CHAPTER XVII
Still another interesting kind of aërial disturbance is the familiar heat thunderstorm. This is not synonymous with those electrified tornadoes and cyclones which are accompanied by thunder and lightning, sometimes of great violence. Most tornadoes are thunderstorms, but not vice versa. The thunderstorm is not essentially a vortex, but rather a wind squall marked by sudden changes of temperature and pressure, bearing with it massive clouds fraught with rain, or hail, and disruptive electric charges flashing frequently to earth, or from point to point in the sky. Its approach is usually announced by rumbling thunder and heavy black clouds along the horizon. Its duration is brief, varying from a few minutes to an hour or two. Further characteristics are thus expressed by Moore:
“On land, thunderstorms occur most frequently at specific hours of the day or night, such as 3 to 5 in the afternoon or 9 to 10 in the evening and sometimes even at 2 or 3 a.m., but no such diurnal period is observed in midocean. The phenomena usually occur in a pretty regular order of succession. After several hours of fair weather, with gentle winds, there comes a calm; the cumulus clouds grow larger, the lower stratum of clouds is seen to be moving rapidly; gusts of wind start up with clouds of dust, rain is seen to be falling at a distance; the movement of rain and dust shows that the wind is blowing out from this rain cloud near the ground no matter which way the rainy region is advancing; a few large drops fall from slight clouds and then suddenly the heavy rain begins. Lightning that may have occurred during the preceding few minutes becomes more frequent and more severe as the rain increases. After the maximum severity of rain and wind, the lightning also diminishes or entirely ceases, and we are soon able to say that the storm has passed by. If we watch its retreat from us in the afternoon we shall see the rear of a great cumulus on which the sun is shining, but through whose dark-blue curtain of cloud and rain nothing save occasional lightning is visible. After the storm has passed, the lower atmosphere soon becomes appreciably cooler and drier, the sky is nearly clear of clouds, and the wind has shifted to some other point of the compass than that which prevailed before the storm.”
The genesis of thunderstorms is varied and manifold. In one simple type, a large tract of heated air in the unstable state and with a high percentage of humidity swells upward at the center, the ascending moist air forming, at the precipitation altitude, a growing cloud which may become very broad, dark and bulky, drifting along over the earth with the prevailing current. Eventually rain begins to form, or may be hail or snow, if the heated column reaches to a great height. The falling shower cools the air from the cloud down to the earth, increasing its density and materially weighting it with the descending liquid or solid particles. The showery column then sinks, especially along its inner part where it is maturest, thus causing an outrush of cool air along the earth, the immediate forerunner and herald of the rain. This outrushing current pushes upward the environing clear moist air, thus forming new margins of massive cumuli around the older nimbus widening within, showering, cooling and sinking. Thus the rain area is broadened and propagated, sometimes with nearly equal speed in all directions, but generally fastest in the direction of the most unstable condition, or of the then prevailing drift of the atmosphere. Indeed, the forward cloud ranks may far outspeed the wind, seeming by their imperious bluster and gigantic gloom to commandeer new recruits, as if by magic, out of the clear sky. Before this solemn mustering and turbulent front of the storm the black vapors suddenly startled into visible shape, rush buoyantly upward in ragged shreds, like smoke from unseen fires, and quickly blend with the general array of compact cloud expanding across the sky. Again, several thunderstorms, merged like a mountain range in solid phalanx, may sweep abreast over a continent, with long horizontal[71] roll, ever rising in front and upheaving the sultry air, thus replenishing perpetually the ponderous cumuli which form the vanguard of this far-flung and titanic march of the clouds. Such a storm is usually powerful and persistent, commonly enduring until the sun’s decline and the shades of night have cooled the lower air, and thus allayed the commotion by enfeebling the forces that favor its progress.
The speed of rise of the air beneath the base of the thunderhead is a question of some interest in aëronautics. If the ascent be so much as a foot or two per second, one may expect the vultures to prefer soaring beneath the thundercloud during its formative period. Here also the aëroplanist might attempt a record flight, if the cloud were high enough to be out of his way. But if he ventured to penetrate the base of the thunderhead, he might find the turmoil too irregular and strenuous for his comfort.
Of like interest is the long aërial swell that leads the advancing storm. When will aviators make this the theater of their adventurous frolic, careering playfully before the brow of the tempest and the harmless rage of the lightning, gay-winged heralds of the coming tumult, sailing perhaps with slackened motive power, yet swift and secure as the storm-riding petrels at sea?
Besides the winds and aërial currents commonly studied by meteorologists, are the minor disturbances which affect more particularly the wayfarers of the sky, whether birds or men. The atmosphere quite usually is vexed with invisible turmoils; most sensible, indeed, over rough territory, but conspicuous also above the smooth terrene, and at all elevations from earth to the highest cloudland. Before sunrise, and generally in weather uniformly overcast, these miscellaneous and nondescript movements of the air are least active, for any given speed of the general drift of the atmosphere; but when the sun shines and the soil is nonuniformly heated, the disturbances become most pronounced. A whole troop of playful zephyrs rise and set with the sun, in addition to the diurnal winds already studied. Over the dusty plain they reveal their presence and shape in those coiling columns that constitute the safety vents of the atmosphere, and obviate the disruptive violence of the uprush that would occur should a considerable region of surface air become excessively heated. Over the city, particularly in winter, the local turmoils of the atmospheric surf are revealed in the play of a thousand smoky columns, and better still, when it snows, by the incessant swell and veering of the flaky flood whose surges and eddies bewilder the vision by their complexity. Over the water the clouds of fog and steaming vapor are the best index of the local zephyrs, where, it must be remembered, the rising and veering of the vapor wreaths accompany like motions in the atmosphere. Over the forest, field and meadow the interminable wandering of thistle down and gauzy shreds of vegetation, now fast, now slow, now high aloft, then sheer earthward, indicate what erratic and perpetual motions prevail throughout the open country even on the stillest days. In the deep bosom of the atmosphere, the parallel ranks of the cirri all across the sky mark the crests of undulations quite as regular and tumultuous as the billows of a wind-swept sea; while the fierce seething and upsurging of the separate cumuli manifest the operation of vortices of prodigious energy. These visible billows and whirlwinds suggest an infinitude of transparent ones hardly less powerful, at the various levels unmarked by clouds. For wherever two streams of abnormally graded densities neighbor each other, a readjustment may occur agitating the entire region with a host of pulsations, squalls, cataracts and fountains which the bird and navigator must parry with proportionate care and skill.
And it is because of the amazing resistance of these wandering zephyrs, waves and eddies that they demand the attention of aëronauts; nay, more, it is because of the substantial labor they can perform when adroitly encountered and duly employed. For the simplest elements of aërodynamic science make clear that a rising zephyr hardly strong enough to support a falling leaf is adequate to sustain the heaviest soaring birds and aëroplanes gliding swiftly through it. In fact, the sailors of fast air ships feel a heavy impulse and distinct shock in plowing those mild cross winds which, to the fixed observer, seem not like blasts, but rather as gentle swells or harmless currents. These, therefore, have been made the subject of investigation by various students of aëronautics.
The first incentive to the instrumental study of the fluctuations of the wind in speed and direction seems to have been the hope to furnish a quantitative basis for various theories of soaring flight. Pénaud,[72] in 1875, had explained this phenomenon by postulating an upward current. Lord Rayleigh,[73] in 1883, had made the more general assumption of a wind having either a variable speed or a variable direction as a necessary and sufficient condition for such flight. Marey,[74] in 1889, and Langley,[75] in 1893, gave elementary qualitative explanations of soaring in a horizontal wind of variable velocity, though neither adduced concrete data to prove that the feat could be performed in an actual wind. Each and all of those theories may be sound enough in the abstract, but to show that they represent realities of art or Nature they should be applied to a concrete instance of soaring of a machine or a bird of known resistance, in a wind of known variability.