SIGNS OF BAD WEATHER IN THE MOUNTAINS
The weather has a strong influence on the experience of your outdoor activity, especially in the mountains. In the Alps, you could easily be surprised by unpredictable weather, even if it seems to be calm and sunny at first sight. For example, you could suddenly be confronted with thunderstorms, or get in an unexpected blizzard during winter. Read this article for some basic weather knowledge to be prepared when going outdoors!
High and low-pressure areas
The basics of meteorology are high and low-pressure areas. Close to a high-pressure area, one can often find calm and sunny weather. In this case, the atmosphere is relatively stable, and rapid changes are not expected.
The opposite is the case close to low-pressure areas, where it is often rainy and windy. These weather systems mostly come from the Atlantic Ocean and are located on the border of warm and cold air masses. These air masses are separated by fronts, which bring changeable weather.
Warm and cold fronts
In this figure, an example of a low-pressure system is shown. On the northern hemisphere, these rotate anticlockwise. The red line indicates a warm front, which is often accompanied by light rain or drizzle. Mostly, the visibility is poor close to warm fronts. Also, the temperature increases a bit. After the passage of a warm front, there could be a short period of mild weather, but often a cold front is following soon.
A cold front (blue) is often associated with (intensive) rain showers and arrives more rapidly than a warm front. Behind the cold front, the air is sensibly colder. During winter, the freezing level drops, and rainfall could suddenly change into snowfall. Often, some showers still develop behind the cold front, but in general, the visibility improves. When the faster-moving cold front passes the warm front, both are merged together into an occlusion front, shown in purple. The accompanied weather is a mixture of both types of fronts.
Bad weather in the mountains
It can be uncertain if, or how fast, a frontal system will pass by in the mountains, as the clouds could be blocked by the ridges. A lot of precipitation can fall on one side of a mountain ridge, while it is dry on the other side. This fickle weather characteristic may result in incorrect weather forecasts.
Apart from fronts, showers can also develop locally without fronts. This often occurs on warm summer days, when thunderstorms appear in the course of the day. Often, this happens very rapidly.
During your outdoor activity in the mountains, it could be useful to observe some early signs of bad weather yourself. Before the warm front arrives, high veil clouds become visible and get thicker (picture below). Also, the wind will become stronger. The clouds, typical at cold fronts, are cumulonimbus clouds. These are shown in this picture and can grow high into the atmosphere, because of the associated vertical motion of air.
Cumulonimbus clouds are also a warning sign for thunderstorms on warm summer days. These could form rapidly in the mountains and before you know it, the sky is rumbling. However, if you see the shelf cloud of the figure below, you know what is coming and you probably won’t need a meteorologist’s warning!
So, keep an eye out for these warning signs. You don’t want to get caught in a thunderstorm or heavy shower. Especially in the mountains.