An avalanche is a rapid flow of snow down a hill or mountainside.

The two things that are needed for an avalanche to occur are: a mass of snow, and a slope for it to slide down. An avalanche can be caused by a variety of factors such as terrain, slope steepness, weather, temperature and snowpack conditions.

Mountain avalanches are large and the conditions that cause them are complex. Large avalanches often occur naturally when a snowpack becomes unstable and layers of snow start to fall, but smaller avalanches can also be caused by human activity – such as skiing.  

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There are three main parts of an avalanche:

 -      Starting Zone: This is the most volatile area of a slope, where unstable snow begins to fracture from the snow around it and slide. This usually happens high up a slope, but it can also happen at any point on a slope

-      Avalanche Track: This is the path that an avalanche takes downhill. An avalanche track will often be clear of trees

-      Runout Zone: This is where the snow and debris, in an avalanche, comes to a stop. The point where the snow and debris pile the highest is called the deposition zone

Avalanches are most common in the winter, but they can occur year-round.

Small avalanches made up for of loose snow sliding down a mountain are called Sluffs. A slab avalanche is made up of a solid block of snow that has cut out of its surrounding by fractures

Why are satellites useful in measuring avalanches?

Avalanches can be deadly due to their unpredictability and intensity. Each year avalanches kill more than 150 people worldwide.

Therefore, it is important for scientists to be able to monitor avalanches, so they can better understand how and why they occur. Data about the snowpack, such as thickness and types of snow layers can be combined with data about weather conditions to make an estimate about avalanche danger levels.

Earth observation can help with monitoring avalanches. In-situ and field observations of avalanches can be difficult and expensive to monitor because they often happen in remote regions.

At the moment, avalanche forecasters widely based their warning on weather models and data delivered by automatic weather stations and in-situ observations. However, these are unreliable as there is often a shortage of weather stations and poor weather frequently disrupts in-situ observation.

Norut Northern Research Institute in Norway uses ESA’s Sentinel-1a satellite to detect and monitor avalanches. They have developed an algorithm that can automatically detect avalanches from satellite images.

They developed a near-real time processing chain that automatically detects avalanches using Sentinel-1 images and processes them into vectorised images that show the release timing, locations, topographical conditions (such as slope aspect, angle and elevation) and morphological features (such as area, length and width) of an avalanche.

Using satellite data means that large regions can be monitored consistently over a long period of time to build up a much richer understanding of why avalanches occur.

The trigger for large avalanches is often unexplained. For example, on 17th July 2016, there was a massive avalanche in Rutog country in western Tibet. 60m cubic meters (the equivalent of 24,000 Olympic swimming pools) of ice and rock killed nine herders and hundreds of sheep and yaks. The debris from the avalanche covered 10sq km. Events on this scale are rare, but scientists have linked it to the 0.4°C per decade increase in temperature in Tibet, which is double the global average. This illustrates the importance of being able to track and predict avalanches in order to prevent the loss of life and livelihoods.