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Language: en

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There is an old saying that "it's too cold to snow".

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Now a lot of people will say, that's obviously rubbish,
it can snow at any temperature.

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which, okay, is sort of true.
Let's rule out some technicalities

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to start off with. First of all, we are talking
about snow made of water here,

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not the methane snow

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that you might get on Titan or some other planet.
And of course, at cryogenic temperatures

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the kind of temperatures where

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oxygen and nitrogen become liquid, it's
certainly too cold to snow unless you start

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having a very strange definition of snow.
We are talking about the regular

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planet Earth, ice water snow, like that's
suddenly started falling

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just here. As air gets colder, it has much less

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capacity to hold water vapor.
It gets saturated

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far far quicker. That's because when it's
cold, the individual water molecules

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are much more likely

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to condense into liquid and much less
likely to evaporate

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out into vapour. So cold air is
much more likely to have already

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gotten rid of its water by forming
clouds somewhere else over there.

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So cold air is drier, and therefore

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it's too cold to snow. But there are
a few other factors involved too.

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The temperature down here -
while it's quite cold at the minute -
is not the same as up there.

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Well, obviously, it's much colder up there. But they don't normally follow each other in lockstep.

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Just because it's cold and dry down here
doesn't mean the ice crystals aren't forming

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up there in the clouds. And that's if there is
any cloud up there, of course.

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That's the other problem. Let's not
forget that snow can only form

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if there's some cloud to form it in.
Clouds are also

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really good at reflecting the heat
from the earth back at the surface.

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If there's no cloud, sure, there definitely won't be snow,

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but it's also going to be much colder

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because the heat isn't trapped
below that cloud layer.

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So the cause and effect is sort of the
wrong way round there, but you can see

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where the old wives' tale comes from.
Yes, for those reasons,

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as the temperature drops further and
further and further snow

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becomes less and less and less likely.
There is some truth to the old saying.

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It can be too cold to snow.

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It is very cold here right now, though,
and I am going back inside.

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Aagh! It's down the back of my neck!

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