British Weather (Part Two)

Winds from the south are not a common feature of British weather, but when they do arrive they can herald fine, warm weather. They may originate over North Africa and pass over France, so they remain fairly dry and are unlikely to bring much rain on their own.

The mixture of warm air and moisture creates thunderstorms at altitude - maybe 3,000 metres (10,000 feet) - bringing thunder and sheet lightning and heavy downpours from France, often at night. Easterly winds can bring very cold spells of weather, in fact it is winds from the east that produce our coldest weather.

The reason for this is that the easterly wind comes from the direction of Russia, a huge land mass which cools down in winter - it lacks the tempering influence of the sea - to extremely low temperatures.

Easterly winds bring us this extreme cold and, unlike a northerly wind, the easterly wind only has a short distance to travel over the warming sea. It was easterly winds, which brought the severe cold of the winters of 1947 and 1962/3, - two of the coldest winters this century in the British Isles.

In Summer, the intense heating in the interior of Europe produces hot, thermal currents so beloved of glider pilots. This may be responsible for the fact that around twenty years ago many of the top gliders were Czechs or Poles, who lived in the path of these dry and warm easterly currents.

Wind directions are usually a good indication of the type of weather that may be approaching, but they do not tell the whole story. If we consider that wind is moving air, then that air must have originated somewhere.

A band of air which has an identifiable uniformity of temperature and humidity is what is known as an air mass and, while these often conform to the north, south, east and west wind patterns, complications can occur when the air masses become mixed up, or approach from an unexpected direction.

The most common air mass affecting the British Isles is the polar maritime which originates in the Arctic and reaches us after sweeping across the North Atlantic. The sea has the effect of warming it from below, producing clouds and showers in the unstable mass of air.

A variation on this air mass, a returning polar maritime, also originates in the Arctic, but sweeps out wide into the Atlantic before making a homeward journey towards the pole. The difference here is that after initial warming in the middle of the Atlantic, the unstable elements are cooled in their lowest layers as the air mass begins to track northwards again.

As a result, when it reaches the British Isles it tends to produce dull, overcast weather often with drizzle. However, enough instability survives in its upper reaches to be able to produce showers and even thunderstorms.

The weather brought by a tropical maritime air mass will be mild and humid rather than hot, except for parts of the east or Southeast of the country. Generally speaking, areas, which are to the east and in the lee of high ground, experience the best weather when such an air mass prevails.

Less common is the Arctic air mass, blowing directly from the pole and which, as mentioned above in the context of northerly winds, can produce heavy snow showers. It was an Arctic air mass that produced a severe spell of snow during late March and early April 1975 - the snow was dry and powdery, indicating it came from a very cold air mass.

The air mass bringing the coldest temperatures is the polar continental mass that comes in from Russia. Fortunately, it is not that common. The polar continental is usually a dry air mass, having little distance to travel over the sea. On occasions it can cross over the North Sea from Scandinavia, which gives it a chance to pick up moisture that falls as snow on the east coast in winter.

The difference between the weather we experience in the Summer and that in the winter is due, in part, to the tilt in the Earth's axis producing shorter days and less sunlight in winter. However, another significant factor is the development of highs and lows out in the Atlantic.

We have seen how the semi-permanent high around the Azores - one of several at that latitude in the northern and southern hemispheres - can help direct winds towards our shores. During the winter months this high tends to recede slightly, so that the lows which form in the Atlantic, instead of passing between Scotland and Iceland, are dragged further south towards the British Isles, bringing with them unsettled, wet and windy weather.

Weather can, and does, develop on a more local level as well. Sea breezes and localised showers or thunderstorms on a Summer's day are regular examples. And of course, the weather that any given spot in the British Isles experiences will depend on local factors of altitude, shelter given by trees or mountains, soil type, and whether we are discussing the open countryside, or built-up areas where temperatures are generally higher anyway.