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CHAPTER X.

LOCAL VARIATION OF WEATHER.

NATURE AND PRINCIPLES.

THE object of this chapter is to explain what is known as the local variation of weather. This term groups conveniently a large class of dependent phenomena, which owe their origin to the influence of local obstacles or peculiarities on the development of weather. We know that in the same country some places are much colder or wetter than others; that some are more exposed to destructive gales; and that others are more frequently ravaged by disastrous hailstorms. We will now endeavour to show why this should be so, and how the products of this variation are related to the general principles of the dependence of weather on the distribution of atmospheric pressure which we have already described so fully.

If we watch the actual occurrence of any local peculiarity of weather, we shall soon find out that in every instance it is the intensity, and not the general character, which is altered. For instance, two places a few miles

apart may differ by 10° of temperature on a frosty morning. No local cause has formed the distribution of pressure which gives the necessary calm. That stillness has been developed by general causes, while it is local peculiarities of exposure, etc., which have enabled radiation to be so much more powerful in one place than another. Similarly, an inch of rain may fall in one place, and only a few drops in another not far distant. But if we think of the day on which this occurred, we shall remember that it was cloudy and showery naturally. The difference of actual rainfall was either due to one place catching a heavy shower which did not affect the other, or else some local peculiarity of the one, which increased the amount of precipitation that would otherwise have been induced by a cyclone of any given intensity.

These instances might be multiplied indefinitely, and it is from the observation of innumerable cases that we are enabled to lay down the general law that the primary character of all weather is given by the general distribution of surrounding pressure; the local variation modifies but never alters this general character. By this means we are able to steer our way through many intricacies of weather which would otherwise present hopeless difficulties, and to explain many phenomena which would otherwise be inexplicable. Hence we see the appropriateness of the word "variation," as applied to the modification due to local causes.

The cases which present themselves in practice are endless. Every country, every part of a country, has a set belonging to itself, and the local meteorologist has to work out the details for his own neighbourhood, just as

the geologist explains the local peculiarities of his own scenery by the combination of general and local causes. Local is also like diurnal weather, in so far that the observed weather is the sum of the local variations and general causes. When the general are strong, the local are entirely masked; when the general are weak, then the local become of primary importance. We shall confine ourselves to a few examples relating to cloud, rain, and hail, so as to exemplify the general principles involved.

LOCAL CLOUD.

By far the most important and difficult source of local variation of weather is found in the development of cloud, rain, and other forms of precipitation by the influence of seas, lakes, rivers, hills, and valleys; some of these phenomena are so interesting that we propose to devote a few pages to their consideration.

We will commence with cloud, though we must remember that in most cases cloud is only undeveloped rain, and that the same cause which, when slight, gives cloud will give rain when more intense. In our diagram of cyclone-weather and prognostics (Fig. 2), we have marked cumulus-cloud in rear of the trough. In England this holds all through the year, but in the drier climate of Continental Europe it is only true during the summer months. The reason is simply that, in cold climates, there is only sufficient vapour to develop that form of cloud in summer. What we have specially to note is, that when cumulus does not form, it is not replaced by

any other kind of cloud, but the sky clears without any cloud at all. For instance, no local variation could ever turn the cumulus of the rear into the cirrus-stratus of the front of a cyclone; the quantity, not the quality, could alone be changed. Similarly for rain. We shall see directly that the actual amount may vary enormously from local causes, but no peculiarity can turn the drizzling rain of a cyclone-front into the heavy, big-dropped shower of a thunderstorm, or vice versa.

A very striking illustration of the influence of local peculiarities in the formation of cloud was once observed by M. Flammarion during a balloon-voyage from Paris to the Rhine. He saw one afternoon that cloud was formed over the rivers and woods, but not over the open plains. The synoptic charts for that evening show that Eastern France was then covered by a large cyclone of moderate intensity; and the explanation of the whole is, that all the air in that part of Europe was then in a rising condition, but that it was only over rivers and damp woods that enough vapour was present to condense into cloud. A more intense cyclone would have developed cloud everywhere, and rain only over the rivers and forests; another still more intense would have brought rain everywhere. In our chapter on Prognostics, we alluded to mist being formed over rivers in fine frosty weather. Here, too, we have local variation, but of a contrary nature to that which we have just considered. This example will serve to call attention to the great importance of differentiating between the various kinds of condensed vapour.

Another very common local cloud is that which rests

on or over hilltops, when blue sky covers the plains. This, of course, is due to the horizontal currents of the air being deflected upwards, and, if sufficient vapour is present, cloud is formed by condensation. The most interesting thing about these clouds is that they remain stationary as a whole, though their outlines and constituent particles are in constant motion. Their prognostic value has been already explained in a previous chapter.

LOCAL RAIN.

We shall now explain a few of the principal causes which affect the quantity of rainfall. One of the commonest and most obvious is that, when the wind which blows over water first meets the land, rain will be precipitated. For instance, in England, with a cyclone of moderate intensity and a westerly wind, rain will only fall on the western coasts and on the high ground inland. With an east wind, on the contrary, the fall will be confined to the most exposed portions of the east coast, and in a less degree to high inland stations. A similar effect is found all over the world. For instance, in Ceylon the rainy seasons on the two sides of the island are in different months, which depend on the time when each coast is exposed to the prevailing monsoon. The south-west monsoon brings rain to the exposed west side of the island, and the dry season to the east coast, which is then a lee shore. The north-east monsoon, on the contrary, first strikes the east coast, and develops abundant rainfall there; while the west coast then enjoys its dry

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