the cyclones which press in from the Atlantic will never get past, so that his country will always be under the influence of the front only of the depressions. All that is necessary for storm-warnings is to watch for signs of the intensity becoming so great as to give rise to a gale. The example we have just given in Figs. 64 and 65 is very characteristic of a gale coming on entirely from increase of intensity, without any motion of a cyclone. This shows the value of any indications of increasing or decreasing intensity which can be derived from any source. An inspection of the illustrative charts will show that the area involved is so large that it is hopeless to trace the cyclones as a whole, but that usually within the area of the British telegraphic reports, and always somewhere, there are localities to the east or north-east where the pressure is steady. Over the Atlantic great variations occur, and the forecaster has, therefore, first to try and discover the area of steady pressure, and then to keep a sharp look-out for any rapid fall of the barometer over the west coast of Ireland, which would produce steep gradients and their associated gales. When once a fragment of a ring of steep gradients is formed, its progress eastwards must be traced by telegraph, and watch must be kept that there is no giving way of pressure over Scandinavia. Since the rate of progress of the steep gradients is usually slow and pretty regular, and since, as has been shown above, the direction of the wind with the general character of the weather is subject to little uncertainty, gales of this type are practically forecast with almost greater success than any other class. The forecaster in Central Europe is not so fortunate. The nature of the changes there are so complex and so ill defined that he can scarcely follow them after they have happened, so that he can do little more than forecast generally unsettled weather while the barometer is falling and secondaries are forming in sympathy with the great Atlantic cyclones. After the mercury has begun to rise, improving weather is certainly indicated. The Russian forecaster has a totally different task. He recognizes the type, and knows that as long as his anticyclone lasts there is no fear of bad weather. We have shown that there is always some isobar, in this case 300 ins. (763 mm.), which remains nearly stationary, and he has to find this out in each case, and watch for any symptoms of a serious change. Thus we see, as the foundation of all synoptic forecasting, that the official in charge of the central bureau must learn by experience the ways of cyclones in his own country, and decide each case on its own merits according to the best of his judgment. The property of any type of weather to continue for any length of time is called the "persistence" of that type. Many phases of weather are due to this principle, and for forecasting it is very important to recognize any signs of this continuance; but, as the indications for this type are the same as for any other, we will describe the details of persistence later on. Then as to signs of change. This type may merge insensibly either into the westerly on one side, or the easterly on the other, the latter change being usually the more abrupt; but it is not possible to give any detailed description of symptoms of change. WESTERLY TYPE. In this type the permanent belt of anticyclones does not extend very far north, and pressure decreases steadily from the tropics towards the north. Under these circumstances, cyclones are developed on the north side of the Atlantic anticyclone, which roll quickly eastwards along the high-pressure belt and usually die out after they have become detached from the Atlantic anticyclone in their eastward course. Their intensity, and consequently the weather they produce, may vary almost indefinitely. When the cyclones are formed so far south that their centres cross Great Britain, and are of moderate size, the intensity is usually great, and severe well-defined storms, with sharp shifts of wind, are experienced. These occur most frequently in spring and autumn, and are the most destructive storms which traverse Great Britain. In another modification, while the pressure is low to the north, and the isobars run nearly due east and west, the whole of the arctic area of low pressure surges southward, with an exceedingly ill-defined cyclone, bringing a rim of steep gradients along the edge of the Atlantic anticyclone, and across Great Britain, in a manner analogous to the phase of southerly type before ex. plained. The indications then are for rain and westerly gales, with very little shift of wind. This phase belongs almost exclusively to the winter months. But the commonest modification at every season, and that which forms about seventy per cent. of European weather, is when the intensity is moderate, and the cyclone paths are so far to the north of the British Islands that the wind merely backs a point or two from the south-west as the cyclone approaches, and veers a point or two towards the west as the cyclone passes, the general direction of the wind being between south-west and west, without rising to the strength of a gale, while rain is moderate in quantity. Sometimes in summer a prolongation of the Atlantic anticyclone covers the southern portion of Great Britain, and distant cyclones of small energy just influence the northern countries of Europe. Then the intensity is too small to develop rain, and only produces cloud in the middle of the day, so that fine, dry weather is indicated, which when very prolonged may give rise to drought. This is by far the commonest of all weather types in temperate regions, and occurs at every season of the year. The existence of this type in Europe is sometimes associated with a similar phase of weather in the United States. That is to say, pressure being high over Mexico, cyclones form over the Rocky Mountains, and then pass along the line of the Lakes into the Atlantic. To this class belong almost exclusively the cyclones which pass from the United States, over the Atlantic, into Europe. At other times, a persistent anticyclone may. cover the American continent, and the whole of the European system of cyclones is born and developed in mid-Atlantic. Before we give more details, it may be well to exemplify some of the leading features of this type. In Figs. 68-71 we therefore give charts over the North Atlantic and Europe for the four days, February 26 to March 1, 1865. These may be taken to represent a fair specimen of ordinary broken weather in Europe, without sufficient intensity to give steep gradients and severe gales. In all, the Atlantic anticyclone was flanked on the west by another over the American continent, and This last on the east by another over Central Asia. |