Wednesday, January 21, 2009

What Happens When Snow Doesn't Accumulate?

Great comments on the blog and I love that you are interacting, trying to help answer each other's questions.

So Greg in Tennessee wrote and said it flurried almost all day, but never accumulated in his rain gauge -- why?

Well my best guess is exactly what John wrote and said in his comment.

  • Either sublimation took place, meaning conditions were just so that the solid snow flurries turned instantly into a gas (water vapor) without ever melting and becoming a liquid

  • Or the wind took the snow flurries away from the gauge

    My best guess, since we are talking about Tennessee -- would be the wind scenario.

    Usually for sublimation to take place you need very cold and very dry air. I doubt it was that dry in Tennessee, but I could be wrong.

    Click here for a link with more info on sublimation.

    Regardless of what happened, if you witnessed snow flurries that never accumulated, either in the gauge or on the ground, you still would report a TRACE of snow, and I would also report a TRACE of precipitation.

    Snow doesn't have to end up in the gauge to be recorded. If you saw it fall, it's a TRACE.

    And then in the comments, record that it flurried all day but never accumulated. If you can, also comment on the general weather, mentioning things like the wind.

    20 years down the road, if someone was doing a paper and ran across your data, it'd help them tell the story.

    And it never hurts to document snow flurries in places that don't see too much of the white stuff!

    WXwatcher from Missouri has been checking out snow conditions in his area at the web site for the National Operational Hydrologic Remote Sensing Center.

    He was asking if much of the country is below normal for snow?

    And without pulling any official data, and just going off what I have observed this season, I would venture to say yes -- with the exception of the northern edges. (i.e. Washington, North Dakota, and the Great Lakes states)

    I know Spokane, Bismarck, Marquette and Chicago have all had snowier than normal winters so far.

    Where places south of there, such as Denver, are well below normal.

    But there are still a few snowy months to go so we'll see how it all shakes out.

    The current poll is really getting an interesting response -- when I looked just now it was a 3-way tie!

    And in just one day we've had half the number of votes that the last poll got in an entire week!

    1. Here in Northeastern Washington State (Lincoln County especially) we’ve been “under” a high pressure system with air stagnation. On top of that freezing fog build-up on power lines has left some without power going on a week now.

      The question is when we get a wind there are frozen fog particles that do accumulate on my snowboard and I’ve had some in my gauge too. Do we count this as a trace?


    2. Hi Bob-

      I had a similar question to Nolan back in December when we were getting really, really damp fog roll in. Here's his answer:

      "Deposition in the rain gauge from fog is commonly handled as measurable and reportable precipitation even if no rain drops are observed. This is in comparison with dew and frost that form from condensation from clear skies. So if you are in a fog and water is collecting and dripping from trees, etc, this situation would result in your reporting what you saw in your gauge as Precip."

      So my conclusion with either freezing or liquid:
      If there's water in or on the guage:
      If the sky was clear, it's zero. If the sky was cloudy or foggy, it's measureable.

      Of course, how clear is clear? This morning we had very light overcast ( I could see the sickle moon), and frost in / on the guage.

      I reported zero, and commented on the frost.

      Personally, I think it would be more appropriate to report EXACTLY what we find in the guage regardless of other conditions or history. Maybe create new fields to indicate whether any precip or cloud was observed in the past 24 hrs.

      I think these questionable "trace" situations would be really good to clarify in the training slides (hint, Chris!), since I think they're more common than not. Evidence Nolan's latest post where the two most common readings are 0.0 and T!

      What about a quick reference guide for established observers? The training is great for promo and setup, but it would be easier for me if there was a quick reference document that answers questions such as this.

    3. Thanks for answering my question in your blog. It's been pretty cold here (below 32 for a high) for a couple of days but I don't think you could consider it dry :-) . It has been windy so that must be the answer. In the future, I will report these as Trace as you suggested. Thanks again for the post.

    4. Last I heard, we here in Central NYS were about 16" ahead of normal for snow. I think I heard that back in December--not sure where we are now. Of course we are a Great Lakes State--but here in Central NYS we are not quite as subject to the Lake Effect Snows as they are just east of the lakes. We get it when the winds are coming from the north or northwest.

      Which brings me to my question--are we seeing a higher than normal number of clipper systems this year? Or am I just paying closer attention to them? It seems like every 2-3 days for the past month or more we've had another one. They bring very cold air, and often snow although very light fluffy stuff that doesn't accumulate a lot.