The gale which has just ravaged the Gulf of Mexico adds another to the long list of disastrous hurricanes. As I write, the effects produced by this tornado are beginning to be made known. Already its destructiveness has become but too certainly evidenced.
The laws which appear to regulate the generation and the progress of cyclonic storms are well worthy of careful study.
The regions chiefly infested by hurricanes are the West Indies, the southern parts of the Indian Ocean, the Bay of Bengal, and the China Seas. Each region has its special hurricane season.
In the West Indies, cyclones occur principally in August and September, when the south-east monsoons are at their height. At the same season the African south-westerly monsoons are blowing. Accordingly there are two sets of winds, both blowing heavily and steadily from the Atlantic, disturbing the atmospheric equilibrium, and thus in all probability generating the great West Indian hurricanes. The storms thus arising show their force first at a distance of about six or seven hundred miles from the equator, and far to the east of the region in which they attain their greatest fury. They sweep with a north-westerly course to the Gulf of Mexico, pass thence northwards, and so to the north-east, sweeping in a wide curve (resembling the letter ∪ placed thus ⊂) around the West Indian seas, and thence travelling across the Atlantic, generally expending their fury before they reach the shores of Western Europe. This course is the storm-track (or storm-⊂ as I shall call it). Of the behaviour of the winds as they traverse this track, I shall have to speak when I come to consider the peculiarity from which these storms derive their names of ‘cyclones’ and tornadoes.
The hurricanes of the Indian Ocean occur at the ‘changing of the monsoons.’ ‘During the interregnum,‘ writes Maury, ‘the fiends of the storm hold their terrific sway.’ Becalmed often for a day or two, seamen hear moaning sounds in the air, forewarning them of the coming storm. Then, suddenly, the winds break loose from the forces which have for a while controlled them, and ‘seem to rage with a fury that would break up the fountains of the deep.’
In the North Indian seas hurricanes rage at the same season as in the West Indies.
In the China seas occur those fearful gales known among sailors as ‘typhoons’ or ‘white squalls.’ These take place at the changing of the monsoons. Generated, like the West Indian hurricanes, at a distance of some ten or twelve degrees from the equator, typhoons sweep—in a curve similar to that followed by the Atlantic storms—around the East Indian Archipelago, and the shores of China, to the Japanese Islands.
There occur land-storms, also, of a cyclonic character in the valley of the Mississippi. ‘I have often observed the paths of such storms,’ says Maury, ‘through the forests of the Mississippi. There the track of these tornadoes is called a “wind-road,” because they make an avenue through the wood straight along, and as clear of trees as if the old denizens of the forest had been cleared with an axe. I have seen trees three or four feet in diameter torn up by the roots, and the top, with its limbs, lying next the hole whence the root came.‘ Another writer, who was an eye-witness to the progress of one of these American land-storms, thus speaks of its destructive effects. ‘I saw, to my great astonishment, that the noblest trees of the forest were falling into pieces. A mass of branches, twigs, foliage, and dust moved through the air, whirled onward like a cloud of feathers, and passing, disclosed a wide space filled with broken trees, naked stumps, and heaps of shapeless ruins, which marked the path of the tempest.’
If it appeared, on a careful comparison of observations made in different places, that these winds swept directly along those tracks which they appear to follow, a comparatively simple problem would be presented to the meteorologist. But this is not found to be the case. At one part of a hurricane’s course the storm appears to be travelling with fearful fury along the true storm-⊂; at another less furiously directed across the storm-track; at another, but with yet diminished force, though still fiercely, in a direction exactly opposite to that of the storm-track.
All these motions appear to be fairly accounted for by the theory that the true path of the storm is a spiral—or rather, that while the centre of disturbance continually travels onwards in a widely extended curve, the storm-wind sweeps continually around the centre of disturbance, as a whirlpool around its vortex.