WEATHER, (Encycl.) Many have laboured with extreme diligence to explore the secret causes of the various changes of weather, and to foretell these alterations with certainty, but hitherto with no great success. Indeed, in this country at least, the alterations of the weather are so many, and frequently so sudden, that we can scarce suppose them to depend on any constant and regular cause. In the warm climates the weather is much more constant and uniform than with us; and even on the Continent in equal latitudes, it is less variable than here. Between the tropics, and probably for a considerable space northward and southward, the sun appears to be the principal agent in determining the weather. At one season of the year immense quantities of vapour are raised into the atmosphere, which, when the solar influence becomes less, are condensed into violent rains, and thus distinguish the year into the dry and rainy seasons. In those climates also the general course of the wind is evidently influenced by the sun, as Dr Halley has observed, (See WIND, Encycl.) Indeed at some times violent storms of wind, called hurricanes, whirlwinds, and storms of thunder and lightning, will occur: but of these the sun can only be accounted the remote cause; the immediate one being most probably the highly electrified state of the atmosphere. The inhabitants of those climates, especially the savages, who are continually exposed to the atmosphere, are able to foretell these tempests some time before. The signs which they observe, however, are in fact only the tempest beginning its operations in the atmosphere; and no
person whatever is able to foretell an hurricane or a storm of thunder a month, or even a week, before it comes. The reason of this is, that the sources of atmospheric electricity are not only invisible to us, but so variable in themselves, that though we knew them at one time, we have not the least security they that would continue the same even for a day. In various parts of this work it has been shown, that the sun communicates the electric fluid to the earth, and that this fluid must escape somewhere from the earth into the atmosphere in the very same quantity in which it is communicated. As the electric fluid, however, cannot easily diffuse itself in the atmosphere, an accumulation of it over any particular spot must soon take place; in which case that spot will cease to emit any more electric fluid. If then a new vent is opened, a current of electric matter may perhaps be directed from that place which emitted it before; and then the atmosphere, instead of receiving electricity from the earth, will communicate its superfluous quantity to it. In such a situation of the atmosphere, storms of thunder may readily take place; but where there is nearly an equal degree of electricity in the earth and atmosphere, an earthquake is most likely to ensue. (See EARTHQUAKE, no 39. art. 10. Encycl.) Besides the mere accumulation of electricity in the atmosphere, there are other causes, such as severe frosts, which may detain the fluid in various parts of the earth at particular times; and thus be indirectly the cause of thunder, earthquakes, &c. in other places. This will easily show the difficulty, nay the impossibility, of predicting these storms; as the causes of them may be very remote from the place where they happen. An hurricane in Jamaica, for instance, may be occasioned by something in New Britain, or an earthquake in Portugal by something in Sweden or Norway.
In our own climate, nothing can be more variable and uncertain than the weather. Some have attempted to deduce its changes from the influence of the moon; but, though it cannot be doubted that the moon as well as the sun has some influence on our atmosphere, the changes in it are by far too many to be occasioned by any regular and permanent cause.—The wind has likewise been thought considerably to influence the weather: but excepting with regard to heat or cold, we cannot find that this is to be depended upon; not to mention that the wind itself makes part of the weather, and ought to have its cause explained as much as the rest. That alterations of the weather, however, do not depend immediately either on the sun, moon, or wind, appears from a circumstance mentioned by Mr Derham, viz. that "moles dig before a thaw." This he ascribes to warm vapours which they feel in the earth; but whatever it is, we from thence learn, that the cause of this alteration lies not in the atmosphere, but in the earth itself, from whence it would seem to be communicated to the atmosphere.
But though it is probable that the action of the electric fluid in the earth is the cause of those sudden changes which are met with in almost all countries; yet the general determination of the weather, or what is called the climate, evidently depends on some settled and permanent cause. The cold of winter and heat of summer are plainly occasioned by the greater or less obliquity of the sun's rays at those seasons. To the sun also are we to ascribe those regular winds which, though most remarkable within the tropics, are yet in some degree to be met with even in our own country.
Weather. Along the eastern coast of Britain, at least to a considerable extent, the wind blows from the south-east almost during the whole spring and summer. This we may naturally suppose to arise from the continual rarefaction of the air by the sun. The cold air rushes in to supply the partial vacuity, but it does not follow the course of the sun directly. The reason of this is, that the air is also rarefied over the continent opposite to Britain; and as the distance is but small, the column of air incumbent on that part of the German ocean which lies to the south-east of this country prevails over that which lies directly to the east, and will not allow it to come in. This continues with occasional variations, owing to unknown motions in the electric fluid till after the summer solstice. The same cause continues still to act; but though the heat be now greater on land than it was before, the sun begins soon to lose his influence on the great Atlantic ocean which lies to the westward. Thus the eastern parts being more rare than the western, a west wind begins to take place. The easterly wind, however, also continues; the two meet and oppose each other; the contest is decided by thunder and lightning, (see the article THUNDER, Encycl.) and seldom fails to end in the total defeat of the east wind. The month of July, therefore, is remarkable in this part of the country for thunder, as the contest generally lasts some weeks. In the month of August the west wind prevails, and continues till the superior density of the cold air in the northern regions counteracts it, and the wind blows from the north, north-east, or north-west. As these winds do not directly oppose each other, the consequence of their interference is not thunder, but rain, which usually falls in great quantities in the month of October; after which the cold northerly winds bring frost and snow.
Thus, notwithstanding the excessive variability of our climate, we may determine the general outline of the weather from evident causes. Thus in the months of January and February we may reasonably expect frost and snow, from the condensed and cold air in the northern regions pressing southward to those parts rarefied by the sun. In March we very often meet with the same, by reason of the weak influence of the sun at that time; but more frequently there are at that time storms of wind from various quarters, but frequently from the west or north. The reason of this is, that the air over the land begins to be rarefied; in consequence of which the denser air rushes in from that quarter where the passage is most free.—In April, the rarefaction of the air being greater, and considerable quantities of electric matter thus passing from the earth into it, the winds cease, or become more gentle, and rains commence. In the beginning of May the wind often blows very cold from the north, though the weather is more settled. Why this wind did not take place in April, seems not so easily accounted for; but most probably the difference of density between the northern and southern air was not then sufficient to allow of a strong current from any quarter. During the latter part of May and the whole of June, the weather is usually settled and clear; the sun gradually exerting his greatest influence. In July we have thunder and rain, for the reasons already given. The latter part of August and the whole of September are usually dry, with westerly winds, in consequence of the latter having prevailed in the conflict with the east wind. The whole month of October, and great part of November, is usually rainy, on account of the interference of the west and north winds; and in the end of November, and throughout the month of December, frost and snow come on, though not with such certainty as in January.
This, in general, is the weather we may expect throughout the year; but by many accidental causes, not easily discoverable, it may be very much altered. Some of these causes we have endeavoured to point out; but, besides the reasons already given, it is not impossible that an alteration may take place in our atmosphere from causes entirely beyond it, and which exist only in the immense mass of ethereal fluid with which the universe is filled. Such causes, however, as they must be totally without the reach of our investigations, can never be assigned as the reason of any kind of weather; yet in some seasons there is such a general cold over a great part of the earth, that we must certainly imagine the cause to have been very high in the atmosphere. Of this the year 1782 furnishes us with a notable example. During the whole of that summer the sun appeared almost totally to have lost his influence. No vegetable, not even grass, came to perfection; and the harvest was scarce finished in the month of December. This was the case in Scotland, and it seems to have been similar all over Europe. The cause of this extraordinary cold, we cannot discover; but its effects were very obvious and dreadful. The electric fluid being confined over so large a space, broke out in the southern parts with fury; and Sicily and Calabria were almost entirely ruined by earthquakes. The earth now poured out such immense quantities of electric matter, that the atmosphere seemed scarce able to contain it. It therefore diffused itself along the lower regions all over Europe. Heat and almost perpetual thunder were the consequence: vegetation was excessively luxuriant; but the immoderate quantity of electricity having occasioned continual fogs which obscured the sun, fruits had not their proper flavour; nor was the crop altogether what might have been expected. The electric fluid, in the mean time, having fully saturated the lower part of the atmosphere, gradually ascended higher; but as the earthquakes ceased to supply new quantities of it, the southern parts of the atmosphere became negatively electrified with respect to the northern parts. To restore the equilibrium, a vast quantity rushed from the north to the south, visible from the north of Scotland to Paris and farther, in the meteor which appeared on the 18th of August 1783. The immediate consequence of this was, that the atmospheric vapours being left without a proper quantity of electric matter to support them, violent rains took place and continued for some time; a very severe winter succeeded an hot summer, and no remarkable alteration in the weather took place till May.
In most parts of the world, the general state of the weather is determined by their situation with regard to the sea, mountains, &c. In general, mountainous countries are subjected to violent rains, storms of wind, very often thunder and lightning. In some places, particularly the southern part of the continent of America, the weather is almost a continual tempest. The Cape of Good Hope is also remarkable for storms, but chiefly of wind; while Terra del Fuego and the opposite point of the continent is deluged with perpetual rains. The Isthmus of Darien is also remarkable for rainy weather, while in the whole empire of Peru
it scarce ever rains. Persia, Egypt, and some other countries, are likewise destitute of rain, without having the inconvenience of earthquakes to which Peru is subject. The reason of these variations cannot be assigned without a more particular knowledge of the situation of those countries than we can boast of. In general, however, it is known, that high mountains have the property of collecting clouds in their neighbourhood, and consequently of producing rains, thunder, and storms of wind. Where a country is nearly surrounded by a vast extent of sea, it is commonly subject to great rains. Hence the climate of islands is more rainy than of continents; and hence probably the reason of the rains in South America and the Islmus of Darien, which approach to an insular situation. The great driness of the air in Peru may be occasioned by the violent determination of the vapours to these parts, which deprive that country of its due proportion: but this can only be hinted as a conjecture, and we have not sufficient data to establish any thing certain.
To conclude this article, we ought now to give an account of those alterations in the air which predict a change of weather, or rather inform us that the weather actually is changing. These, however, are but few, and not always certain. The brute creation are much more sensible of them than we; and give information by various means, according to their peculiar instincts. Those who are much in the open air also will be much better able to perceive these alterations, than such as are not. The barometer is the most certain index we have, and will frequently inform us of a change before the least sign of it can be otherwise perceived. But though we are thus informed that some change is about to take place, we are at a loss to know what it is, and can only guess from the season of the year what kind of weather we may reasonably expect. Thus, in the summer-months, if the wind is easterly, the barometer falls, and the clouds in the west seem stationary, or move a little to the eastward, we may certainly expect thunder at no great distance. If the wind is brisk, and the clouds move before it heavily and sluggishly, we are also to expect thunder, but at a distance. Indeed, we may judge of the approach of thunder-clouds by the time of the day in which they appear. When the sun has reached the meridian, his power of rarefying the air above any particular spot is then greatest. Hence, as this luminary is continually moving to the westward, the increase of the rarefaction towards that quarter must prevent any current of air from blowing from the west, and at the same time solicit a current from the east: but if by some other cause a westerly wind shall prevail, and drive the clouds directly against the easterly one, it is then plain that nothing further can stop their progress; they will be violently electrified, and discharge their contents directly over our heads. Hence the vulgar observation in this country, that if thunder begins at eleven in the forenoon, we will probably have but little; but a great deal, and very near, if it begins at twelve, or between twelve and one.
Another pretty certain indication of an approaching change of weather is a great clearness and transparency in the atmosphere. This is commonly taken notice of when the moon is newly changed. If at that time we see distinctly the part of the moon which is not en-
lightened by the sun's rays, we are pretty certain that rough stormy weather is soon to ensue. In warm climates this transparency is not confined to the atmosphere, but extends even to the waters of the ocean: and when the bottom is visible at a great depth, it is a certain prognostic of an hurricane. This transparency denotes some internal agitation of the electric fluid; for by a certain motion of it even the most solid bodies may be made transparent as glass, (see ELECTRICITY, Enzyc. no 4.); and the agitation becoming more and more violent, at last produces the most terrible commotions in the atmosphere. Something similar may be observed with regard to the transmission of sound; for in such states of the atmosphere sounds are heard much more difficultly and at a greater distance than others; but a sufficient number of observations have not yet been made to determine this matter with any accuracy.
It would be tedious to enter into a detail of all the prognostics which are drawn from the various appearances of the clouds, of halos, parhelia, aurora borealis, &c. Every person expects rain when the sky begins to grow hazy and dull; and halos and parhelia only appear in such cases. As for the aurora borealis, it is often seen for many nights together without any remarkable change in the state of the atmosphere. It is the natural effect of a constant and regular cause, and is probably too high in the atmosphere to produce any remarkable effect; though it has been thought in some cases to be productive of a current of wind from the south-west, for which a probable reason is alligned under the article AURORA Borealis, no ult.