the mellow tigger's musings

I've seen occasional confusion from people over the last few weeks "Why is it so cold, isn't it Spring now?" - and I thought I should say a bit about one of the major causes that I almost never hear people talk about - the polar vortex.

This is a swirling wind around the Arctic that exists for basically the whole arctic night. One of the things it does is keep the freezing polar winds from coming further south in to Europe. But when it finally collapses in the Spring, it finally allows those winds out, and you get a sudden burst of cold air as all of that freezing weather escapes down to us.

Normally this happens some time in late February, but this year the collapse seems to have been a month later.

The other major factor is largely down to circulating high pressure areas (imagine slow large hurricane shaped wind "objects") that constantly move around the North Atlantic. Put one of these off of the west coast of Ireland, going clockwise, and it will pull air down from the North even further/faster. See this short video I took from the NullSchool site (my favourite wind visualisation site). In it you can see cold winds pouring down from the North Pole, funneled further by the circulation. And if you click on the link there you can see that currently the wind is instead being pulled off of the Altantic, where it's a few degrees colder.

British weather tends to be more chaotic than the weather north or south of us. This is because Spain (for instance) is fairly reliably in the warm weather caused by the heating tropics. And Norway is fairly reliably cold, due to proximity to the North Pole. But Britain can be part of either weather system, as the "barrier" between them is pulled North or South by a few hundred miles depending on the movement of the high pressure areas in the eastern part of the North Atlantic, either funnelling the warm air up to us or channeling the cool air down to us.

You can see that at the moment the warm weather is being slowly blown North-East, now that the cold weather isn't pushing its way down to us:


So, next time we get a period of warm weather at the end of Winter/start of Spring followed by a sudden burst of freezing weather for a few days, that's the polar vortex collapsing. And if we suddenly go from warm weather to cold (or vice versa)  it's because we've switched weather system.

If you'd like to read more, then this is quite good.

(And apologies to anyone who actually knows anything about the weather for any appalling mistakes I've made.)