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Too Cold to Snow

Posted by Peder on 5 January 2009

Growing up in the upper Midwest, there was a little piece of household wisdom that seemed to always ring true.  When the temps were down in the single digits or colder (Fahrenheit scale, here) it would never snow.  It was too cold to snow. This was pretty much the worst that winter had to offer because the snow was often replaced by wind, and Cold + Wind = Stay Indoors. Stupid Alberta clippers.

But thanks to this presentation of snowflakes (via TYWKIWDBI) I now know it is not quite that simple.  Different kinds of snow are produced by different weather conditions, as depicted by the final slide:

Kind of interesting how they cycle between plate-like and tube-shaped flakes as you go down the temperature scale. If I’m reading this correctly the curved line titled “Water saturation” represents the amount of water molecules in each flake. That is, the big dendrites and sectored plates made around 0-10F contain the most water molecules per flake. And if there’s not enough water vapor in the atmosphere, smaller plates would be made at those temps. When you consider that colder air doesn’t hold as much water vapor as (comparatively) warmer air, it makes sense why so little snow is produced below 15F.  But if we had lots of moisture and cooled it to that level, we’d make the biggest snowflakes possible — as the scientists that made this graph did. That’s how I’m reading this, anyway. I found some more information on the topic here and here.

Generally when snow falls around here and the weather is over 20F, well, that’s the best.  Big flakes, lots of them, and you can stand being outside to enjoy them.  If the weather is down in the teens you get whispy, trace amounts of snow.  The snow looks smaller because it is smaller; the needles, columns and prisms are a lot smaller than the plates and dendrites formed at warmer temps.  Much colder than that and the air doesn’t have the carrying capacity for any real precipitation.  Hence, too cold to snow.  At least in practice, if not in principle.


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