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[♪♩INTRO]
If you imagine a blizzard, you probably picture lots of snow, bitter cold, wind biting at
your nose and ears...
But one thing you probably do not imagine is thunder. [THUNDER SFX]
Thunder is usually reserved for summer rainstorms, instead.
But every once in a while, thunder shows up in what seems like the wrong season -- and
when it does, it can be kind of terrifying.
It’s known as thundersnow, and it’s one of the coolest rare quirks of weather.
It’s easy to think of winter snowstorms and summer rainstorms as pretty much the same
thing, it’s just at different temperatures and with different kinds of precipitation.
But there’s more to it than that.
Before your classic summer storm, sunlight warms the ground and makes the air at ground
level warmer than the air a couple kilometers above it.
But before winter storms, the ground is already pretty cold, and the more indirect sunlight
doesn’t warm it very much.
So there aren’t the same sorts of big temperature differences between layers in the atmosphere
that there are in the summer.
That might not sound like a big deal, but it makes all the difference in the world,
for both the type of storm and whether there’s thunder.
To understand why, it helps to know how summer thunderstorms happen in the first place.
See, warm air can hold more moisture than cold air, so in the summer, water in the environment
tends to evaporate into that heated air near the ground.
That’s part of why it feels so sticky and humid outside right before a summer storm.
Then, those pockets of warm, wet air quickly rise into the cooler layers above them.
And ultimately, the water condenses into these tall, dark storm clouds.
Those jostling air pockets also generate lots of wind -- another familiar sign of an impending
thunderstorm.
The wind whips up the water droplets, and ice crystals, and dust in the atmosphere.
And as those different materials rub against each other, electric charge gets separated,
with one part of the cloud becoming more positively charged and the other more negative.
This is known as the triboelectric effect, and it’s the same reason you sometimes get
zapped by static electricity.
Eventually, the charges in the cloud get so out-of-balance that a lightning bolt streaks
across the sky to even things out.
That lightning super-heats a column of air, which expands as it’s heated and then contracts
back together after the bolt is finished.
And that vibration of the air, finally, gives you thunder.
It’s a pretty specific series of events.
You need warm, moist air rising through turbulent winds into cooler, drier air.
And then, that needs to generate the electric charge imbalances that lead to lightning and
thunder.
This happens all the time in the summer, but it usually doesn’t happen in the winter.
Because winter storms tend to fail at the first step in this process.
Since the ground generally stays pretty cold in the winter, the air above it stays cold
and dry.
So any rising moisture tends to condense pretty uniformly throughout the sky instead of all
in one place.
Instead of the tall, dark, and handsome clouds that precede a summer storm, you get shorter,
grayer, gloomier ones that cover the whole sky.
And instead of electric charge building up, the charge is dispersed or hardly separated
at all.
So things tend to stay pretty quiet.
But under just the right conditions, there still can be thunder during a winter snowstorm
-- or thundersnow, to be exact.
It sometimes happens near lakes or other large bodies of water where the water warms the
air right above it just like the ground does in the summer.
Thundersnow can also happen during a powerful nor’easter -- a kind of storm along the
Eastern Coast of the U.S.
These tend to have strong winds that churn the atmosphere and can separate enough electric
charge for lightning even in the winter.
Thundersnow can be pretty alarming when it happens, because we’re so used to blizzards
just silently raging while we huddle at home with tea or hot chocolate.
Still, it’s definitely cool, and it gets meteorologists pretty excited -- there are
even some viral videos to prove it.
But even though we know the kinds of conditions that can cause thundersnow, scientists still
don’t quite know how common it actually is.
So they’re working on strategies to better figure it out.
Recently, scientists have found that, if winter lightning is likely, the ice crystals in a
cloud will all get tilted in a certain way by the electric forces in an impending strike.
That means radar will bounce off the clouds in the same way, which could help scientists
track and predict thundersnow.
So this winter, they’ve been looking carefully at weather patterns, radar, and even reports
on Twitter to help us understand if thundersnow is coming to a blizzard near you.
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[♪♩OUTRO]