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SnowFlurry 21 2016/17 | Things to know about the spring situation

The avalanche situation in the Eastern Alps is heading towards a spring situation

by Lukas Ruetz 03/23/2017
Moisture penetration and nocturnal radiation now become the primary avalanche-forming factor. What about temperature reserve, original layer sequence, humidity and isothermality?

The temperature reserve and spring situations

Snow only melts as soon as it has warmed up to 0°C. However, snow layers have different temperature ranges due to their varying initial temperature when snowing in, in other words, they store cold. After the first warm spring days, the snow cover slowly begins to warm up from the surface. The temperature reserve is therefore slowly replenished and the snow heads towards 0°C. This happens relatively slowly due to the high insulating capacity of snow in deeper layers. While the surface is already heavily soaked, deeper layers can still have reserves of, for example, -10°C.

This explains why a spring situation from an avalanche perspective, i.e. a completely soaked snow cover from the ground to the surface, must first form and requires some time with heat input. These are often the last days in March or April, when you can go on a ski tour all day and only observe a very slight to non-existent increase in danger during the day due to warming - even though you already feel like you're in an infrared cabin during the sporting activity.

The already superficially moistened snow cover can form a snow cover on the surface, but this is stored on a still "cold" or only very slightly moist foundation. If you break through such a cover, this has nothing to do with the increase in danger in a "classic spring situation".

Before this classic spring situation occurs, however, existing weak layers, which may already be classified as "benign" again (because they are now better connected again), become relevant. This is the first stage of the overall avalanche spring situation: the increase in danger due to the weakening of existing weak layers as a result of initial moisture ingress. As a result, the bonds between the crystals become weaker again and the triggering readiness increases again. In addition to storms and heavy snowfall, the most active time for avalanches in winter is usually when the snowpack is first soaked through to the ground and the associated weakening of old weak layers.

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Once we can no longer recognize the initial layering of the high winter snow cover, but instead find a fairly uniform mass of melting forms, we speak of the "classic spring situation". The classic spring situation is characterized by a continuous snow cover consisting of melting forms, where the formation of a harsch cover at night, i.e. a superficial solidification, determines the avalanche danger. Underneath the snow cover, you will find snow melt - i.e. melt forms with a high water content - down to the ground. If there is no superficial snow cover, it is referred to as a "swamp".

The spring situation is therefore divided into a first part, with the increase in danger due to the first soaking of old layers, and a second part, the classic situation.

The isothermal snowpack

The term "isothermal" actually describes a snowpack with a completely uniform temperature, i.e. the same prevailing temperature from the first layer on the ground to the snow surface. In theory, this can be -5°C, -11°C or -2°C throughout. In practice, however, a snowpack is only described as isothermal if 0°C prevails throughout, i.e. the snowpack no longer has a temperature reserve. One reason for this is that there are normally no layers of snow near the ground that are significantly colder than 0°C due to the ground heat flow. An isothermal snow cover at a temperature range other than 0°C is therefore hardly possible due to the layers near the ground, which have already warmed up to this point anyway.

What remains for the term "isothermal snow cover" is the continuous 0°C, i.e. the melting point - this is where the snow cover can remain almost constant over long periods of time (weeks to months). If the energy balance remains positive, i.e. the supply of heat continues, more and more snow melts. As a snow/water mixture cannot become warmer than 0°C, the snow cover remains at this temperature until it has melted away. Although there are also influences from above (atmosphere) and below (ground), the temperature no longer changes, only the water content and the thickness, because only more heat is added.

The water content (liquid water content, LWC) can reach about 15 percent by volume, after which the water begins to flow away at the very latest, i.e. to seek paths vertically and horizontally down to the ground.

Only during night-time radiation do the uppermost 20 cm cool down again. The water content in the mixture of ice and water that is now present freezes again and a superficial layer of snow forms. This is only slightly colder than 0°C. A change in the weather can only allow the snow cover to freeze completely again with very long-lasting, cold temperatures, or even "replenish" the temperature reserve. In practice, this hardly ever happens, as long-lasting cold spells with temperatures well below 0°C rarely occur in spring. In addition, fresh snow usually falls first due to the cold front. This fresh powder snow provides excellent insulation for the snow underneath. This means that the cold air that follows can no longer cool the soaked old snow cover

The thaw

The term "thaw" is generally only used to describe a weather situation with warm temperatures that leads to thawing. In snow and avalanche science, it is understood to be warm and humid "dirty weather". The higher the humidity, the more the snow cover can become soaked and subsequently soaked and melt. The daily reduction in snow depth at warm temperatures combined with high humidity is many times greater than at warm temperatures but dry air. If there is also rain and/or overcast nights, you can almost watch the snow cover melt away. In thawing weather, it is not just the air temperature and solar radiation that work together to melt the snow - the two join forces with a few accomplices: The increased heat input of diffuse radiation, the lack of heat radiation and thus cooling of the surface, the lack of cooling due to weak evaporation and sublimation on the surface and over time. In dry, high-pressure weather, the energy balance of the snow cover is negative at night; it only continues to melt during the day. In thawing weather, the snow cover melts continuously 24 hours a day and night.

"The accumulation of snow cover" in cool, dry high-pressure weather

After extensive soaking or soaking of the snow cover at many altitudes and exposures, a cold, fair weather phase with extremely dry air follows. The energy balance of the snow cover can thus be negative in total from day and night. The radiation (which of course also takes place during the day) is extremely strong due to the low humidity and the cloudless sky, in addition there is a high proportion of evaporative cooling or high energy loss due to strong sublimation on the surface, also due to the low humidity. In addition, the air temperature is a few degrees below the snow temperature, i.e. below 0°C. This means that the radiant energy supplied by the sun is not sufficient to completely soften the existing snow cover from the night, as all other parameters in this case cause the snow cover to cool down rather than warm up. This cover becomes thicker and thicker night after night, so the moist snow cover continues to freeze through to deeper layers. The temperature reserve slowly fills up again from top to bottom. This allows a dozens of centimetres thick layer of snow to form - which does not soften despite the fine weather, meaning that the risk of avalanches hardly increases during the day.

If such a period is followed by "normal" spring weather again with warmer, above all more humid air, or simply cloudy weather with diffuse radiation and a lack of radiation, it can take a few days for the very thick layer of snow to become completely soaked again from top to bottom and thus bring with it a daily increase in avalanche danger.

Case studies

Cool, dry, high-pressure weather at the end of April 2015

A classic spring situation with a sharp daily increase in avalanche danger was followed by a few days without a daily increase in danger: in the morning, cool temperatures and humidity reminiscent of ski tours in the Central Andes meant that even south-east-facing slopes were still slithering down on a frozen surface. In the afternoon there was the best firn on steep south-facing slopes. There was nowhere to break through all day. The snow cover on the slush got a few centimetres thicker every day and reached a thickness of over 30 cm.

Thaw in early April 2016

On 1 April 2016, the first massive warming of spring arrived. Summery conditions prevailed for a week, which caused the sparse snow cover to melt away in combination with humid air and overcast nights. Within 24 hours on the first of April, the snow quality changed from "spring conditions with a load-bearing snow cover with a slight increase in danger during the day" to "bogging down to the knee or hip area up to altitudes of 2500m".

Harsch cover formation in the morning in a classic spring situation on 23.3.2017 in the Allgäu

In the morning hours, the arrival of significantly warmer, drier air masses led to a rapid decrease in relative humidity with a simultaneous increase in temperature. Despite the first rays of sunshine and rising temperatures, the snow cover was able to harden quickly during the morning. On the ascent, there was broken snow on the swamp, but on the descent, the cover was just fine. Thanks to Kristian Rath for describing this example.

Note: Humidity has just as great an influence as temperature, as it has a strong impact on radiation in and out. In spring, there does not necessarily have to be an avalanche spring situation with a pronounced, diurnal increase in danger.

This article has been automatically translated by DeepL with subsequent editing. If you notice any spelling or grammatical errors or if the translation has lost its meaning, please write an e-mail to the editors.

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