Snow and weather
The influence of temperature, and particularly substantial changes to temperature, on avalanches is a complex process that is still under debate.
An excerpt from the book ‘Lawine. Die 10 entscheidenden Gefahrenmuster erkennen’:.
For too long in avalanche science it was assumed that large temperate changes during snowfall (warm to cold or the other way around) have a positive influence on the avalanche situation. This is only the case under certain circumstances. Generally such a temperature change has a negative effect because it causes a change in the construction of the snowpack: a weak, soft layer can come into existence. These layers are often found in terrain with southern expositions. It's dangerous because the weak layer is not directly present after the snowfall and only develops in the days after.
In short, temperature has a huge influence on the snowpack and therefore contributes to the creation of avalanches. The problem, however, is that the influence is extremely complicated and the exact consequences are difficult to predict.
The temperature of the snowpack plays a larger role than the air temperature. The snowpack itself is not susceptible to changes in the air temperature. Especially in shaded areas. This has to do with the excellent insulating quality of snow. This is why weak layers located deep in the snowpack can exist for weeks or even months during winter.
Air temperature does not affect the snowpack because of the insulating quality of snow. These elements have the most power to heat up the snowpackl:
- Relatively warm air and clouds. The clouds transmit warmer, more humid air into the snowpack
About the sun
Rain and clouds influence all aspects of the mountain. This is not the case for the sun. The path of the sun is a consistent one. She rises in the east, moves south and sets in the west. The winter sun doesn't hits northern faces leaving them shaded. Northern faces stay cold for longer periods. This helps create weak layers and preserves them for longer periods. Northern faces are generally more dangerous when it comes to avalanches than southern faces.
A run which is square to the sun will be exposed to more powerful solar radiation than a lower angle run. In other words, a steep south-facing run will be affected more by the sun than flat runs during winter. That also explains why tree runs are less affected by the sun—the branches keep the snow shaded. When the sun gets higher in the sky later in the season this difference becomes less noticeable and flat runs more affected by the sun.
Changes in temperature can influence the avalanche hazard. The next scenarios are important to recognise:
- Temperature rises
- Temperature drops
- Stabile temperatures
- Shifting temperatures
Rise in temperature
A significant rise in temperature increases the avalanche hazard quickly. When the temperature of the open air rises more than ten degrees the snow on top of the snowpack, logically, gets heated up first. These layers become heavier. This results in more tension in the snowpack, which can increase the avalanche hazard before the weak layers in the snowpack are effected by the temperature rise.
- A notable rise in temperature causes a rise in the avalanche danger on a short notice.
- In the depth of winter the rise of the air temperature has a very little impact on the snowpack on the northern slopes. Due to the lack of direct sunlight it stays cold there.
- When is stays warm for a longer period of time the snowpack becomes wetter. This results in a weaker snowpack and a larger risk of wet snow avalanches.
A drop in temperature
A drop in temperature is usually positive for snow stability, meaning it does not heighten the avalanche danger. Because the air temperature drops, the snow temperature drops. As a result the snow freezes again. The tension between the different layers wanes and the avalanche danger decreases.
Of course there are periods in the Alps with stable temperatures. How does that influence the avalanche danger?
Continuous cold temperatures (between -5 and -10 degrees Celsius) do not change the avalanche danger. It stays the same. The bonding between the different layers in the snowpack only improve slowly or will get worse if the temperature differences in the snowpack increase.
- During continuous cold temperatures the avalanche danger stays the same.
Continuous warm temperatures (around the freezing point or warmer) have a negative effect on the avalanche hazard, because warmer temperatures weaken the snowpack. The snow becomes wetter and therefore heavier and this makes the weaker layers in the snowpack carry a larger weight. It may be clear that during a longer period of warm temperatures the snowpack loses strength.
- During continuous warm temperatures the snowpack weakens and the avalanche danger rises.
A constantly shifting temperature leads to a continuous heating up and cooling down of the snowpack. The rise in temperature leads to a heightened risk of avalanches. When the actual temperature shifts happen, the avalanche danger rises and lowers. Over time, continuous warming and refreezing of the snowpack will result in a decline in the avalanche danger.
- At first the avalanche danger rises during warmer temperatures, but after a period of fluctuations the avalanche danger declines.
A change or no change in temperature during winter always influences the snowpack and the avalanche danger. The important issue is whether or not the air temperature is actually affecting the temperature of the snowpack.