This month’s tip should help you understand wind speed in the mountains. Of all the weather phenomena (temperature, precipitation as rain or snow, etc), wind has the greatest influence on us as mountaineers. At its worst, it is ‘the killer’ among them all. The importance of the wind is even greater in winter, when it has a huge part to play in the formation of avalanches.
Main Influence on Wind Speed in the Mountains
The principle influence over what the wind speed in the mountains will be on any given day is the current weather system.
Air will flow from regions of high pressure to regions of low pressure – this is ‘wind’. The speed of the wind is governed by the ‘pressure gradient’. This describes how steeply the drop off is between high and low pressure. The steeper this gradient, then the faster the wind will move.
If you look at a weather map, there are lines that resemble contour lines on a mountain map – and they have much in common. These lines (isobars) link points of equal pressure (at a moment in time). You could imagine areas of high pressure as summits and areas of low pressure as dips in the ground. If you poured water over this imaginary surface, then it would flow downhill from the summits to the dips. The steeper the gradient, then the faster the water would flow. Air behaves in a similar way to water, and so we have wind.
However, the wind doesn’t blow directly from high to low pressure – it is deflected off course by the rotation of the earth. In the northern hemisphere, wind circulates in anti-clockwise direction (and slightly inwards) around a low pressure area. This is the same as water running down a plug-hole. Therefore, when you look at a weather map, the wind direction will be more parallel to the isobars rather than crossing them.
Wind Speed and Altitude
Apart from the pressure gradient being the starting point for wind, altitude also affects the flow of air.
Moving air that is in direct contact with much of the earth’s surface is slowed down by friction. In comparison, air that is not touching the surface of the earth is not held back and is therefore free to move faster. So wind speed increases above sea level.
In addition, gravity also affects the air. But the further away from the earth’s surface, the less this is the case. So at very high altitudes, the flow of air is free to move even more quickly.
Importance of Wind Speed and Direction
When planning a day in the mountains, it is vital to seek out forecast information on what the wind might be doing. Most generic forecasts (eg TV news etc) describe sea level conditions. Be sure to seek out more specific information on the mountainous areas or at the very least make sensible allowances in your expectations. In the UK, expect wind speed on the mountain summits to be up to 2 or 3 times what they are in the valley.
Armed with this knowledge, you can plan a route that will be safe and achievable in the current conditions. Avoiding exposed ridges (Striding Edge, Crib Goch etc) is one basic safety precaution. But you could also plan a whole day that seeks out the sheltered ‘lee’ side of the mountain. Or you could pick a route that means you will spend more time with your back to the wind. Another tactic is to walk into the wind in the morning so that when you are more tired in the afternoon, the wind is behind you. If there are no other good options, you could simply stay low for the day and avoid the strongest winds.
Local Effects on Wind Speed in the Mountains
Local terrain features (the mountains themselves) can also affect the wind speed (and to an extent the direction). Imagining the air as water can again help to understand this better. Picture a fast flowing mountain stream filled with boulders.
Acceleration of Wind over Summits
As a block of air moves towards a mountain summit, the mountain is in the way. The air which would have flowed ‘through’ the summit area is forced upwards, squeezed into the air above it. But in order for all this air to move over the summit (in a smaller space available), the flow of the air has to accelerate. The same basic science is the reason that the water in a stream must accelerate over rocks on the stream bed.
So on mountain summits, not only is the average wind speed faster than in the valley below, but it is also stronger because this squeezing effect forces the wind to strengthen.
Acceleration of Wind through Cols
In a similar way, the wind speed will often be much stronger in a mountain col or saddle.
Picture the stream flowing and suddenly being forced between two boulders. Because less space is available again, the water must accelerate to get through. The wind behaves in exactly the same way at a col, accelerating through it. The winds in cols are often much stronger than anything else nearby, either higher or lower. Avoiding cols on windy days might therefore be a good choice.
Eddies on Lee Slopes
Once again, think of the mountain stream. After the water has tumbled over a series of rock steps or waterfalls, powerful eddies are created with the water circulating around until it finally breaks free and flows down stream once more.
In the mountains, a fast wind passing over a summit with a steep cliff may drop suddenly down after it has passed the cliff face. This creates a circular eddy in which the air moves back towards the cliff (counter to the main wind direction). It then blows up the cliff face before hitting the main air flow at the top once again.
Other Causes of Wind in the Mountains
One other cause of wind in the mountains is differential heating and cooling. This means that some areas can heat up in the daylight more quickly than others. They can also cool down more quickly too.
On a coastline, it is this differential heating and cooling that can create onshore and offshore winds. This is because the land and the sea heat up and cool down at different rates. These winds can sometimes be very strong depending on the relative speeds of heating and cooling
In the mountains, a similar phenomenon occurs because the summits of mountains can heat up more quickly than the valleys on warm days. They also cool more rapidly at night. This leads to:
- Anabatic Winds – An uphill wind, usually in the afternoon. The sun has warmed the mountain slopes causing warm air above them to rise. This in turn draws up air from the valleys below to replace it.
- Katabatic Winds – A downhill wind, usually at night. The mountain summits cool down more quickly then the valleys causing the cold (and denser) air on them to rush down towards the valley floors.
These winds can be strong, and might be a factor in deciding if, where and in what direction to pitch a tent for a wild camp the mountains.
Wind is the most important aspect of a mountain weather forecast. Mountain-goers should research it carefully. Both wind speed and direction are important in the mountains.
- The pressure gradient between high and low pressure areas is the main influence on wind speed.
- Wind speed increases with altitude due to loss of friction and the effects of gravity. The wind speed on a UK summit can be 2-3 times stronger than in the valley below.
- The shape of the mountains can significantly affect the wind, making it accelerate over summits and through cols, and causing eddies.
- Different rates of heating and cooling cause anabatic (uphill in the day) and katabatic (downhill at night) winds in the mountains