If you've spent enough time in the mountains, you've probably noticed that the strongest winds typically occur at higher elevations compared to cities and towns in the valleys below. This is especially true for areas above treeline.

Why is this?

There are two main reasons. The first reason has to do with the change in temperature, pressure, and density that occurs with rising altitude. The second has to do with friction near the earth's surface.

Pressure Gradient Increases and Air Density Decreases With Altitude

Wind is a function of the change in air pressure over a horizontal distance. The more rapidly pressure changes over a horizontal distance (known as the pressure gradient), the stronger the wind.

Air pressure and temperature are also related. As temperature changes, pressure also changes. In other words, sharp differences in temperature over a horizontal distance also result in sharp changes in pressure and stronger wind speeds.

As elevation increases, temperature and pressure gradients also tend to increase. This is partly due to the fact that air masses aloft are less influenced by the terrain and solar heating of the earth's surface.

Also, as elevation increases, the density of the air decreases. Air that is less dense is able to "move around" more easily, which can result in faster wind speeds.

Friction From the Earth's Surface Decreases With Altitude

At the earth's surface, terrain changes, tree cover, vegetation, and even buildings all act as wind barriers which cause wind speeds to slow. This is known as surface friction.

As elevation increases, there is less friction to cause wind speeds to slow down. If you imagine an invisible horizontal line at an elevation of 12,000 feet in the Western U.S. for instance, there is only a limited amount of mountainous terrain that exists at and above this elevation – and thus less friction to slow down winds, compared to 6,000 feet where significantly more surface friction exists.

The same example applies to any mountain range. Consider the White Mountains in New Hampshire, for example, where limited terrain exists above 5,000 feet, and thus these areas are more subject to high winds due to less friction compared to surrounding lower terrain.

Vegetation and tree cover also influence friction and wind speed. Have you ever noticed that wind is often stronger above treeline? Once you rise above treeline, there is less friction due to a lack of trees to "interrupt" the wind flow.

A lack of tree cover along with locally flat terrain also help to explain why large bodies of water such as lakes, as well as large flat plains or valleys are more prone to gusty winds. 

Forecast Anywhere

OpenSnow's Forecast Anywhere feature (available to All-Access subscribers) is an excellent tool for viewing hourly wind forecasts out to 10 days for any location and elevation in the world. With this feature, you can get an idea of expected winds for both the base and summit of a ski resort, hiking trail, or climbing route.

Alan Smith