Foehn occurs when moist air flows over a mountain range or a mountain. This forces the air masses to rise, causing them to cool down. Moist air becomes around 0.5 degrees C colder every 100 meters of altitude. However, cold air can absorb less moisture than warm air, which is why the clouds become denser and denser until they can no longer hold the moisture. This often results in heavy precipitation in the congested areas of the mountains and the air loses a large proportion of its water - making the air drier. The decisive effect for the Föhn is that the already quite dry air on the leeward side of the mountains sinks into the valleys - and warms up by about 1 degree C per 100 meters of altitude. The warming is about twice as strong as the previous cooling. And due to the strong warming, the air can absorb more moisture, which means that the clouds (almost) completely dissolve. Foehn is a (relatively) warm and dry wind. And not for nothing was the foehn in America christened "Chinook - the snow eater" by the Indians.
Local foehn effects can be found behind every mountain / mountain range
Whether the Föhn breaks through (as described above) depends on the weather situation and the size of the terrain obstacle. It should now be clear why in the mountains it is usually the highest or the upstream mountain ranges that receive the most intense precipitation, while in the inner Alpine and sheltered valleys in particular - e.g. Engadin, Valais - it is often quite dry or at least has snowed significantly less.
