In most weather forecasts which are aimed at mountain weather, wind is a dominant factor. Wind in the mountains are different from winds in valleys or plains and have a more typical diurnal character. Some winds are typical for the season, whereas some winds are dominantly present and can suppress the diurnal rhytm. In this post, I will try to explain the different mountain winds and what causes them.
Special attention is for prevailing winds, upslope & downslope wind, but also föhn winds and the venturi-effect.
First of all: what is wind?
The air on earth has a lot of mass, even though you don’t feel it usually. It’s a gas, and gases expand greatly when heated up, and consequently contracts when cooled down. This is the driving force for all weather on earth: the sun heats up the surface, and the heated air rises. Low pressure is formed. But as there needs to be an equilibrium (after all, we don’t live in a vacuum), the air needs to be replaced: wind is simply moving air. With larger temperature differences, the moving air has more force.
With prevailing winds in this context, we do not focus on seasonal patterns, but rather the strength of the over-all force and direction. When the prevailing wind is sufficiently strong, the diurnal effect is diminished.
Wind at altitude is in most locations of the world particulary strong: the jet stream. There is a strong connection between the strength and location of the jet stream and weather systems, for example. But also between altitudes: close to the ground, at 1500 meters, 5000 meters and 10.000 meters; the wind speeds are related to one another, to the extent that there is a prevailing wind pattern.
High mountains are exposed to the wind at altitude by the simple fact that they are at a high altitude. This has a double-effect. First of all, the winds are stronger at altitude. But the air at lower altitudes is forced against the mountain range and needs to go somewhere: uphill is the only option, exacerbating the effect of the already stronger wind.
Mountain wind: the diurnal pattern
Mountain winds are very typical for their own location. Each valley will have a different characteristic, which is determined by altitude difference, slope angle, exposure, cloud cover and the presence of snow or glaciers.
Below profile is a profile of the Mont Blanc Valley, albeit quite schematic. On the left-hand side you have the higher part of the Mont Blanc range, which is west-facing. On the right-hand side you have the Aiguilles Rouges.
Assuming calm conditions (so no overriding prevailing wind pattern), the air temperature drops due to radiation – especially when snow or ice is present. Colder air is heavier, and starts flow downhill, along the valley floor. When the air cannot escape swiftly enough, so-called frost hollows occur. These are typical conditions for an area: it’s an area where it colder than what you might expect when you look purely at the altitude and latitude of a location.
The Mer de Glace in the Alps could be a frost-hollow. Cold air from the surrounding mountains (all above 4000 meters) flows downhill to the glacier, where there is relatively little room to “escape”. Only at the very bottom of the glacial valley, there is an opening into the larger valley.
Such winds are called : katabatic wind, coming from the Greek word Katabaino which means going down.
Daytime: the reverse happens
When the sun starts to heat up the valley, the rising air breaks the movement of the katabatic winds. It’s the typical morning calm of a summer morning. When the temperature increases further, the air starts to move uphill: anabatic winds.
During the day, the sun is shining more vertical, and also the other part of the mountains lies in the sun. The wind speeds along the mountain ridges increase, as the air is heated up. This air movement is what’s driving the birds of prey and storks, but also the parapenters, along the mountains or above heated plains.
At the end of the day, the wind on one side of the mountain will decrease, as less solar energy is available to heat up the surface. In general you can state that a dry mountain area will have stronger winds, as the surface heats up faster. Glacial area‘s will have less effect from the heating by the sun, as the air temperature does not increase as much as without the snow and ice: snow and ice reflect a lot of solar energy.
These breezes can be quite intense, and are the strongest on a south-facing slope such as in the Tarentaise between Moutiers and Bourg Saint Maurice.
Föhn winds are typical for all mountain areas and can be incredibly strong. They are synoptically driven: a result of the overall weather pattern. The föhn winds blow on the lee-side of a mountain, whereas the windward-side is exposed to the moist air.
Moist, stable and relatively mild air is forced over a mountain slope: the air cools down due to adiabatic cooling: the air expands or decompresses. This cooling forces the moist to condensate and clouds form. This condensation actually causes the air to warm up, relatively. It still cools down when flowing uphill, but less fast as the air is saturated. Rain (or snow) sets in, releasing the moisture from the air.
On the other side of the mountain, the air descends. However, with no moist being present anymore, the air heats up with 1 degree per 100 meters. As a result, on the lee-side it will be significantly warmer, drier and sunnier. How significant?
Well, imagine we have a situation where the wind is coming from the west. It’s 20 degrees Celsius in the Saone-Valley, west of the Alps. Moist air comes in, with a saturation level at 2500 meters altitude. Until the air is saturated, it cools down with 1 degree per 100 meters. At 2500 meters, it’s now 5 degrees Celsius. The air is forced further upwards, all the way over the Mont Blanc at 4800 meters.
As the air is saturated, the cooling slows down to 0.6 degrees per 100 meter: resulting in a summit temperature of -9 degrees. On the lee side of the mountain, the moisture is gone: it has been released as snow and rain on the mountains and what’s left of the moisture evaporizes when descending.
As there is no moisture, the resulting process is adiabatic heating, at a rate of 1 degree per 100 meters. The result is a very sunny, dry, warm 25 degrees Celsius in Courmayeur at 1400 meters. Significantly warmer than the 20 degrees in the Saone-valley.
Venturi-effect: extremely high wind speeds
Under some circumstances, the winds can pick up really bad. This is particularly the case when there is a so-called Venturi-effect. This effect can occur when the wind is moving through a (sudden) smaller channel, like a canyon. Funneled through the sides of the canyon or valley, the air is constricted and thus forced to speed up. It’s similar to a river: when the river gets more narrow, the water flows faster as the same amount of fluids need to pass.
These effects are very much depending on local geography and wind direction. The most famous (European) example is the Mistral in the Rhone-valley in France. Under certain conditions (low-air pressure above Genua, high pressure west), the wind is forced through the Rhone Valley, causing it to speed up. The Rhone-Valley is long and unidirectional: it goes north-south.
However, venturi-effects also play a role at more local scales – but can be highly unpredictable and needs to be seen at the location.
How does wind affect my hiking routes?
You might be wondering: nice to know. But now what. Well, there is an applicability to this, for you as a hiker (or any other outdoor sport).
Knowing whether a föhn-wind will pick up and what the effect may be, can be crucial. Föhn winds can be strong, but do represent usually very good (sunny & dry) weather. It also diminishes a large snow pack in a matter of days or even hours. The high wind speeds, dry air and high temperatures are “the best” recipe to decrease snow cover. As the air is dry, it also means that the snow field that is still present might not be very wet.
Knowledge of the katabatic and anabatic winds can be useful when planning ridge-crossings or finding a place to pitch the tent. Exposed ridges on the side of a valley with afternoon sun can cause strong winds and make it more difficult to pitch the tent.
Whereas setting up your tent in a frost hollow can be easily prevented by having a slightly different look on your surroundings and being able to spot the “risks” of the katabatic winds.