SnowFlurry

SnowFlurry 19 2019/20 | The types of spring - Part 2

Classic and non-classic spring situation

by Lukas Ruetz • 03/31/2020

Another quarantine walk. In the afternoon on a steep south-facing slope at 1500m, not even the ice melted in the All Freeze, No Melt situation last week.

Lukas Ruetz

Avalanche warning products often only refer to the "classic spring situation". But what are the "non-classical spring situations"? This time it's about the three non-classical of the four different snowpack scenarios in spring.

All Melt, No Freeze

This situation is the spectre of all winter sports enthusiasts in spring. Either the air is so warm or so humid or the sky is so overcast - but usually all in combination - that the snow surface cannot freeze on the surface even at night. The snow cover then melts not only during the day, but also at night. No snow cover forms. The snow remains moist/wet, usually slowing down and sucking on the ski base, and the avalanche risk remains at a consistently high level. As a rule, the snow is also so bad for skiing that you don't even want to go off-piste anymore.

The extreme form of the All Melt, No Freeze scenario is the thaw. In thawing weather, the dew point - a measure of the absolute humidity - rises above 0°C. The snow then stops sublimating and only melts. At a dew point below 0°C, snow melts and sublimates. At an even lower dew point, snow only sublimates. With All Melt, No Freeze, all the snow that disappears remains on the snow cover as meltwater, nothing turns into water vapor and is transported away through the air. This causes the snow cover to soak even faster. The energy input is many times higher. And that's not all: thanks to the positive energy balance, it then has time to melt even at night, not just during the day as in the classic spring situation. In thawing weather, the snow cover disappears as if in fast motion.

Even a dry snow cover from the height of winter soaks through so quickly that you soon find yourself in a swamp. You sink through a wet blanket of snow all the way to the ground. Not only dangerous in terms of triggering avalanches, but also completely fun-free on the ascent and descent!

In the All Melt, No Freeze situation, there is no daily increase in avalanche danger. It's just always constantly precarious. Days to clean up at home and go for a run.

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Penitential snow - also known as penitentes — Penitent snow - also known as penitentes

Lukas Ruetz

All Freeze, No Melt

If the air is very cold and preferably also very dry, the energy balance can be negative throughout the day, even on sunny slopes, despite strong spring sunshine. This means that the snow surface cools down considerably and remains well below 0°C. Long-wave radiation, sublimation cooling (a lot of snow evaporates in dry air and thus cools the snow surface) and low air temperatures of below -10°C prevent the snow from melting. The surface snow cover remains frozen all day, i.e. hard.

The snow cover can even freeze through completely again in the event of a sustained onset of winter with very low temperatures. This means that not only does a superficial snow cover form, but the entire, damp-wet snow cover freezes into a hard, compact stick - a complete melting crust. Then we find a completely dry, frozen and anhydrous (after all, all the water has turned to ice) snow cover, which can theoretically build up temperature reserves again. This means that the cold penetrates deeply and not only freezes the snow cover through and through, but cools it down to well below 0°C. However, this requires more than a week of freezing cold conditions, preferably without fresh snow. This is because fresh snow would insulate the old snow cover far too well from the cold air, preventing the cold from penetrating deeper. But even without fresh snow, depending on the thickness of the snow cover, it takes a long time for the snow cover to freeze through again. More on this in this SnowFlurry under "Harschdeckel-Akkumulation".

In the All Freeze, No Melt situation in spring, there is no diurnal increase in avalanche danger.

In most cases, danger level 1 prevails throughout the day. When skiing, you rattle over a hard crust that does not soften.

In the past week, this situation has sometimes occurred. WeatherBlog reported. The north-easterly current brought very cold and extremely dry air. The snow cover remained hard all day even on higher sunny slopes despite intense radiation. Finally, on March 27, more humid and warmer air moved in again and the snow surface was able to warm up again to 0°C with the combo of intense radiation + humid air + relatively warmer air and thus firn up. The SchneeStöberer can observe this from home and was not on a ski tour.

By the way: In the Alps, penitential snow is most likely to form in the All Freeze, No Melt situation. A type of snow, also known as jagged firn, which is created by the sublimation of the snowpack. Its formation can also be observed in the last few days. Some of the jags were 15 cm deep. At least for those who still have snow in the quarantine garden, like the snow pusher at home.

High winter situation in spring

Then there is the normal high winter situation in spring, which will return at high altitudes this week.

It was always relatively cold with regular fresh snow until well into March or April. The snow cover then only ever becomes superficially moist due to diffuse radiation from the clouds or direct sunlight during a brief intermittent high. A deeper soaking, which can lead to one of the three spring situations in the first place, has simply not yet taken place. You may still have to watch out for old, persistent weak layers (= old snow problem close to the ground). In most cases, however, these have already bonded well again due to the thicker snow cover in late winter and the associated degrading transformation and are no longer an issue.

The high winter situation in spring is usually more about the classic drift snow problem. The fresher and colder the drifting snow, the more susceptible it is to disruption. If it is still rather cold in spring and there is fresh snow and wind, then you simply have a high winter situation with its specific dangers. The advantage: The intense radiation warms the drifting snow very quickly and the problem is usually solved extremely quickly, often in one or two days. Sometimes even in a few hours.

The high winter situation can also develop again after a spring situation has already taken place. If it gets cold and snows for several days (in contrast to the all-freeze, no-melt situation, in which there is no or only minimal fresh snow), the soaked old snow cover freezes again, at least on the surface. The wet snow problem is then more or less solved and a classic drift snow situation arises again. Possibly in combination with a fresh old snow problem, because a striking, built-up, transformed weak layer forms at the interface between the warm old snow surface and cold new snow (large temperature difference leads to the formation of weak layers).

In the high winter situation in spring, there is no or only a very slight increase in avalanche danger during the day.

Summary

There are four different snowpack scenarios in spring with different effects on snow quality and avalanche risk:

Classic spring situation: Daily Melt-Freeze Cycles
All Melt, No Freeze
All Freeze, No Melt
High winter situation in spring

Red - Extremely dry (dew point almost -60°C!!) and cold air (partly below -20°C) is coming in. Blue - This prevents the snow cover from firn up despite the almost unclouded sunshine (All Freeze, No Melt; the gray curve of the surface temperature remains far below 0°C for days) Green: Only on 27.03. does the air become so warm and humid again that, in combination with the radiation, the snow cover firns up again in the course of the morning. — Red - Extremely dry (dew point almost -60°C!!) and cold air (partly below -20°C) is coming . Blue - This prevents the snow cover from firn up despite almost unclouded sunshine (All Freeze, No Melt; the gray curve of the surface temperature remains well below 0°C for days) Green: Only on 27.03. does the air become so warm and humid again that, in combination with the radiation, the snow cover firns up again in the course of the morning.

LAWIS/Lukas Ruetz

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