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Incoming fronts

Snow and weather

Clouds bring snow and that is generally good news. But not every cloud is the same. One could arrive fast, while the other sneaks in and the third one brings his friends. You won't learn all the ins and outs of the weather in this module, but you learn what the basis is for fronts and clouds. 'Front' is a meteorological term that people use all the time. It is the collective term for a warm front, cold front and occlusion.

1. Warm front

Warm air will be blown into the mountains with a warm front. Because warm air is lighter than cold air, it will be first noticeable in the higher aerial stratums. As the warm front comes closer, the warm air will also be noticeable on lower levels. The passage of a warm front will not go unnoticed. The snow lines rise and in principle, it could be warmer on top of the mountain than in the valley. The wind will turn to the south and the level of humidity will rise and the visibility will get worse.

In short, a warm front brings snow, warmer air, southerly and stronger wind and a rising snow line. Also the visibility gets worse.

You can actually see a warm front coming rather easily. On weather forecast maps, your altimeter and with the naked eye, because warm fronts are easily spotted from afar. The first higher clouds create a diffused light and maybe even a halo around the sun. At that moment the front with precipitation is still 600 kilometers away. At the same time the air pressure drops. This is something you can measure with the altimeter on your watch.

 The red lines with the half-spheres mark the warm fronts on the weather maps.

The red lines with the half-spheres mark the warm fronts on the weather maps.

 In real life the warm fronts are recognised by the high clouds which you can see coming in from afar.

In real life the warm fronts are recognised by the high clouds which you can see coming in from afar.

Because you can see the warm front coming in, you will have more than enough time to head to safer terrain. But do not wait too long! Once the warm front has arrived, stay close to the groomed slopes, or even better, on the groomed slopes, because of the poor visibility. If the snow line lies low enough, try to find powder in the trees.

2. Cold front

The opposite of the warm front is the cold front. During a cold front the warm air is violently driven away by the cold air. This heavier cold air slides, as a matter of speaking, underneath the warm air. The warm humid air will suddenly rise up. This results in strong clouds with heavy showers and strong winds. This happens often in spring and autumn.

 Blue lines with triangles indicate the cold front on weather maps.

Blue lines with triangles indicate the cold front on weather maps.

 The cold front has arrived without any signs. The temperature and snow line drop.

The cold front has arrived without any signs. The temperature and snow line drop.

A cold front is not easy to see coming and it is over before you know it, making it almost impossible to anticipate in time. Cold fronts are easily spotted on the weather maps and are always mentioned in the better weather forecasts. Stay close to the groomed slopes due to poor visibility. Or head lower on the mountain to the trees to ride some powder. Once the cold front has passed, there can still be some heavy isolated showers which can cause some trouble. These clouds arrive fast and can surprise you. If you are certain you are located in the tail end of a cold front and got stuck in a white-out because of that, then wait for the clouds to clear. But even better is it to find shelter in the trees.

3. Occlusion

A cold front moves faster than a warm front, meaning the cold will catch up with the warm front eventually. This creates the so-called occlusion front, combining the cold and warm front and causing heavy showers. The temperature differences are less intense than during a warm front, making the snow line not as high.

 Left of the mountain Stau originates. The Föhn originates on the right side.

Left of the mountain Stau originates. The Föhn originates on the right side.

The arrival of the occlusion front looks the same as the arrival of a warm front. But do not wait too long! Once the occlusion has arrived, stay close to the groomed slopes, or even better, on the groomed slopes due to poor visibility. If the snow line is low enough, head for the trees to search for powder.

 The purple lines with half-spheres and triangles indicate the occlusion on a weather map.

The purple lines with half-spheres and triangles indicate the occlusion on a weather map.

Once the occlusion has passed, there can still be some isolated showers which can be very heavy. These clouds can arrive fast and may surprise you. If you are certain you are located in the tail of an occlusion and got stuck in a white-out, wait for the clouds to clear. But even better is it to find shelter in the trees.

4. Föhn storm

A Föhn is a wind which usually becomes warmer and relatively more dry due to a descending movement on the leeward side of the mountains. Earlier we mentioned fronts. Front usually bring precipitation and when they stay long enough we call them ‘Stau.' On the other side of the highest mountain originates the so-called Föhn. This Föhn can be strong and can arrive quickly.

Mountains have an unmistakable influence on the weather. The effect is very noticeable during the formation of a Föhn or Stau. On the above picture you can see a mountain with clouds on one side and on the other side beautiful weather. The clouds are coming from the front system of a passing depression. This depression literally pushes her front to the Alps and the air has to rise because of that. The air expands and cools down. But cold air is less capable of holding humidity, creating condensation. 

Think of a cold day. You will see condensation. The moment you breath out a small white cloud of condensation forms. This does not happen on a warm day. Et voilà. 

With the formation of condensation, warmth and liquid water is being released. Water that, due to gravity, wants to go down. In this case the rule applies: the stronger the propulsion, the more intensive the precipitation will be. And if the build-up of clouds keeps on long enough, then it can snow and rain for a long time: the Stau. And Stau can be created as a result of a passing occlusion, warm or cold front.

The opposite is the Föhn. The air stream does not want to go up and neither is it cooling down. No, the air stream moves down in this case. A Föhn or a Föhn wind is a relatively warm and dry fall wind. Besides the difference between a wet windward side and a dry leeward side there is another remarkable difference, namely the temperature. Once the atmospheric humidity on the windward side has reached 100 percent, than the air temperature will only drop about 0.6 degrees Celsius per 100 metre. This differs from the leeward side. Here the air descends, making it shrink and making it warm up again. In this process the air temperature will rise about 1 degree Celsius per 100 metre. As a consequence of this phenomenon it is possible to experience different temperatures on a similar altitude on either side of the mountain.

Draw particular attention to the temperature when you translate the above picture to reality.

It is logical that almost all streams from the south will bring warmer air with them than the streams from the north. This is why a Southern Föhn is much more well known than a Northern Föhn. Not only can this bring heavy storms, but the temperature in so-called Föhn valleys can rise up to ten, even twenty, degrees Celsius. These temperatures are above zero and this can happen in the depths of winter. A Southern Föhn is a literal snow eater and has large consequences for the snow. Luckily it is rather easy to predict where the phenomenon will occur. Föhn valleys are valleys (on the leeward side of the mountain) which are lying in the same direction as the current wind direction. Preferably they are narrow and will fan out when you descend. The wind can speed up in such valleys to maximum speed and the air can warm up to the maximum.

 Strong winds at the peak!

Strong winds at the peak!

With a Föhn the wind blows strongly and the visibility will be good. You will not notice it immediately due to the sunshine, but on nearby crests and peaks huge amounts of snow will be transported and immediately wind-drifted snow will be created.

 A cloud at the peak we call a Föhn fish. It is a sign that the wind is blowing and that somewhere in the Alps a storm is coming.

A cloud at the peak we call a Föhn fish. It is a sign that the wind is blowing and that somewhere in the Alps a storm is coming.

A Föhn is often announced in the weather forecast, but is also easy to spot it with the naked eye. The so called Föhn fish clouds and the snow blowing from the peaks are often a good indication of a strong Föhn. So watch out for freshly blown in snow!

5. Beautiful weather

There are two moments which will create beautiful weather. On the one hand there is the classic high-pressure area that can remain above the Alps for days and which can bring sunny days and cold nights. There will be, literally, no cloud in the sky. On the other hand there is the temporary tail of high-pressure between two fronts. Far away the new clouds are already forming.

 Cloudy, humid and cold in the valley. Sunny and warmer higher up on the mountain.

Cloudy, humid and cold in the valley. Sunny and warmer higher up on the mountain.

There are a few things to keep in mind when the weather is beautiful. Firstly, how long will the beautiful weather last? Do you need to keep an incoming front in mind? Secondly, there is the so-called inversion. It can remain cold and humid in the valleys. In that case the valley will be covered in clouds and it will be cold. Check the webcams on the mountain to see if it is already sunny there. In addition to this you need to be aware that the clouds can climb even more, making it still possible for you to get stuck in a white-out. In short, be alert of clouds. They can cause you to get stuck in a white-out.

Summary
Clouds and wind are easy to predict using a proper weather forecast. Be extra alert when a Föhn is being announced or when there is a cold front coming. Within a few hours your nothing-the-matter situation can change into a huge-trouble situation. You need to keep this in mind when planning powder trips further away from the ski area.

Videos, graphics, guidance, other information, or user generated content (the “Content”) on this site is presented for general educational and information purposes only and to increase overall backcountry safety awareness. The Content is neither intended to be expert advice or a substitute for expert advice, nor is it a substitute for a ground course offered by qualified avalanche educational/certification centers. The Content contained in this site should not be considered exhaustive and the user of this site should recognize that Backcountry activities carry inherent risks of serious injury or death. The user of this site should complete a ground course from a qualified avalanche center before engaging in any backcountry activities.

This information is part of the Salomon Mountain Academy. All the knowledge for all the skiers and snowboarders who want to ride out of bounds or in the backcountry. A great foundation for beginners and the perfect refresher for experts. 

START FREE TRIAL!