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How cold does it have to be for rain to become snow?

(6 Posts)
SuePurblybiltByElves Thu 02-Dec-10 16:09:49

Or is that a silly question? Just ask because the BBC Snow Man says that there's a big band of rain/sleet/snow approaching tomorrow. I'm wondering why they can't pin it down as they seem to be able to predict the temperatures.

OhYouSnowySnowyKitten Thu 02-Dec-10 16:23:48

Not a silly question at all - in fact it is really interesting and complicated. It depends on a few factors:

-The temperature of the sky all the way from -cloud level to ground level
the humidity of the air the precipitation is falling through
-the rate at which the precipitation is falling.

sometimes what can happen is if it starts out as rain and it fall through drier air some of the rain evaporates, as it evaporates it cools the air and turns what was initially a rain situation into a snow situation. The heavier the rain the more likely this is to happen.

SuePurblybiltByElves Thu 02-Dec-10 16:26:35

So it can start off as snowflakes and turn into rain? What about those bits of snow (getting technical, me) that look like polystyrene balls? No good for snowmen? What makes them weirdy shaped?

If you look at a map and know it will be cold enough for snow, there's no way of telling accurately if it will fall as sleet?

I'm getting interested in all this weather malarkey.

OhYouSnowySnowyKitten Thu 02-Dec-10 17:12:19

yes - can start of as snow and turn to rain or can start of as rain then the subsequent precipitation be snow.

The polystyrene balls are graupel. They are soft hail - a kind of cross between snow and hail.

OhYouSnowySnowyKitten Thu 02-Dec-10 17:14:25

In the uk it can be quite hard to tell if it will be snow or not. If the upper air temperatures are cold enough, like they have been recently then you can say that yes, away from coasts it will be snow. When the upper temperatures are a bit warmer the situation becomes more marginal and it becomes very very difficult to predict.

SuePurblybiltByElves Thu 02-Dec-10 18:12:37

New fave word: graupel. Excellent.I will sound so clever on the school run (only w [adult contact likely tomorrow grin)

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