Winter weather has settled in, although it is officially still autumn. Meteorological observations, however, point to our having not four seasons of the year, but five.

When it snows in the first week of December, and it never really feels like spring until April at the earliest, one could be forgiven for thinking that the season of winter takes up more than its fair share of the year. Officially, however, winter has not yet begun.

The seasons, if you want to be scientific about it, begin and end at the solstices (when the sun is furthest from the earth and we experience the longest and shortest days of the year) and the equinoxes (when the sun passes directly over the equator, and day and night are of equal length all over the world). Winter will begin this year on 21 December; next spring will start on 20 March, and autumn on 23 September. In terms of our weather, however, it would make more sense to divide the year into five seasons, not four.

A study of 50 years of British weather, published in 1950 by H.H.Lamb, indicated the following seasonal patterns:

1) Spring to early summer: From the beginning of April to the middle of June, British weather is at its most variable. According to Lamb there are fewer long-lasting spells of consistent weather conditions in this period than at any other time of year. Surprisingly, it is also the time when we are most likely to have long stretches of northerly winds.

2) Mid- to late summer: From mid-June to early-September, we are fairly likely to have settled weather, especially in the period from the middle of July to the middle of August - but whether it will be good weather or bad is in the laps of the weather gods. Long spells of high pressure (bringing hot, dry summers) occur almost twice as often as long periods of low pressure (bringing rain).

3) Autumn: While the end of August and the very start of September tend to bring unsettled conditions, things settle down between the second week in September and mid-November. In September, high pressure fine weather is more likely than poor weather. October is the most settled month of the year, most commonly seeing westerly winds bringing rain and low pressure areas across the Atlantic to deliver the potential for stormy conditions. More rarrely, cold easterly winds may settle in for long periods.

4) Early winter: From the second half of November until the middle of January, there is another period of changeable weather. Arctic winds of the type that brought snow to most of the country last week rarely last for long. If the weather does settle down, however, it is most likely to be rainy westerlies.

5) Late winter and early spring: This can be the true winter of our discontent - or surprisingly mild. From the second half of January until the end of March long periods of stable weather are quite common, but anything is possible, from an early spring in February, to persistently rainy weather, or icy winter conditions prevailing until the end of March.

And why is it cold in winter? Not - as too many people believe - because we are further from the sun (which we aren't), but because of the earth's eccentric habit of spinning at an angle of some 30 to our plane of rotation about the sun. In winter, the sun's rays hit us obliquely and for fewer hours each day. Result: cold weather.