On the Origins of Thundersnow

So today there was a flurry of snow crossing my part of the UK. I say a flurry because it was hardly thick enough to settle, although I hear up north it’s a different story.

Anyway, the snow did not settle but seemed to gather and bunch up in the sky, bits of flake swirling around in the air, whipping up a treat. Before I knew it there was a massive storm forming! A huge banging sound – it was more like an explosion than a traditional rumbling peal of thunder – tore through the house and outside I witnessed massive blue flashes of ball lightning, originating from within the cloud of snow. It highlighted the snowflakes in a way I’ve never seen before, then minutes later the storm had dissipated and I was left stunned. My neighbours had rushed outside too and were staring up at the sky and chattering excitedly.

Thundersnow is very rare, and usually associated with freak storms in Northern parts of the US. It’s not really something that happens here that often.

thundersnow infographic depicting warm air convecting after contact with cold air
[Washington Post thundersnow infographic – click to go to original WashPost article…]

The reason it’s so rare is that for storm clouds to even form at all you need some kind of convection mechanism, and the heat required for this is not usually present in the kinds of temperatures experienced in snowy weather. But sometimes a mass of cold air can slide in atop a mass of warm air, and if the warm air is very humid, close to the dew point but still cold enough to be icy, a weird kind of instability occurs and it will start to convect.

The nimbus head of the storm won’t usually be as high as your average cumulonimbus because these precise unstable conditions won’t last long enough to let it rise so high. And the energy will probably be dissipated relatively quickly, just as I saw today.

A thundersnow-storm will likely be accompanied by a shower of hailstones (which also happened today). This happens when the convective potential is very high, causing strong updrafts and rapid motion. What was interesting was that the hail hit about ten minutes before the thunder began. And when the thunder did start up, I don’t think I’ve ever seen ball lightning so fierce and blue!

The whole thing was so short and violent but it was an epic experience.

2 thoughts on “On the Origins of Thundersnow

  1. We had thundersnow here in North Carolina last night. It had been in the high 50’s all day and around 6pm the temps began to drop and by 8pm it was thundering and snow was coming down. The winds were strong and blew the storm away by midnight. It was our first snow of 2013 and beautiful. Today is a beautiful, sunny day.
    Marie

    1. Hi Marie, lovely to hear your account of thundersnow. I understand that the US has more dramatic occurrences of this phenomenon so I’m quite jealous!

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