The change of the monsoon—the periodical shifting of the wind—the most singular feature of the case, is a gradual process, usually occupying about a month, which reduces the reign of the two annual monsoons, north and south of the equator, to five months each, the remaining two months being spent in the transitions. In each interval of change, calms, light variable breezes, alternate with storms of tremendous violence. Mr. Caunter thus describes the scene at Madras, in the interim between the cessation of one monsoon and the setting in of another: “On the 15th of October, the flag-staff was struck, as a signal for all vessels to leave the roads, lest they should be overtaken by the monsoon. On that very morning some premonitory symptoms of the approaching “war of elements” had appeared. As the house we occupied overlooked the beach, we could behold the setting in of the monsoon in all its grand and terrific sublimity. The wind, with a force which nothing could resist, bent the tufted heads of the tall, slim cocoa-nut trees almost to the earth, flinging the light sand into the air in eddying vortices, until the rain had either so increased its gravity, or beaten it into a mass, as to prevent the wind from raising it. The pale lightning streamed from the clouds in broad sheets of flame, which appeared to encircle the heavens as if every element had been converted into fire, and the world was on the eve of a general conflagration, whilst the peal, which instantly followed, was like the explosion of a gunpowder magazine. The heavens seemed to be one vast reservoir of flame, which was propelled from its voluminous bed by some invisible but omnipotent agency, and threatened to fling its fiery ruin upon every thing around. In some parts, however, of the pitchy vapor by which the skies were by this time completely overspread, the lightning was seen only occasionally to glimmer in faint streaks of light, as if struggling, but unable, to escape from its prison, igniting, but too weak to burst, the impervious bosoms of those capacious magazines in which it was at once engendered and pent up. So heavy and continuous was the rain, that scarcely any thing, save those vivid bursts of light which nothing could arrest or resist, was perceptible through it. The thunder was so painfully loud, that it frequently caused the ear to throb; it seemed as if mines were momentarily springing in the heavens, and I could almost fancy that one of the sublimest fictions of heathen fable was realized at this moment before me, and that I was hearing an assault of the Titans. The surf was raised by the wind and scattered in thin billows of foam over the esplanade, which was completely powdered with the white, feathery spray. It extended several hundred yards from the beach; fish, upward of three inches long, were found upon the flat roofs of houses in the town, during the prevalence of the monsoon, either blown from the sea by the violence of the gales, or taken up in the water-spouts, which are very prevalent in this tempestuous season. When these burst, whatever they contain is frequently borne by the sweeping blast to a considerable distance over-land, and deposited in the most uncongenial situations; so that now, during the violence of these tropical storms, fish are found alive on the tops of houses; nor is this any longer a matter of surprise to the established resident in India, who sees every year a repetition of this singular phenomenon. During the extreme violence of the storm, the heat was occasionally almost beyond endurance, particularly after the first day or two, when the wind would at intervals entirely subside, so that not a breath of air could be felt, and the punka afforded but a partial relief to that distressing sensation which is caused by the oppressive stillness of the air so well known in India.” It is an extraordinary but well-ascertained fact, that as soon as one monsoon ceases, though a month may elapse before the succeeding one appears, the clouds take the direction of the approaching monsoon, and thus from the regions of the atmosphere herald its advent to the dwellers below.
We naturally inquire concerning the origin of these peculiar movements, but must be content with a very scanty measure of information upon the subject. The laws which nature obeys in these periodical changes are undoubtedly identical with those which give rise to atmospheric currents in general, but their mode of operation is in this case obscure. The north-east and south-east monsoons, the former on the north and the latter on the south side of the equator, may be considered as trade winds, explicable upon the same principles, but counteracted for a certain time by causes which produce winds from a different quarter, the south-west and north-west monsoons. It has been observed that the south-west monsoon, which prevails to the north of the equator, is coincident with the sun being vertical to that region, when Hindûstan, Siam, and the adjacent countries receive their maximum of heat. Consequently, the incumbent air, being rarefied, ascends, and a rush of colder air to supply its place, is produced from the southward, which is then receiving the oblique rays of the sun, and which presenting a surface of water is immensely less heated than the lands to which the luminary is perpendicular. In like manner, the north-west monsoon, which prevails south of the equator, is coincident with the sun being south of it likewise, and vertical to the region, when the sandy plains of New Holland become powerfully heated, and the air over them rarefied, creating a wind by the rush of the colder northern air toward the point of rarefaction. These are the explanations commonly given, and though in several respects they do not account for all the phenomena, yet the probability is, that they present the correct theory, anomalous circumstances arising from the influence of causes which are local and as yet unknown. The monsoons are more valuable as auxiliaries to commerce than the trade winds, owing to the change in their direction, for a ship may proceed to a distant port with one monsoon and be aided on its return by its successor.
3. Land and sea breezes. A line in one of our popular songs,
“How sweetly the breeze blows off the shore,”
refers to the wind which begins at evening to blow from the coasts situated between and near the tropics: and an equally grateful breeze blows by day from the sea to the shore in those warm climates. The inequality of the solar action on the land and water, together with the tendency of the atmosphere to preserve an uniform density, is the cause of these periodically shifting currents. During the day the land acquires a temperature higher than that of the ocean, and the air over it is therefore rarefied and ascends, and the cooler air from the sea glides in to fill the partial vacuum produced. At night, the land rapidly cools with the atmosphere over it, but the sea and the air in connection with it retain a nearly equal temperature, in consequence of which, the colder and heavier land-air displaces the less dense or lighter air over the water, and a wind from the shore is created. The smoke of Vesuvius beautifully exemplifies this diurnal change in the direction of the atmospheric currents along the shore, its long tail stretching landward for a few hours, and then veering round to seaward. In the Mediterranean and the West Indies, the land breeze usually begins at six or seven o’clock in the evening, and blows until eight in the morning, when the sea breeze begins, increasing till mid-day, and gradually dying away in the afternoon, a period of stillness occurring between the changes, as between the ebbing and flowing of the tide. The sea-breeze of the Mediterranean in summer is said to be perceptible sometimes as far north as Norway. These draughts of the cool air of the ocean are important benefactions to various countries, where the heat would otherwise be insupportable. Along the coast of Malabar, the alternate breezes are powerfully felt, the land wind extending in summer a considerable distance out to sea, redolent with the roses and spices of the shore. Though the land and sea breezes are most sensible in tropical countries, yet in far remote latitudes, and especially around lakes, the same diurnal shifting in the direction of the wind is experienced. The change of temperature in the air over a spacious lake, caused by the succession of day and night, has been computed to be about thirty times less than that which takes place in the atmosphere of the surrounding land—the air over the land being much more heated during the day, and much less heated during the night, than that over the lake—an inequality of temperature which necessarily occasions a breeze from the lake by day, and toward it by night.
Vesuvius from St. Elmo.
The old and faithful voyager, Captain Dampier, in a quaint but pleasing style, has given the most exact description of these remarkable winds, as they occur in tropical latitudes. “These sea-breezes do commonly rise in the morning about nine o’clock, sometimes sooner, sometimes later; they first approach the shore so gently, as if they were afraid to come near it, and ofttimes they make some faint breathings, and, as if not willing to offend, they make a halt, and seem ready to retire. I have waited many a time, both ashore to receive the pleasure, and at sea to take the benefit of it. It comes in a fine, small, black curl upon the water, whereas all the sea between it and the shore, not yet reached by it, is as smooth and even as glass in comparison. In half an hour’s time after it has reached the shore, it fans pretty briskly, and so increaseth, gradually, till twelve o’clock; then it is commonly strongest, and lasts so till two or three a very brisk gale; about twelve at noon it also veers off to sea two or three points, or more in very fair weather. After three o’clock, it begins to die away again, and gradually withdraws its force till all is spent; and about five o’clock, sooner or later, according as the weather is, it is lulled asleep, and comes no more till the next morning.
“Land-breezes are as remarkable as any winds that I have yet treated of; they are quite contrary to the sea-breezes; for those blow right from the shore, but the sea-breeze right in upon the shore; and as the sea-breezes do blow in the day and rest in the night, so, on the contrary, these do blow in the night and rest in the day, and so they do alternately succeed each other. For when the sea-breezes have performed their offices of the day, by breathing on their respective coasts, they, in the evening, do either withdraw from the coast, or lie down to rest. Then the land-winds, whose office it is to breathe in the night, moved by the same order of divine impulse, do rouse out of their private recesses, and gently fan the air till the next morning, and then their task ends, and they leave the stage. There can be no proper time set when they do begin in the evening, or when they retire in the morning, for they do not keep to an hour, but they commonly spring up between six and twelve in the evening, and last till six, eight, or ten in the morning. They both come and go away again earlier or later, according to the weather, the season of the year, or some accidental cause from the land. For, on some coasts, they do rise earlier, blow fresher, and remain later than on other coasts, as I shall show hereafter.
“These winds blow off to sea, a greater or less distance, according as the coast lies more or less exposed to the sea-winds; for, in some places, we find them brisk three or four leagues off shore; in other places, not so many miles, and, in some places, they scarce peep without the rocks; or if they do sometimes, in very fair weather, make a sally out a mile or two, they are not lasting, but suddenly vanish away, though yet, there are every night as fresh land-winds ashore, at these places, as in any other part of the world. Indeed, these winds are an extraordinary blessing to those that use the sea in any part of the world within the tropics; for as the constant trade-winds do blow, there could be no sailing in these seas; but by the help of the sea and land-breezes, ships will sail 200 or 300 leagues, as particularly from Jamaica to the Lagune of Trist, in the Bay of Campeachy, and then back again, all against the trade-wind. The seamen that sail in sloops or other small vessels in the West Indies do know very well when they shall meet a brisk land-wind by the fogs that hang over the land before night; for it is a certain sign of a good land-wind to see a thick fog lie still and quiet, like smoke over the land, not stirring any way; and we look out for such signs when we are plying to windward. For if we see no fog over the land, the land-wind will be but faint and short that night. These signs are to be observed chiefly in fair weather; for in the wet season fogs do hang over the land all the day, and it may be neither land-wind nor sea-breeze stirring. If in the afternoon, also, in fair weather, we see a tornado over the land, it commonly sends us forth a fresh land-wind. These land-winds are very cold, and though the sea-breezes are always much stronger, yet these are colder by far. The sea-breezes, indeed, are very comfortable and refreshing; for the hottest time in all the day, is about nine, ten, or eleven o’clock in the morning, in the interval between both breezes; for then it is commonly calm, and then people pant for breath, especially if it is late before the sea-breeze comes, but afterward the breeze allays the heat. However, in the evening again, after the sea-breeze is spent, it is very hot till the land-wind springs up, which is sometimes not till twelve o’clock or after.”