"On Sunday (refers to the week before last [5.1.14]) it snowed wet in the south up to over 2000 m in places and after a starry night, dry powder was also to be found much lower down on Monday. (...) I have been thinking about a build-up transformation of the wet fresh snow. Is that conceivable?"
Short answer: No.
Long answer: What happens again when the transformation builds up? Let's assume we have a temperature gradient in the snow cover. This creates a relative humidity gradient and a vapor pressure gradient. When individual molecules detach from the crystals during sublimation processes, they migrate upwards along the various gradients until they meet the next crystal and dock there again. The infamous cup crystals are formed because, due to certain properties of the vapor pressure, the molecules do not dock onto the pointed arms of the crystals, but between them (perhaps these are the crystal axillary cavities?). So far so good. Bastl now assumes that a corresponding temperature/vapor pressure gradient has arisen between his moist, relatively warm fresh snow and the cold air and that the slushy snow has been transformed in a building-up manner. This doesn't work for two reasons: Firstly: When the air cools down, the top few centimetres of snow also cool down quickly. This means that there is no longer a sufficient gradient between these top few centimetres and the air and the build-up transformation as just discussed cannot take place. With very wet snow, it would be more likely that a crust would form when it is shock-frozen. Secondly, and this is the actual and more process-related reason: the sublimation process that makes the build-up transformation possible is simply far too slow to make any difference overnight in terms of downhill quality. In sunshine and normal wind and moisture conditions, an order of magnitude of 1 cm of snow can sublimate from the snow surface per day (http://www.wsl.ch/fe/schnee/projekte/sublimation_schneedecke/index_EN). In the snowpack, where the sun does not reach, there is much less energy available to sublimate snow molecules, so the process is even slower there. Another limiting factor regarding the transport of water vapor molecules in the snowpack is, according to various studies, the limited vapor diffusion in the pore spaces.
The complete publication can be downloaded here as a pdf.
So what happened at bastl?
Let's start by quoting our colleague Orakel, who announced the snowfall mentioned by bastl as follows: "In the east of the Southern Alps, it should remain snowy above about 1400-1600 m, but this will depend on the strength of the precipitation there and the magical precipitation cooling that it produces. (...) In the north, the high precipitation limits will apply until the snow line also drops below 1000 m with the onset of the cold frontal cold. 1. precipitation cooling - For well-known reasons, snow normally falls more or less from top to bottom. At the top, where the snowflakes form, it is colder than at the bottom, where it may even be slightly above zero. The snow therefore turns into rain at a certain height because it is simply too warm. If you want to melt snow, you have to use energy to do so. In this case, this energy is extracted from the warmer ambient air. If, for example in a narrow Alpine valley, the air volume is not too large and it is snowing heavily or raining below, so much energy is lost from the air when the snowflakes melt that the air temperature and thus the snow line gradually falls. 2. cold front cosmetics - It is warmer in front of the cold front than behind it, which means that the temperature drops somewhere in between, sometimes quite sharply. The snow line therefore usually drops during the passage of the front. We conclude: It cooled down at some point during Bastl's snowfall, due to precipitation cooling, or with the cold front, or both.
The current weather...
...this time the WeatherBlog leaves it largely to our colleague Orakel, who has already announced a new alarm. It won't be a lowland winter for the time being, but at least it's snowing at higher altitudes, once again mainly in the west and south.
