Meteorology Part 8

Sometimes the rainbow colours a.s.sume the shapes of crosses instead of circles. Occasionally a bright halo will be seen above the shadow-head of the observer, concentric rainbows enclosing all. In some recorded cases the grand effect must have been simply glorious.

Scientific observation has done much to dispel the superst.i.tion which has clung so tenaciously to the Highland mind. The lonely grandeur of the weird mountain giants has been clearly explained as perfectly natural, yet the awe-striking feeling cannot be entirely driven off.

CHAPTER XXVIII

THE WIND

Once was the remark pointedly made: "The wind bloweth where it listeth."

And that is nearly true still. The leading winds are under the calculation of the meteorologist, but the others will not be bound by laws.

Yet there are instruments for measuring the velocity and force of the wind, after it is on; but "whence it comes" is a different matter. A gentle air moves at the rate of 7 miles an hour; a hurricane from 80 to 150 miles, pressing with 50 lbs. on the square foot exposed to its fury.

Some of the gusts of the Tay Bridge storm, in 1879, had a velocity of 150 miles an hour, with a pressure of 80 to 90 lbs. to the square foot.

Before steamers supplanted so many sailing vessels, seamen required to be always on the alert as to the direction and strength of the wind, and the likelihood of any sudden change; and they chronicled twelve different strengths from "faint air" to a "storm."

In general, the wind may be considered to be the result of a change of pressure and temperature in the atmosphere at the same level. The air of a warmer region, being lighter, ascends, and gives place to a current of wind from a colder region. These two currents--the higher and the lower--will continue to blow until there is equilibrium.

The trade winds are regular and constant. These were much followed in the days of old. A vast amount of air in the tropics gets heated and ascends, being lighter, and travels to the colder north. A strong current rushes in from the north to take its place. But the earth rotates round its axis from west to east, and the combined motions make two slant wind directions, which are called the "trade winds," because they were so important in trade navigation.

Among the periodical winds are the "land and sea breezes." During the day, the land on the sea coast is warmer than the sea; accordingly, the air over the land becomes heated and ascends, the fine cool breeze from the sea taking its place. Towards evening there is the equilibrium of temperature which produces a temporary calm. Soon the earth chills, and the sea is counterbalancingly warm--as sea-water is steadier as to temperature than is land--the air over the sea becomes warmer, and ascends, the current from the land rushing in to take its place. Hence during the night the wind is reversed, until in the morning again the equilibrium is restored and there is a calm, so far as these are concerned. These are therefore called the "land and sea breezes." Of course, it is within the tropics that these breezes are most marked. By the a.s.sistance of other winds, a hurricane will there occasionally destroy towns and bring about much damage and loss of life; but better that hundreds should perish by a hurricane than thousands by the pestilence which, but for the storm, would have done its dire work.

In countries where the differences of pressure are more marked than the differences of temperature, in the surrounding regions the strength of the wind thereby occasioned is far stronger than the land and sea breezes.

The variable winds are more conflicting. These depend on purely local causes for a time, such as "the nature of the ground, covered with vegetation or bare; the physical configuration of the surface, level or mountainous; the vicinity of the sea or lakes, and the pa.s.sage of storms."

Among these winds are the simoom and sirocco.

The _east_ winds, which one does not care about in the British Islands during the spring time, are occasioned by the powerful northern current which rushes south from the northern regions in Europe. Dr. Buchan points out a very common mistake among even intelligent observers who shudder at the hard east winds. It is generally held that these winds are damp. They are unhealthy, but they are dry. It is quite true that many easterly winds are peculiarly moist; all that precede storms are so far damp and rainy; and it is owing to this circ.u.mstance that, on the east coast of Scotland, the east winds are searching and carry most of the annual rainfall there.

But all of these moist easterly winds, however, soon turn to some westerly point. The real east wind, so much feared by invalids, does not turn to the west; it is exceeding dry. Curious is it that brain diseases, as well as consumption, reach their height in Britain while east winds prevail.

Once in Edinburgh, during the early spring, I had rheumatic fever, and during my convalescence my medical adviser, Dr. Menzies, would not let me have a short drive until the wind changed to the west. The first thing I anxiously watched in the morning was the flag on the Castle; and for nearly two months it always waved from the east. How heart-depressing!

Creatures are we in the hands of nature's messengers. We so much depend upon the weather for our happiness. Joyful are we when the honey-laden zephyr waves the long gra.s.s in June, or when

"The gentle wind, a sweet and pa.s.sionate wooer, Kisses the blushing leaf."

Compared with this, how terrible is Shakespeare's allusion to the appalling aspects of the storm:--

"I have seen tempests, when the scolding winds Have rived the knotty oaks; and I have seen The ambitious ocean swell, and rage and foam, To be exalted with the threat'ning clouds; But never till to-night, never till now, Did I go through a tempest dropping fire."

CHAPTER XXIX

CYCLONES AND ANTI-CYCLONES

The criticism of the weather in the meteorological column of our daily newspapers invariably speaks of "cyclones." It is, therefore, advisable to give as plain an explanation of these as possible. Cyclones are "storm-winds." Their nature has to be carefully studied by meteorologists, who are industriously at work to ascertain some scientific basis for the atmospheric movements.

What is the cause of the spiral movement in storm-winds? In their centre the depression of the barometer is lowest, because the atmosphere there is lightest. As the walls of the spiral are approached, the barometer rises.

Dr. Aitken has ingeniously hit upon an experiment to ill.u.s.trate a spiral in air. All that is necessary is a good fire, a free-going chimney, and a wet cloth. The cloth is hung up in front of the fire, and pretty near it, so that steam rises readily from its surface; and, when there are no air-currents in the room, the steam will rise vertically, keeping close to the cloth. But if the room has a window in the wall, at right angles to the fireplace, so as to cause the air coming from it to make a cross-current past the fire, then a cyclone will be formed, and the vapour from the cloth will be seen circling round. When the cyclone is well formed, all the vapour is collected into the centre of the cyclone, and forms a white pillar extending from the cloth to the chimney. This experiment shows that no cyclone can form without some tangential motion in the air entering the area of low-pressure.

Now to ill.u.s.trate the spiral approach. Fill with water a cylindrical gla.s.s vessel, say 15 inches in diameter and 6 inches deep. Have an orifice with a plug a little from the centre of the bottom. Remove the plug, the water runs out, pa.s.sing round the vessel in a vortex form. But, as the pa.s.sage between the orifice (or centre of the cyclone) and the temporary division is narrower than in any other place, the water has to pa.s.s this part much more quickly than at any other place. And this curious result is observed: the top of the cyclone no longer remains over the orifice, but _travels_ in the direction of the water which is moving most speedily. Similar to this is the cyclone in the atmosphere; its centre also moves in the direction of the quickest flowing wind that enters it.

Dr. Aitken is of opinion that, in forecasting storms, too little attention has been paid to the _anti-cyclones_. They do more than simply follow and fill up the depression made by the cyclones. They initiate and keep up their own circulation, and collect the materials with which the cyclones produce their effect. Neither could work efficiently without the other.

Suppose a large area on the earth over which the air is still in bright sunshine. After a time, when the air gets heated and charged with vapour, columns of air would begin to ascend in a disorderly fashion. But suppose an anti-cyclone is blowing at one side of this area. When the upper air descends to the earth, it spreads outwards in all directions; but the earth's rotation interferes and changes the radial into a spiral motion.

The anti-cyclonic winds will prevent the formation of local cyclones, and drive all the moist, hot air to its circ.u.mference, just above the earth.

The anti-cyclone forces its air tangentially into the cyclone, and gives it its direction and velocity of rotation, also the direction and rate of travel of the centre of depression. The earth's rotation is the original source of the rotatory movements, but both intensify the initial motion.

Accordingly, the cyclone must travel in the direction of the strongest winds blowing into it, just as the vortex in the vessel with the eccentric orifice travelled in the direction of the quickest moving water. This is verified by a study of the synoptic charts of the Meteorological Office.

The sun's heat has always been looked upon as the main source of the energy of our winds, but some account must also be taken of the effects of cold. It is well known that the mean pressure over Continental areas is high during winter and low during summer. As the sun's rays during summer give rise to the cyclonic conditions, so the cooling of the earth during winter gives rise to anti-cyclonic conditions. It is found during the winter months in several parts of the Continent that as the temperature falls the pressure rises, producing anti-cyclones over the cold area; whereas, when the temperature begins to rise, the pressure falls, and cyclones are attracted to the warming area.

Small natural cyclones are often seen on dusty roads, the whirling column having a core of dusty air, and the centre of the vortex travelling along the road, tossing up the dust in a very disagreeable way to pedestrians.

Sometimes such a cyclone will toss up dry leaves to a height of four or five feet. They are very common; but it is only when dust, leaves, or other light material is present that they are visible to the eye.

CHAPTER x.x.x

RAIN PHENOMENA

The soft rain on a genial evening, or the heavy thunder-showers on a broiling day, are too well known to be written about. Sometimes rain is earnestly wished for, at other times it is dreaded, according to the season, seed-time or harvest. Some years, like 1826, are very deficient in rainfall, when the corn is stunted and everything is being burnt up; other years, like 1903, there is an over-supply, causing great damage to agriculture. The year 1903 will long be remembered for its continuous rainfall; it is the record year; no year comes near it for the total rainfall all over the kingdom.

Rain is caused by anything that lowers the temperature of the air below the dew-point, but especially by winds. When a wind has blown over a considerable area of ocean on to the land, there is a likelihood of rain.

When this wind is carried on to higher lat.i.tudes, or colder parts, there is a certainty of rain. Of course, in the latter case the rain will fall heavier on the wind side than on the lee side.

For short periods, the heaviest falls or "plouts" of rain are during thunder-storms. When the raindrops fall through a broad, cold stratum of air, they are frozen into hail, the particles of which sometimes reach a large size, like stones. Of course, water-spouts now and again are of terrible violence.

One of the heaviest rainfalls yet recorded in Great Britain was about 2-1/4 inches in forty minutes at Lednathie, Forfarshire, in 1887. Now 1 inch deep of rain means 100 tons on an imperial acre; so the amount of water falling on a field during that short time is simply startling. The heaviest fall for one day was at Ben Nevis Observatory, being fully 7-1/4 inches in 1890. In other parts of the world this is far exceeded. In one day at Brownsville, Texas, nearly 13 inches fell in 1886. On the Khasi hills, India, 30 inches on each of five successive days were registered.

At Gibraltar, 33 inches were recorded in twenty-six hours.

The heaviest rainfalls of the globe are occasioned by the winds that have swept over the most extensive ocean-areas in the tropics. On the summer winds the rainfall of India mainly depends; when this fails, there is most distressing drought. Reservoirs are being erected to meet emergencies.

From Dr. Buchan's statistics it is found that the annual rainfall at Mahabaleshwar is 263 inches; at Sandoway 214; and at Cherra-pungi 472 inches, the largest known rainfall anywhere on the globe. Over a large part of the Highlands of Scotland more than 80 inches fall annually, while in some of the best agricultural districts there it does not exceed 30 inches.

Of all meteorological phenomena, rainfall is the most variable and uncertain. Symons gives as tentative results from twenty years'

observations in London--(1) In winter, the nights are wetter than the days; (2) in spring and autumn, there is not much difference; (3) in summer, nearly half as much again by day as by night.

The wearisomeness of statistics may be here relieved by a short consideration of the _splash_ of a drop of rain. Watching the drop-splashes on a rainy day in the outskirts of the city, when unable to get out, I brought to my recollection the marvellous series of experiments made by Professor A. M. Worthington in connection with these phenomena. Of course, I could not see to proper advantage the formation of the splashes, as the heavy raindrops fell into these tiny lakes on the quiet road. There is not the effect of the huge thunder-drops in a stream or pool. The building up of the bubbles is not here perfect, for the domes fail to close, nor are the emergent columns visible to the naked eye. It is a pity; for R. L. Stevenson once wrote of them in his delightful "Inland Voyage," when he canoed in the Belgian ca.n.a.ls, as thrown up by the rain into "an infinity of little crystal fountains."

Beautiful is this effect if one is under shelter, every dome seeming quite different in contour and individuality from all the rest. But terrible is it when out fishing on Loch Earn, even with the good-humoured old Admiral, when the heavy thunder-drops splash up the crystal water, and one gets soaked to the skin, sportsman-like despising an umbrella.