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  1. #21
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    Hi Karel.
    welcome in this forum, on behalf of all of us.

    Talking of Gerolstein crater I think you're right. In mountain areas the frost hollows placed at higher altitudes normally can cooling the air that is colder than lower altitudes (during a cold wave) but on plain of central Europe, following a cold wave, the first hundreds of meters have the same temperatures and often the first 50-200 m are the coldest.

    With regards to your dobt about the "opening" of this topic, let Stephan to decide...

  2. #22
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    Citazione Originariamente Scritto da Bruno Visualizza Messaggio
    Thank you Stephan to share the interesting animation and the beautiful and useful program (I've just downloaded it).

    Talking again about the effect of the cold air avalanches, in our "Buse di Collato" frost hollow (2276 m), placed on Pale di San Martino plateau, there are normally 4-6 air avalanches during an undisturbed night. See following images ("Sponde Alte" is an adjacent frost hollow a little higher - 2399 m - that is normally very sensitive to the synoptic winds)
    Dear Bruno,

    The Sonne v4.01 from www.helson.at is really helpful, I appreciate the work of the programmer! Unfortunately the export functionality for plots is limited but hey, it is free...

    Thanks for your elaboration on the cold air avalanches. Do you have an explanation why cold air accumulates over quite a long time (from 1h to 2 and more hours) until the parcel accelerates downslope?

    Best regards, looking forward to the next (not so massive) cold spell in the coming week...

    Stephan

  3. #23
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    Citazione Originariamente Scritto da Karel Visualizza Messaggio
    Hi all,

    It's very nice to follow the work that Stephan does in Switserland. Nice frost hollows with all different characteristics. I am curious for further measurements in hopefully perfect conditions. It is good to see how every frost hollow is different and that makes it very difficult to predict the potential. Thanks for sharing the sun diagram. You use several good tools to study the frost hollows very well. I learn every day!

    Hopefully the coming weeks will give new possibilities. Unforutnately I don't have frost hollows in Belgium but there is an interesting crater just over the border in Germany, in Gerolstein. There was an interesting day on January 22nd with -13.0 degrees on the top of the crater and -24.8 degrees at the bottom that lies 14m lower. The doline lies a little too high in the landscape on 500m altitude. I think this takes some degrees away to be even colder. The same day I had in a valley in Neidingen (Near Sankt-Vith, Belgium) -22.9 degrees on almost 400m. I wasn't aware that with more then 20cm of snow, normal valleys can also be very cold.

    I hope this information in this topic is not open to everyone? The work that Stephan does must be protected.

    Have a good evening,
    Karel
    Hi Karel!

    Nice to meet you here ;-)

    Thx for your flowers - the two frost hollows in the Alpstein performed very well on January 6th and 7th, especially Hintergräppelen is very promising and seems (until evidence to counter is produced) to be the coldest place in Northeastern Switzerland.

    Interesting values from Gerolstein! -24.8 °C is not bad and is probably not the end of the story... Extremly low temperatures are also possible in lowland areas. There is a report in the "Annalen der Schweizerischen Meteorologischen Zentralanstalt 1929" discussing the extreme cold spell in 1929. The lowest temperature measured in the plains on the north side of the Alps was -35 °C on February 12th at the airport in Dübendorf (just outside of Zurich).

    Thanks for your care regarding my work. I see both sides:
    • There are apparently people working in frost hollows who made bad experiences (I remember vaguely examples in the French Jura), but Bruno mentioned that he never had problems within the extensive network in Triveneto. I avoid to provide exact coordinates but if there is a criminal intent, then even a rough statement of place is enough...
    • On the other hand I try to give enough information in order to make my measurements and results transparent and comprehensible. I strongly believe that a measurement and data is useless without the belonging metadata.


    Best regards, Stephan

  4. #24
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    Citazione Originariamente Scritto da Stephan Visualizza Messaggio

    Do you have an explanation why cold air accumulates over quite a long time (from 1h to 2 and more hours) until the parcel accelerates downslope?
    Dear Stephan,
    the cold air avalanches has not been (well) studied and monitored yet, I would like do it, sooner or later.

    As you have probably read in my report “Le fabbriche naturali del freddo” cold air avalanches is formed on slopes, covered by snow, with at least 25 degrees grade (interesting…almost the same for the snow avalanches) and 150-200 m long, placed above the cold air pool. This what I have noticed in a frost hollow monitored for one winter.

    The strong radiative cooling of snow cools down slowly also the air just above it and a “cold air slab” gradually forms (I don’t know what are its sizes…may be depth of 2-5 m?). When its mass/weight reaches a critical value, the “slab” fall down. It probably happens same as you have a very steep glass roof and, during a snowfall, the fresh snow glides whenever it reaches a depth, for example, of 5 cm.

    But there are a lot of things to know and discover regarding the cold air avalanches...

    Kind regards

  5. #25
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    Dear Stephan and Karel, again a welcome from me in our forum. Sorry for my english, I hope you understand.
    I follow the project of the monitoring temperatures in frost hollows since 2008, and always there are something to learn. The sites bring surprises when do you not really await. In particular few days ago I spoke with Bruno, about the frost hollow Avalina Regnalt, at 1964 meters above sea level, the minimal temperatures was -35,9°C, but the depth is only 8 meters, Avalina Regnalt has a great sky view factor. http://forum.meteotriveneto.it/showt...TN)-1964-m-M3V
    Stephan, Hintergrappelen is very interesting -38°C at this altitude.
    Which instruments do you uses Stephan?
    I do not know if you have got to talk whit Bruno, about the monitoring on Pale di San Martino, but for years we installed at Busa di Manna a datalogger on the bottom, on one side and one on the sella of outflow.
    In future we would also monitor the wind, but for now the costs are quite high.
    About the bad experiences, in 120 monitored sites, only two times I have suffered the theft of dataloggers, always in the same place, sometimes we lost the equipment due to excessive snow.

    Best regards.

    Giampaolo.
    Freddofili si nasce "dolinatori" si diventa


    stazione online con Weather Display Live
    http://www.meteolevicoterme.it/Dati-...ne-Online.aspx

  6. #26
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    Dear Giampaolo

    Don't worry about your English, I think I understand you perfectly and my English is far from being perfect!

    Your observation in the frost hollow of Avalina Regnalt is also covered by litterature: a good sky view factor is by far more important than the depth of a frost hollow (e.g. http://journals.ametsoc.org/doi/pdf/...A%3E2.0.CO%3B2).

    Regarding the instruments I use: Bruno's report "Le fabbriche naturali del freddo" was very inspiring for me. For quite a while I was thinking about working with HOBO Onset dataloggers, which have a very good reputation. A colleague of mine who is also working in frost hollows suggested me to try Tinytag dataloggers from Gemini because he made very good experiences with this loggers.

    Now I have two types of loggers in use:

    http://www.geminidataloggers.de/data...lus-2/tgp-4104 together with http://www.geminidataloggers.de/prob...es/pb-6005-1m5 at the bottom of my two frost hollows. They have a measurement range down to -50 °C.

    For tests campaigns and at the upper part of my frost hollows I a using http://www.geminidataloggers.de/data...talk-2/tk-4023. They are small and a little bit cheaper, measurement range goes down to -40 °C. The also fit in passive radiation shields like Davis 7714 or http://www.geminidataloggers.de/acce...other/acs-5050.

    Wind would also be perfect... If my financial possibilities would allow it then this type of weather station would be an option: http://www.lufft.com/de/produkte/all...sorik-8370u01/

    It is good news to hear the you have so few problems with vandalism or theft. Do you have an idea why it happened twice at the same place? Did you took protection measures?

    Best regards, Stephan

  7. #27
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    Citazione Originariamente Scritto da Bruno Visualizza Messaggio
    Dear Stephan,
    the cold air avalanches has not been (well) studied and monitored yet, I would like do it, sooner or later.

    As you have probably read in my report “Le fabbriche naturali del freddo” cold air avalanches is formed on slopes, covered by snow, with at least 25 degrees grade (interesting…almost the same for the snow avalanches) and 150-200 m long, placed above the cold air pool. This what I have noticed in a frost hollow monitored for one winter.

    The strong radiative cooling of snow cools down slowly also the air just above it and a “cold air slab” gradually forms (I don’t know what are its sizes…may be depth of 2-5 m?). When its mass/weight reaches a critical value, the “slab” fall down. It probably happens same as you have a very steep glass roof and, during a snowfall, the fresh snow glides whenever it reaches a depth, for example, of 5 cm.

    But there are a lot of things to know and discover regarding the cold air avalanches...

    Kind regards
    Dear Bruno

    Very interesting! What kind of monitoring setup did you had in this frost hollow where you monitored the cold air avalanches? Do you also work with thermal imaging cameras or smoke experiments?

    I agree with you that there seem to be certain analogies with snow avalanches. On the other hand: material properties are quite different (eg. viscosity)... do you have any hints regarding literature which you can recommend?

    Best regards, Stephan

  8. #28
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    Citazione Originariamente Scritto da Stephan Visualizza Messaggio
    Dear Bruno

    Very interesting! What kind of monitoring setup did you had in this frost hollow where you monitored the cold air avalanches? Do you also work with thermal imaging cameras or smoke experiments?

    I agree with you that there seem to be certain analogies with snow avalanches. On the other hand: material properties are quite different (eg. viscosity)... do you have any hints regarding literature which you can recommend?

    Best regards, Stephan
    Hi Stephan,
    I didn't setup any special monitoing in the frost hollow affected by cold air avalanches...I had only the datalogger at the bottom of it (that has cleary revealed the nearly regular disturbance caused by the arrival of air avalanches at the bottom).

    I already thought to the thermal camera (only thought...unfortunately my regional enviromental agency isn't interested to the pure scientific research) but I had a doubt regarding its capacity to detect the differences of temperatures inside the hollow. What do you think about?

    More useful could be a smoke experiment that could be "make visible" the air avalanches.

    A workable experiment can be done (and I would like to do it, sooner or later, as I said) is using velocity/direction wind sensor (data every 15-30 seconds) in two sites along the slope, both coupled with temperature/humidity datalogger (data every 15-30 seconds). Regarding the temperature along the slope I also expect to see a sudden increase in temperature immediately after the start of avalanche and then a new drop.

    Talking of the specific literature I didn't ever find any study or experiment about this topic, in my opinion there is an empty place in this sense. I found only some mention in mountain meteorology book, for example this one
    https://books.google.it/books?id=Mz_...hes%22&f=false

    We have to fill this empty place!

    Bye

  9. #29
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    Dear Bruno

    Just one short link: http://www.conrad.it/ce/it/product/1...e&rt=home&rb=1 --> I would love to make some tests with this gadget... it is not a professional camera, but the price-performance ratio seems quite reasonable...

    More in the next days!

    regards, Stephan

  10. #30
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    Citazione Originariamente Scritto da Stephan Visualizza Messaggio
    Dear Bruno

    Just one short link: http://www.conrad.it/ce/it/product/1...e&rt=home&rb=1 --> I would love to make some tests with this gadget... it is not a professional camera, but the price-performance ratio seems quite reasonable...
    Thank you for the link, Stephan.
    I suspect that thermal cameras, in our case, detect the temperature of the upper layer of snow and not the air temperature,
    Very cold air is very dense and has a behaviour somewhat similar to the water, but is transparent for the thermal cameras, in my opinion

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