like this: just as famous is the Chinook, blowing on the eastern slopes of the Rockies (U.S.A.), Uri (Swizerland), Samum (Iran).
When it appears the temperature undergoes a rapid and significant increase, while the air becomes clear; the clouds are absent, apart from those of a lenticular, almost always isolated and with jagged edges (altocumulus).
The foehn wind blows mainly in winter and spring, especially along the mountain valleys of northern Italy and also in full Valpadana, from north to south or from northwest to southeast. The frequency of this event is variable:
Northern Lombardy and Piedmont, the regions most affected by the phenomenon, the average ratio is about ten days per year [40 days even if we consider the micro foehn lasting a few hours] . At the confluence of the Po valley with alpine valleys (eg the entrance to the Val di Susa) the foehn can exceed the speed of 100 km / h. About Italy this northern wind occurs when the difference of atmospheric pressure, or pressure gradient, between the north and south of the Alps is high.
The foehn can cause an increase in temperatures of 30 ° C in a few hours. For this reason it is also called "eating snow", because the snow melts rapidly due to the combined effect of raising temperature and low humidity.
The classic situation of foehn is accompanied by a massive accumulation of heavy rainfall and cloudiness ("Stau") on the southern slopes of the Alps, the cluster of clouds is also called favonico wall and may be visible from
the northern part of the Alps with the gradient adiabatic dry air masses down along the northern side of the Alps, the clouds disperse and create a "window favonica" characterized by extremely dry, clear skies and almost cloudless, and superb visibility. Features of this powerful wind are the beautiful arc clouds.
Physical Laws.The theoretical model which describes more fully the physical process of the hair dryer is that of an adiabatic transformation, ie without heat exchanges with the outside. For this reason it is the First Law of Thermodynamics.
In our case the work is done on the system (compression) and thus will be beneficial. Of course, always regard our experiment, we must assume that air behaves as an ideal gas, and everything will necessarily lead to errors in the theoretical prediction. Due to the first law of thermodynamics we can deduce that in our case to a compression (decreased level) will correspond to an increase in temperature and vice-versa.
At this point, assuming always to be in the presence of an adiabatic transformation, we can assume as valid the Poisson's equation. where the value is the ratio Cp / Cv where Cp represents the specific heat of air at constant pressure and Cv is the specific heat at constant volume, while V p represent respectively the pressure and volume of the air mass in question.
In the figure is shown, on a graph pressure - volume (P, V), also called Clapeyron diagram, the trends of three isotherms (dashed lines, which represent as many branches of hyperbola) with T3> T2> T1, while a solid line is plotted one adiabatic.
But it looks like an isotherm is steeper, imagining that compress the gas is observed that the temperature increases, and that is what practically happens when you experience the phenomenon of the foehn.
A sky unbelievably clear.One of the main characteristics of foehn wind, is to make the sky clear and incredibly colorful.
Any cloud or background is enhanced by contrast and saturation.
The U.V. filter sometimes hard to keep track of the incredible brightness of the air.
One can safely say that the Foehn wind is a great friend of us, is able to clean up the air in a flash from all the rubbish we produce us humans.
Curiosity(1)The Foehn wind is misnamed. In practice it is an effect due to the difference in pressure that occurs downstream and upstream of a mountain range.
(2)The foehn can be generated without creating the phenomenon of Stau in the north. It 'a situation typical of winter.
(3)The west wind and the stau can be reversed. That occurs in the Italian stau and the Swiss foehn occurs. It is in these cases that the two phenomena are more "violent" so much water in the Italian region and very hot wind in the Swiss region with very high daily temperature.
(4)The foehn is known as a hot wind. But this can also be cold. It depends on the air masses.
(5)It 'a very dangerous wind. Such as broken branches and uprooted trees also depends on its intensity. During the days of foehn occurs swollen streams to the rapid melting of snow and promotes the detachment of avalanches.
(6)The foehn is not a phenomenon unique in our regions, but occurs in any area of the planet Earth. The mecanism is always the same. For example, the Adriatic coast are subject to the wind fall called Garbino and not foehn.