What are they?

Why and where do they blow?

And finally, what are their effects?

The term faune describes a warm, dry wind that blows down the lee or downwind side of a mountain.

The term was first used to describe a wind that often blows in springtime in the Alpine valleys in Switzerland and Austria, but the name is now used for a variety of dry warm winds around the world, including the Chinook in the Rocky Mountains, the Santa Ana Wind in California, the Zonda in South America and the Loo in India.

So let's see how faune winds are formed.

When winds encounter a mountain high enough to block their passage, the air is pushed up along its flanks, and as the air rises, it cools down.

In fact, as the air rises, it experiences less and less pressure.

This is because the higher the air is, the less air molecules are above it, so air particles experience less and less weight on them.

So when air is pushed up a mountain, the drop in pressure allows the air to expand.

But to expand, the air has to give out some energy to its surroundings, and this causes it to become colder.

This process is known as adiabatic cooling, and causes the temperature of the rising air to drop at a rate of approximately 1°C for every 100 meters of elevation.

This rate is known as the dry adiabatic lapse rate.

But that is not all.

Cold air can contain less water vapor than warm air.

So as the air cools below a certain temperature, it cannot retain all the water it initially had when it started rising.

We say it becomes saturated, and droplets start to form.

This leads first to clouds, and eventually to rain.

The elevation at which this happens is known as the lifting condensation level.

Now, because condensation causes latent heat to be released, as the air rises above the condensation level, it keeps on cooling adiabatically, but at a much slower rate, since part of the cooling effect linked to expansion is offset by heat released by condensation.

So, instead of cooling 1°C every 100 meters, the rate drops by almost half to approximately 0.6°C per 100 meters.

This reduced rate of cooling is known as the wet adiabatic lapse rate.

Eventually, the rising air reaches the top of the mountain.

By this stage, it will have cooled down a lot, and likely it will have lost most of its humidity, so we are now dealing with cold, dry air.

Cold and dry air is heavy, and will start to sink, rushing down the lee side of the mountain.

Now, the opposite phenomenon happens.

The air that sinks becomes compressed, and it warms up.

By how much?

Well, because the air is now dry, it will follow the dry adiabatic lapse rate, and warm up by 1°C per 100 meters, or almost twice as fast as it cooled down on the upwind side.

By the time fern winds reach the valley, they will be not only dry, but also much warmer than they were on the other side of the mountain.

Fern winds can have some beneficial effects, as warmer climates can favour certain cultures.

It is, for example, partially thanks to fern wind that wine can be produced in the Alps.

But often, the effects of fern can be devastating.

In fact, fern winds can attain considerable speeds and cause a lot of damage.

People in California particularly dread the Santa Ana winds, because they are known to promote wildfires.

The Loo causes a lot of sufferance in India too, where it makes temperature rise to dangerous levels.

Finally, in springtime in the Alps, the sudden increase in temperature linked to fern wind can cause the rapid melting of snow and cause avalanches and inundations.

This concludes our today's video.

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