You may have heard the phrase: “It’s too cold to snow”. Yet as snow is so utterly synonymous with cold, this seems a rather counter-intuitive statement, doesn’t it?
Well, unlike many other weather sayings, there’s actually more than a grain (or should I say a flake) of truth in this one.
Antarctica is the ‘driest’ place on the planet
One ingredient of snow is obviously freezing temperatures. But, in fact, even if it’s not quite freezing at ground level, it’ll normally be freezing not far above your head. That’s why snow can reach us when we’re actually just above zero.
The other ingredient is water – and this is crucial. The ‘sweet-spot’ for snowfall is when it’s cold, but still warm enough to hold sufficient moisture to allow flakes to form.
The colder the air is, the drier it also tends to be. Air at freezing point can hold just a quarter of the moisture that air at room temperature can. By the time we get down to a bone-chilling minus 12 degrees C, it can hold only a tenth of the moisture. At absolute zero (minus 273 Celsius to be precise) there is no moisture, so snow is impossible.
Well it may not be quite as cold as minus 273C, but this is the reason Antarctica is the ‘driest’ place on the planet. The snow and ice there has taken tens of thousands of years to accumulate because the air is just so cold.
It’s often the arrival of warmer, moister air – not too warm of course, but just warm enough – that triggers the development of snow flakes. So after several days of bitter cold, listen out for the arrival of a warm front, not a cold front, in the forecast. Ironically, it’s this that can often ultimately usher in the white stuff.