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.
