The incredible dryness of cold

Frosted gingerbread roof

I’m not about to express surprise at the fact that it’s cold in winter. By the standards of a LOT of people I can think of, the temperatures we’re facing this week – an average of about -7°C during the day – are temperate, mild, even balmy by comparison with quite extensive other parts of the world.

But what never ceases to amaze me in Southern Germany is just how dry cold can be. Having spent my formative years in the insular climate of lowland areas of the UK, especially on the coast, I’m used to cold being accompanied by wet. I expect a daytime thaw even when there’s snow on the ground, and so the fact of the snow just staying the same for days on end, simply because it remains frozen and can’t go anywhere or change its consistency, is quite weird. It strikes me as eerily Groundhog-Day-like, so maybe it’s appropriate that I’m writing this post today of all days!

What’s struck me particularly today is that even though there is absolutely no snow or visible patches of ice on the streets here in the middle of the city (the picture above is of the North-facing roof of the house), the street surfaces nevertheless have an odd look to them, a pale, dusty, almost mildewed-looking grey. It’s like a thin salt crust or sprinkling of talcum, very dull and matte, and not in the least slippery. I’m guessing the tiny moisture droplets in the air are simply freezing on contact with the frozen ground and causing this strange effect. It’s definitely not salt as it darkens and then disappears if you warm it.

It’s odd how little details of a place’s climate can astonish you even after many years of living there, but at the same time it’s pleasing to be able to marvel at something that must strike many other people as ordinary or insignificant.

Postscript: Following on from a discussion of this with Rolf on Twitter, I have now been reminded that this phenomenon is called deposition or desublimation – in this case the direct transformation of water vapour (i.e. gas, not droplets) into ice in sub-freezing conditions. Thanks Rolf!

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9 Comments

Filed under Intercultural & interlinguistic, Out & about

9 responses to “The incredible dryness of cold

  1. Hi, Bexxi!

    I know just what you mean about the difference between a “wet cold” and a “dry cold.” I grew up in the southern part of the United States (Mississippi) and then lived up north (Michigan) for six years. Talk about a contrast in climate!

    I love the image you’ve posted. I always look forward to seeing your work when you share it.

    ~Bryant

  2. 30 km’s west of you it seems much more wet and a fair deal of snow as well.

  3. I might be wrong but I think the grey crust on the downtown streets that you describe as “like a thin salt crust” is actually that—a crust left from the salt used for thawing the snow.

    • Thanks for the comment – that was my immediate thought, but I was then surprised when it went briefly darker and disappeared when I warmed it up just slightly. There is often a lot of salt on the roads when it’s been snowy or icy, but thus cold weather came from nowhere and what I described had a different quality and was very finely distributed.

  4. Kelly

    We just call it frost.

  5. Ok, my promised update. It’s salt(s).

    I hadn’t seen this in ages and so thought of desublimated water (ice formed from water vapour in the air) in my tweet. But then I visited friends in Bremen and saw it there on all the roads.

    Some observations:

    1.) It’s everywhere, no difference between shadows and sunlit areas. Ice should sublimate away quickly in the sun and stay in the shadows longer.

    2.) It’s only on the road, not on private parking lots with similar surfaces. Physics shouldn’t care about property lines, municipal workers should.

    3.) The white stuff is creeping up from the road surface into the first meter or so of crossing red brick paved bike paths and fades away there. No reason why there should be a different

    4.) It was everywhere in Bremen, nowhere in Berlin. Basically the same weather, perhaps a bit dryer in Berlin.

    So it can’t be ice. But what else?

    There are different ways to de-ice a road surface. Here in Berlin they use a solid salt or a mixture of salt and grit. Bremen seems to be a bit more variable in its means, they also use a spray on salt solution. It spreads evenly on the road surface and has not to be distributed by the car tires. And it uses less salt per area. But it’s only working with a little bit of ice or snow. With more on the road they also use the solid stuff.

    The ice melts because of the freezing point depression caused by the salt. The resulting liquid spreads on the road surface and is carried away by bike tires onto the first meters of the bike paths. Then the dry air sucks up the water and it evaporates away. Left behind is a thin salt crust made up by very tiny crystals.

    But why is this sensible to heat? Well, it isn’t. By touching it you give off enough water (70% of you is water) to solve these crystals. Even more water is deposited by blowing on the surface. But a hair dryer should have no effect. Would be a nice experiment, but I had neither a hair dryer nor an electrical outlet in reach.

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