Hairdryer winds

If you’ve ever spent time near a major mountain range, then you might be familiar with downslope winds. These impressive blasts of air rush down from mountain crests and get warmer and warmer as they descend. By the time these air masses reach the leeward foothills, they can be warm enough to raise the ambient temperature by 20 degrees Fahrenheit (11°C) or more in just a few minutes. It is no coincidence, then, that in German-speaking regions of the Alps, the words for hairdryer and for downslope winds are identical: Föhn.

Known as Chinook winds in North America, Föhn winds are sometimes called “snow eaters” because of their ability to make abundant snowfall disappear in a matter of hours or even minutes. Such winds may go by different names around the world, but the meteorological principles behind them are the same.

  1. Wind impinges upon a mountain range, forcing air up and over the crests of the peaks.
  2. As the air rises in altitude on the windward side, it expands thanks to decreasing atmospheric pressure, which causes it to rapidly cool. Think of how the propane cylinder on your barbecue grill becomes cold as it depressurizes: It’s the same principle.
  3. As the rising air cools, moisture condenses out and falls on the mountain slope as rain or snow. This is why the windward faces of mountain ranges usually collect much more precipitation than leeward sides.
  4. Liberated of its moisture, the air begins its descent down the leeward slope. But dry air changes temperature more rapidly with altitude than an equivalent amount of moist air. That’s because the energy that would normally go into heating both air and water is now fully available to heat only the air.
  5. As the dry air rushes down the leeward slope, it warms quickly and emerges in the foothills as a warm, dry wind.

Chinook Wind

Föhn winds can cause major wind damage and their hot, dry nature makes the areas they affect particularly susceptible to wildfires. Local folklore around the world also blames such winds for health problems, both physical and emotional.

Southern California’s famous Santa Ana winds are also downslope winds, in that they travel down the slopes of mountains, but the physical mechanism behind them is somewhat different. While Föhn winds arise as a result of a mountain range’s rain shadow, Santa Ana winds occur when a high pressure system settles over the Mojave Desert and Great Basin east of Los Angeles. Seeking the path of least resistance, air is forced down toward the coast through the only channels available: the canyons and arroyos of the San Bernardino mountains. As the air descends, it compresses, warms, and dries. The result is a powerful wind that fans the flames of wildfire-prone areas.

Downslope winds are just one example of the kinds of weather phenomena created or controlled by topography. In directing airflow and forcing wind to accelerate and decelerate, terrain can affect snow deposition, local wind speeds, and the formation of fog.

And in Germany, it’ll even dry your hair.