Mountain weather: things to consider

The weather in the mountains can be very treacherous. Circumstances change very quickly: one moment it seems sweltering hot, but as soon as clouds appear in front of the sun it can be very cold. This depends on the altitude you are at. Here I will try to explain briefly what the dangers could be and how to recognize them.
After all, the following applies: the sooner you recognize a danger, the faster you can work on a solution. The biggest dangers are:

Why is the weather so changeable in the mountains

First, however, there is a brief introduction to why the weather changes so quickly. This has to do with two factors.

The first factor is deception. Because you can’t see what’s happening behind the next mountain, it seems to go very suddenly. Seemingly out of nowhere, a thunderstorm makes it over the mountain behind, which could have hung there for hours.

The second element has to do with the structure of our atmosphere. On a bright summer day the atmosphere seems to be calm: there is hardly any wind, there is not a cloud in the sky. What you cannot see, however, is how the atmosphere behaves. This way, a large amount of moisture can be supplied unnoticed, which you cannot see.

However, as soon as it collides with a mountain wall, this air is forced to rise. As it rises, the air expands and cools. Due to the cooling, the moisture condenses and clouds are formed. Due to the erratic shape of the mountain walls, this can happen very suddenly and cause very heavy showers. This type of thermal showers  often arise in warm and calm weather, when there is sufficient moisture present and there is an inversion.

More or less the same goes for gusts of wind. The wind at high altitude is always stronger than on the ground. Wind gusts of 200 km/h or more regularly occur on the highest peaks. The Mont Blanc, the westernmost and highest mountain in the Alps, is particularly susceptible to this.

In warm summer weather (but sometimes also in winter) a convergence line can arise. This line forms a separation between 2 types of air on which squall lines often arise. The high mountains can act as an extra trigger in the case of a convergence line, causing heavy showers that can linger for a long time. The advantage of a convergence line is that they are often easy to see due to the cloud formation and can be estimated fairly well in advance, often in contrast to isolated thermal showers.

Thermal showers occur during the day, as a result of rising air temperatures. This causes a flow of air to occur, and when there is sufficient moist availabe, showers can form. With more moist and a larger temperature gradient, the showers will be heavier. Also in these conditions, mountains are a trigger as they tend to “capture” the moist: the summit influences wind flow and creates a natural event for the air to rise.

Snow showers

When the conditions are good enough for showers to occur, it remains to be seen what kind of shower it will be. During heavy showers, precipitation will often fall as thick raindrops or as hail. The heavier the shower, the greater the precipitation intensity and the hailstones. Hailstones can be a danger, rain is often just annoying. It only gets really annoying when your tent is right next to a river…

When you are located high enough or the upper air is cold enough, the precipitation can also fall as snow. This happens very regularly in the Alps, even in summer. Take into account snow above 2500 meters in the Southern Alps and from 1800 meters in the Northern Alps, as worst-case snow line in summer. Above 3700 meters, almost all precipitation falls as snow or hail, regardless of whether you are in the northern or southern Alps, and whether the shower is heavy or not. There are exceptions: its been known to rain on top of Mont Blanc, during extremely hot weather events.

A typical temperature difference is 6 degrees per kilometer (0.6 per 100 meters), but can be more than 10 degrees per kilometer. In winter (mainly, but also in the morning) it also happens that it is warmer on top of the mountains than in the valley. The coldest places are the high valleys, where you also have the greatest chance of waking up with a pack of snow.

If you get stuck in a snowstorm, it’s best to find shelter. When it snows and the wind has picked up, it can cause white-out conidtions. You can’t distinguish horizon from ground and have no orientation marks. Stay where you are and prepare yourself as best as possible for what is to come. Try to suppress the urge to keep going. There have been reports of people dying in fog or snow just yards from a hut that could have protected them – and that they might have reached if they would have waited a little bit.


A thunderstorm is just annoying at best, deadly at worst. This mainly depends on your environment and your own common sense. Standing on top of a mountain top with a thunderstorm, for example, is not a good idea. Try to take cover during a thunderstorm. If that is not possible (on a bare mountain slope for example): make yourself as small as possible (squatting on your haunches) and keep as little contact with the ground as possible. The advantage of a thunderstorm is that you may not see it coming quickly, but you often hear it well in advance.

Thunderstorms arise in unstable air as a result of strong updrafts: warm air rises rapidly and the moisture in this warm air condenses into clouds. This so-called “updraft” is easily recognizable, but at that moment there is already a storm. And another condition is that you must have an unobstructed view. A mountain that takes away your view is therefore “difficult”.

Recognizing a (forming) thunderstorm

So we are actually looking for another method to determine whether it can lead to heavy showers with possible thunderstorms. As mentioned, these showers arise in unstable air. An unstable air means a lot of moisture and heat and with that a lot of “potential energy”.

When this energy is used, showers are formed. You can often tell how humid the air is through so-called thunderstorm indicators: Altocumulus castellanus. This has limited predictive power: it is especially if these clouds already form in the morning that you can infer something: there is change in the air and there is energy in the atmosphere. Other “sniffs” are airplane trails that don’t disappear and turn the whole sky into a milky white. This indicates a lot of moisture.
In this “convective” variant, showers mainly occur in the afternoon.

In the mountains you often have to deal with a so-called stratiform variant: due to the location of mountains (and valleys), the air (and possibly moisture) is pushed upwards. As a result, less convection (rising air as a result of rising temperature) is required. This also causes the typical “fair-weather clouds” around the tops of the mountains. They cause their own weather and weather systems – and are not always fair-weather clouds. Less forcing is needed to create showers.

Fog in the mountains

Fog in the mountains is very different compared to low-land fog. In the low-lands, fog is created by the cooling and saturation of the air, which creates a cloud on the ground. In the mountains, fog consists of real clouds, which are often much thicker. Visibility may be reduced to less than 2 meters. This can be dangerous, especially on snow-covered ground or a glacier: disorientation is often the result.
Do the same here as with snow: nothing. Stay where you are, only move on when you can see where you are and where you are going again. Keep in mind that mountain fog can be accompanied by storms and snowfall and so is not a sign of calm weather, as is often the case in valleys and low-lying areas.

Inversion in the mountains

A very characteristic phenomenon in the weather, anywhere in the world, are inversions. An inversion means that in a certain part of the atmosphere the temperature course works exactly the other way around. Normally it gets colder as you get higher in the lower layer of the atmosphere (troposphere up to +/- 11km).
In an inversion, it gets hotter the higher you go. This is especially noticeable in windless weather in both summer and winter. During the summer, however, they are mainly morning and night inversions, while in winter they can always occur.

Cold air tends to descend, while warm air tends to rise. The cold air descends to the ground, while the warm air tries to escape. However, because of the colder layer of air higher up in the atmosphere, the warm air is trapped: an inversion occurs. These inversions are seldom thicker than a few hundred meters and are especially a problem above large cities due to the smog formation that arises.

In the mountains, inversions actually occur in the same way: at the end of the afternoon cold air thunders down the mountain slopes, causing it to cool quickly in the valley (even thought the descending air is heating up by compression).
The upper layer of air remains cold due to the radiation (clear weather is often a characteristic of an inversion), so that the warmer air can no longer escape from the valley. Halfway up the mountain you will find an inversion. See schematically the situation of Chamonix below.

At the end of the afternoon the sun sets behind the mountains, cooling the air. Cold air is heavier and will descend.
Cold air is flowing into the valley: katabatic winds are starting to flow as the colder air is moved by gravity downhill.
The inversion occurs (depending on the circumstances) between 1600 and 2300 meters altitude. For example, on December 14, 2007, it was -11°C in Chamonix, while at 2330m it was -5°C. So this is 6 degrees warmer, where you would normally expect it to be 8 degrees colder.
Inversion in the mountains. Colder air flows from the mountains into the valleys, causing the temperature there to sink lower than they would otherwise have done. Higher up the valley, the temperature rises. This means it could freeze 20 degrees on the summit, and 12 on the valley floor. But with an intermediate layer where it might be just -5: an inverted temperature profile.

But why is an inversion in the mountains of any importance?
When many clouds are formed in the valley (due to stoves, cars or ordinary cloud formation), they rise. Thanks to the inversion, the clouds cannot rise further than the inversion height: smog is created. The inversion thus acts as a lid in the atmosphere. At higher inversions, the resulting sheep clouds can string together to form a spread gray cloud slurry, so that it can also be gray for days on end in an inversion.
When a cloud manages to break through the typical inversion, heavy showers often occur in the summer: once the lid is off, a cloud can quickly develop into a thunderstorm that can hang over the same place for a long time.

Subsidence inversion

The discussed inversion should not be confused with a subsidence inversion. In a subsidence inversion, a parcel of air descends, causing it to heat up and dry out. This warmer air is very clear, but keeps the clouds trapped in the lower layer. With a very mild upper air temperature, a subsidization inversion can still result in a few degrees of frost in winter, thanks to the thick package of clouds that are difficult to dissolve just below the inversion. When this occurs in low countries like Belgium or The Netherlands, the Ardennes & the Limburg hilly landscape are often bathed in sunlight and have spring-like temperatures.

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