If you have not done so already, today is the day to switch to warm clothing and winter coats. Icy winds are expected to blow across Britain from the east, to ensure that the last week of autumn brings our first real cold spell.

There was a wonderful graphic on the BBC television weather forecasts at the weekend that for once enabled Michael Fish and his colleagues to give a real impression of what was going on in the sky. Like a flock of ravens, arrows appeared over a deeply frozen area of Russia, first pointing directly at Britain, then making their way across Europe, before inclining south west to cross the English Channel, and finally bending around our western coasts to swoop off again to the north.

That is the icy wind that is about to hit us. There has been an area of intense high pressure over eastern Europe drifting towards us, and the usual low pressure areas in the Atlantic are moving in our direction. Those arrows produce a vivid impression of the winds that result from such conditions.

Since air tends to move from high to low pressure - that is, outwards from the centre of a high-pressure area, and inwards towards the centre of a depression - and the Coriolis effect diverts the wind from a direction perpendicular to the isobars to one more or less parallel with them, we get a clockwise swirling wind from the high pressure, and an anti-clockwise swirling winds from the low pressure.

Combine this with the polar maritime air moving towards us from the east, and the systems moving towards us across the Atlantic from the west, and you get just the picture that the arrows on the weather forecast were pointing out: two great masses of wind, moving towards each other and swirling in opposite directions, with their meeting-point somewhere around the west of Britain, where they both decide to travel north together.

Looking just at the more powerful high-pressure system coming from the east, however, you get the impression that Britain is the plug hole around which all these winds are swirling.

So prepare yourselves for high, icy winds, which will feel even colder than they are because of the wind-chill factor. It's from Russia with furry hat, coat, scarf and gloves.

It will be double-0 seven below zero if you're lucky.