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December 17, 2010 / Dave Gorham

Thundersnow. Really?

Searching for interesting weather news the other day, I came across a video of a person being interviewed during a snowstorm and then – Boom! Thunder and lightning! Or, to be correct: Lightning, then thunder (of course).

What gives? Could this be? Oh, it be (in the words of Elaine from a particular Seinfeld episode). In fact, right here in Houston we experienced a thundersnow storm last winter during a passing snow shower (not much more than a flurry, really). Oddity of oddities, indeed. However, if you consider what a thunderstorm is and how it forms, then you shouldn’t be surprised that thundersnow exists. Perhaps you should be surprised it doesn’t happen more often.

And we can base its existence on one scientific fact: Thunderstorms don’t necessarily form in hot air, they form on the rising currents of air hotter than the surrounding air. Perhaps the earliest scientific fact you remember is, “Hot air rises.” This is not quite correct as it is hotter air (air that is hotter or warmer and less dense than the surrounding air) that rises. With that in mind, if you surround a parcel of 32F air with air that is 20F, then that 32F parcel is considered warmer (or hotter) and less dense than the surrounding air so it will indeed rise. And the greater the contrast in temperatures the faster the less-dense air will rise. If everything is just right, thunderstorms will form while it’s snowing. Lightning, thunder, snow — thundersnow! And by the way, following the same logic and meteorology, we can’t rule out tornadoes in a snowstorm.

The particular news story I watched was in the Great Lakes region which creates even more interesting science. What could be more interesting about the formation of winter thunderstorms near the Great Lakes? The Great Lakes, themselves. That is, as long as they’re not frozen over. Unfrozen, the lakes not only provide a source of abundant moisture but as the cold air from the Canadian Plains sweeps across the lakes, the lakes provide a dramatic contrast in temperature. It doesn’t matter that the water is capable of inducing hypothermia in mere minutes. What matters is that the air immediately above the water may very well be 20-25 degrees (or more) warmer than the advancing Canadian air. As the colder, more dense air thrusts the warmer air aloft thunderstorms are likely to form. And of course, the abundant moisture allows for dramatic snowfall accumulations on the lee side of the lakes in the classic lake-effect snow scenario. Once the lakes freezes over, the temperature contrast and the moisture source are locked up until the ice melts; thundersnow and dramatic snowfalls become much less frequent.

The two graphics below, meteorologically, are almost identical. One explains the formation of thunderstorms, one the formation of thundersnow.

The formation of thunderstorms occurs when more-dense air undercuts less-dense air forcing the less-dense air aloft where it condenses into clouds, rain and possibly thunderstorms. Image: Wikipedia.

The formation of thundersnow occurs when more-dense air undercuts less-dense air forcing the less-dense air aloft where it condenses into clouds, snow and possibly thundersnow. Image: Wikipedia.

Perhaps Homer's "Mr. Plow" business would've been more successful if Springfield was within the infamous snow belts of the Great Lakes? Is it safe to say we can guess where Springfield isn't? Image: Wikipedia.


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