Rime
When the air freezes
What we see on the trees in the photo taken in the Three Peaks ski area in San Candido Sesto Pusteria, is not snow fallen from the sky but rime.
There are two types of frozen humidity : rime, the subject of these lines, and surface hoar, which we will discuss in a future blog.
Rime generally forms in late autumn but also in winter during a period of intense cold and presence of moisture in the air (low clouds or fog for example).
In the absence of wind, the moisture freezes all around cold objects with which it comes in contact, for example branches, grass or the snowpack itself, as seen in the photo with the trees, taken at 11 a.m. with a temperature of -10° at an altitude of 2200 meters.
If, on the other hand, there had been wind, the moisture contained in the air would have been pushed by the wind and would have frozen as soon as it touched a cold surface, thus growing upwind and giving rime that can reach even considerable size and weight, as can be seen in the photo of the weather station.
What observations can we draw from the photo of the trees?
Certainly negative temperatures in the last few days (the rime did not melt even in the hottest hours of the day and remained on the trees and ground for 2/3 days)
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Absence of wind or very weak winds at that altitude for the last 2/3 days.
Rime also deposited on grass and snowpack; normally at first warming the rime should melt but if it is covered by new snowfall it could become a small disconnection layer between old and new snowpack
What about from the photo of the weather station?
Here the presence of much moisture and wind are well evident.
The wind direction is also clear; the wind blew from right to left.
The wind speed does not allow the moisture to freeze easily on the snowpack but only by impacting colder objects.
If you would like to see the observation that is the source for this article, click the link below:
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Ernesto Bassetti
Ernesto is one of the leading snow and avalanche experts in Italy and beyond.
By passion and by profession, he has been involved in mountain safety for over 40 years, with particular attention to the avalanche issue.
He is co-founder, and until 2020 Vice President, of MND Group; important international player in the field of mountain infrastructure and world leader in controlled avalanche release systems.
He is a ski instructor, National Ski Mountaineering Instructor, former director of the Mario Righini Ski Mountaineering School.
He was promoter, co-founder and soul of the "Servizio Valanghe Italiano" (Italian Avalanche Service), of which he held the Presidency for 18 years.
