Weather in the UK page 2

The passage of such a small depression is often, one might say usually, followed by the passage of a ridge of high pressure, ushered in by the continuation of the cold northerly winds and a steadily rising barometer. Fine sunny weather may result in the summer, but as the winds decrease and calm conditions prevail the passage of such a ridge of high pressure is often marred in winter by the occurrence of fogs.


It is now possible to re-state what has been said in the last few pages in modern terms by stating that the British Isles are affected by six principal types of air or air masses. Most of the air reaching Britain can be traced to one of three main source-regions—regions where the air has had sufficient time, such as a number of days, to acquire a degree of homogeneity of temperature and humidity throughout its mass. These three source regions are the Arctic, eastern Europe or Eurasia and the Atlantic high pressure area of the Azores—precisely the areas already clearly indicated ot

Fig. 47. The Arctic yields maritime Polar air (mP) which may be intensified as a bitterly cold northerly air stream in winter when it is designated maritime Arctic (mA). Northern Russia and Siberia give us our most severe winter weather through the mass of contin ental Arctic air (cA) contrasted with the rather rare continental polar (cP) and continental Arctic (CA) of Summer where air masses from the same quarter have been warmed in their passage eastwards. The rarity of such air masses reaching Britain is clear from Figs. 49 and 50 which show th& normal movement away from Britain. The Atlantic area over the Azores yields maritime Tropical (mT) air; much rarer (alm ost unknown in winter) is continental Tropical(CT) from North Africa or the summer Mediterranean.

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The Equinoxes

In the analysis of the general conditions in Europe we referred to the formation of a great high pressure centre over eastern Europe in winter and the formation of a low pressure centre over the same area in summer. Naturally there must be two seasons of the year when the change from one to the other takes place, and it is very largely the resulting disturbance of atmospheric conditions which is responsible for the well known equinoctial gales experienced in this country. Whilst in the early part of the year high winds are associated with the month of March and the fame of March winds is perpetuated in many a nursery rhyme and popular ballad, the change indicated by these winds does not always take place at exactly the same time. In some years March may “come in like the lion and go out like the lamb,” in others it may come in like the lamb when the winter conditions still prevail, but go out like the lion. The corresponding high winds associated with the disturbances at the autumnal equinox are the equinoctial gales of September and October. After the equinoctial gales of March come the still

Temperatures
So far we have been dealing with the weather conditions of the British Isles, and owing to the irregular succession of weather it is sometimes said that the British Isles have no climate, since climate is described as the average state of the weather. This may at least serve as a useful reminder that we should use the averages mentioned in the paragraph below with care owing to the variability from year to year, and even during one year. Taking, first, temp erature conditions in the winter, it may be said that in general conditions in the British Isles reflect in detail those prevailing in Europe as a whole. In winter the west is warmer than the east and the isotherm of 40 degrees in January roughly divides the islands into two and its curve should be carefully noted. As one would expect at this season, the extreme south-west of Britain and south-western Ireland are, taking the average conditions in January, the warmest parts of the islands. The Scilly Isles have an average temperature in January of no less than 45 degrees, whilst snow and frost are both rare.. The important effects which this has on the products which are possible in these areas, and on the use of warmer parts of Britain in winter as winter resorts, may be mentioned. Taking the evidence afforded by the isotherms alone the coldest parts in the British Isles in Winter are certain tracts down the east coast, and it would seem that the east coast of Scotland in the neighbourhoo1 of Aberdeen is not as cold as some parts of the coast of East Anglia farther south, but the ordinary dry- bulb thermometer is scarcely an adequate measure of temperature in so far as it affects human beings and there is perhaps a rawer quality in the air in northern tracts. In summer, by way of contrast, the south of the British Isles is warmer than the north. The isotherm of 60 degrees in July runs roughly from east to west. The south-east quarter is the warmest of all in July in the neighbour hood of London, but the average along the south coast is high. The coolest parts of the islands at this season of the year are the extreme north of Scotland, the Orkneys and the Shetlands Though geographers have long been accustomed perhaps too slavishly—to take January and July as the typical Winter and summer months, they are not in the British Isles the Coldest and warmest months respectively. It frequently happens in oceanic or insular climates that there is a considerable lag between the period when the sun’s rays strike least obliquely or most obliquely on the surface of the ground and the time when the highest and owest average temperatures are reached, so that for many part

So far we have been considering temperature as it affects lowlands, and the isotherms which have been discussed are sea level isotherms. If one considers the actual surface temperatures as recorded the theoretical difference between actual temperature and temperature reduced to sea level is 10 F. for every 300 feet. Since the greater part of Lowland Britain is less than 600 feet above sea level the theoretical difference does not exceed 2° F. Elevation begins to exert an important effect on the higher ground of Highland Britain and on the top of the highest point in the British Isles, Ben Nevis, at 4,400 feet, the temperature if reduced to sea level equivalent would be nearly 150 F. higher.

The effect of elevation justifies us in regarding the Highlands of Scotland in January as the coldest part of the country. The factor of elevation is readily apparent when snowfall is considered (see Fig. 62). Whilst in the south-west of the country, in Devon and Cornwall, snow does not normally fall on more than five days in each year, and is not found to be lying for more than five days, such lofty tracts as Dartmoor are frequently seen to have a powdering of snow. Indeed, on the higher parts of Dartmoor snow normally lies for more than twenty days in every year.
The effect of elevation on temperature is however far from simple. Cold air behaves much like cold water. It flows downhill and will collect in a valley or basin where there is no outlet and gives rise to a “frost pocket” or “frost hollows.” Some extremely low temp eratures have been recorded in such obstructed valley situations amongst the Chiltern Hills. On the other hand, where cold air can drain freely as it does down river valleys and out over the sea, such frost pockets are absent.
Studies which have been undertaken in connection with the growing of fruit and the siting of fruit orchards have shown how very important it is to secure free drainage of air. Even a high hedge can hold up air movement and increase the hazards of frost. In addition aspect plays a much greater part in the determination of local climates than has been suspected in the past. If one attempts to determine the upper limit of cultivation in such areas as the Pennines, one finds that cultivation is almost invariably to a higher level on the northern sides of valleys, that is to say on those slopes facing southwards towards the sun, than it is on the southern sides of the valleys. Dr. Alice Garnett1 has carried out elaborate studies showing how important this fact is in the Pennine valleys of West Yorkshire. Hills running from east to west afford a remarkable protection from cold northerly or north-easterly winds. The attraction of