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There is an old saying that "it's too cold to snow".
Now a lot of people will say, that's obviously rubbish, it can snow at any temperature.
which, okay, is sort of true. Let's rule out some technicalities
to start off with. First of all, we are talking about snow made of water here,
not the methane snow
that you might get on Titan or some other planet. And of course, at cryogenic temperatures
the kind of temperatures where
oxygen and nitrogen become liquid, it's certainly too cold to snow unless you start
having a very strange definition of snow. We are talking about the regular
planet Earth, ice water snow, like that's suddenly started falling
just here. As air gets colder, it has much less
capacity to hold water vapor. It gets saturated
far far quicker. That's because when it's cold, the individual water molecules
are much more likely
to condense into liquid and much less likely to evaporate
out into vapour. So cold air is much more likely to have already
gotten rid of its water by forming clouds somewhere else over there.
So cold air is drier, and therefore
it's too cold to snow. But there are a few other factors involved too.
The temperature down here - while it's quite cold at the minute - is not the same as up there.
Well, obviously, it's much colder up there. But they don't normally follow each other in lockstep.
Just because it's cold and dry down here doesn't mean the ice crystals aren't forming
up there in the clouds. And that's if there is any cloud up there, of course.
That's the other problem. Let's not forget that snow can only form
if there's some cloud to form it in. Clouds are also
really good at reflecting the heat from the earth back at the surface.
If there's no cloud, sure, there definitely won't be snow,
but it's also going to be much colder
because the heat isn't trapped below that cloud layer.
So the cause and effect is sort of the wrong way round there, but you can see
where the old wives' tale comes from. Yes, for those reasons,
as the temperature drops further and further and further snow
becomes less and less and less likely. There is some truth to the old saying.
It can be too cold to snow.
It is very cold here right now, though, and I am going back inside.
Aagh! It's down the back of my neck!
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