CHAPTER XVI

Besides the periodic winds so far treated, there are prominent aërial movements having no regular course or season. These are the nonperiodic winds which so exercise or perplex the weather forecaster and those who confide in him. In general such winds are of a temporary character, arising from an unstable condition of the air in some locality, or from unequal heating, either of which causes may generate, or briefly sustain, an updraught, with its attendant gyration. Owing to the whirling character of such ascending currents, they have received various significant names, such as cyclone, tornado, whirlwind; the three terms applying to vortices in decreasing order of magnitude. Each in turn may be treated briefly.

The cyclone is a temporary large gyratory wind. It may last a few hours or a few days. It may measure fifty to a hundred miles across, or it may measure more than a thousand miles. On the weather map it is in general marked by a group of closed isobars, showing a considerable pressure gradient toward a small internal area where the pressure is a minimum. To an observer looking about the earth’s surface and lower levels of the atmosphere, the cyclone appears merely as an ordinary wind, accompanied perhaps by rain or snow. It is not a swiftly rotating narrow column, or cone of air, like a tornado or whirlwind, full of gyrating dust and débris.

The motive power of a cyclone, though in general due to the buoyancy of heated air, may spring from more than one set of conditions. Notice has already been taken of vortices due to a hot column of air at lower barometric pressure than its lateral environment. Take another case. If a dry atmosphere is of uniform temperature and pressure at various levels, but has a vertical temperature gradient a little greater than the normal cooling of an ascending gas, a portion of air started upward in any casual way becomes warmer than its lateral environment, and hence continues to rise until the unstable condition due to abnormal temperature gradient ceases. Again, while the surface stratum is in stable equilibrium, it may happen that the second mile of air is abnormally hot, and the third mile abnormally cold, and thus a vortex may occur in mid air, without disturbing the face of the earth.

Whatever be the initial atmospheric condition causing the vertical uprush, the nature of the resulting circulation is in general that of the cyclone, illustrated, in part, by the whirling vortex of water in a basin. As the current ascends, an indraught occurs in all the lower regions of air, and an outflow in all directions above, sometimes at the height of a mile or two, again in all the region next to the isothermal layer. As the earth has at all places above the equator a component of rotation about the vertical line, it follows that in northern latitudes all the air flowing toward the vortex is in a whirl opposite in motion to the hands of a watch lying face upward, and all the outflowing air above has a like angular motion, but gradually diminishing until it is reversed. At the lower portion of the vortex the air whirls inward and upward with increasing velocity, while above, it whirls outward and upward, with waning velocity, thus moving in a double-spiral path shaped like a cord wound on an hourglass. In the constricted part, or neutral plane of the vortex, the air moves neither outward nor inward, but spirals straight upward. To match the upflow, and complete the closed circulation, there must be a downflow on the exterior of the cyclone, and since the whirl is reversed in direction, this outer mass of downflowing reverse-whirling air embracing the cyclone is called the anticyclone.

Between the inner and outer vortex the air is comparatively calm and the pressure is a maximum, with steepest gradient toward the center of the cyclone. Also the air is calm just at the axis of the vortex, while for some distance away its speed increases as the radius of its whirl, so that the central mass rotates practically as a solid column, thus still further lowering the pressure near the axis. This solidly rotating central column of air is sometimes called the core of the vortex.

High above the center of the cyclone, where perhaps the air is sucked downward, clarified by compression, then whirled outward, the sky is usually clear, or thinly fogged, while without this central patch are heavy clouds. The obscure or clear central part is called the “eye[66] of the storm.” Through this the cirrus clouds may sometimes be seen high above, either stationary or radiating away, if the vortex extends so high. Sailors on the deck of a vessel passing through a cyclone have often noticed the eye of the storm overhead, perhaps ten or twelve degrees in diameter, and with special clearness in the tropics. To the white, feathery cirrus clouds, scurrying away radially from the top of the vortex, they have given the name “plumes of the storm,” or “mares’ tails.” In sailing their vessel through the center of a cyclone, they have observed the circulatory motion of the winds and clouds, and frequently have found the deck covered or surrounded with cyclone sweepings, such as land and water birds, insects, butterflies, etc., brought into the quiet core of the vortex from the incurving winds beyond. Further details of the motion in a cyclone vortex are given as follows by Ferrel, §178:

“In Fig. 49 is given a graphic representation of the resultant motions and of the barometric pressures for both the surface of the earth and for some level high up in the atmosphere and above the neutral plane, where the motions in the vertical circulation are outward from the center. The solid circles represent isobars at the earth’s surface and the solid arrows the directions, and in some measure, by their different lengths, the relative velocities of the wind. The heavy circle represents the circle of greatest barometric pressure at the earth’s surface, say 765 mm., while the pressure of the outer border is 760 mm., and the dividing line between the cyclone and the anticyclonic gyrations. Within this limit the pressure diminishes to the center, and the gyrations are cyclonic, and the direction of the resultant of motion inclines in toward the center, but beyond that limit the gyrations are anticyclonic, and the direction of resultant motion inclines toward the outer border of these gyrations. The heavy dotted circle represents the circle of maximum pressure at some high level, and is much nearer the center than that at the earth’s surface. It is also the dividing line between the cyclonic and anticyclonic gyrations at that level. The dotted arrows indicate the directions and in some measure the relative velocities, of the wind at this level. The arrows in the cyclonic part represent the direction of the wind as declining outward, because the plane here considered is supposed to be above the neutral plane, where the radial component of motion is outward, but for any level below the neutral plane the inclination is still inward. The arrows are shorter above in the cyclonic part and longer in the anticyclonic part than they are at the earth’s surface, since the cyclonic gyratory velocities decrease and the anticyclonic increase with increase of altitude.

Fig. 49.—Velocity Diagram in Horizontal Section of a Cyclone.

“The upper part of the figure is a representation of a vertical section of the air, very much exaggerated in altitude, in which the solid curved line represents a section of an isobaric surface near the earth’s surface, say of 740 mm. barometric pressure. The lowest part corresponds with the center of the cyclone and the highest part with the heavy circle in the lower part of the figure, and the steepest gradients with the longest solid arrows, since the greater the gyratory velocities at the earth’s surface the greater the gradients, though they are not strictly proportional. The second dotted curved line from the top represents a section of the isobaric surface of high altitudes, in which the highest parts correspond with the heavy dotted circle below, since the highest pressure at all altitudes is very nearly where the cyclonic gyrations vanish and change to the anti-cyclonic. The depression here is smaller because the cyclonic area is smaller, and the gyratory velocities less, than at the earth’s surface. The upper dotted line belongs to an isobaric surface still higher, where the gyrations are supposed to be all anti-cyclonic, and here, consequently, the greatest pressure is in the center, as indicated by the curved line.

“As the interior of the whole cyclonic system is warmer than the exterior, and consequently the air less dense, the distances between the isobaric surfaces are necessarily greater in the interior than the exterior part, and so, however much the isobaric surface at or near the earth’s surface may be depressed by the cyclone gyration there, at a considerable altitude, if the temperature difference is great enough, it must become convex instead of concave.

“The track of any given particle of air in a cyclone, resulting from the vertical and gyratory circulation, is that of a large converging and ascending spiral in the lower part, but of a diverging and ascending spiral in the upper strata of the atmosphere, and the nearer the earth’s surface the more nearly horizontal is the motion, since the vertical component gradually decreases and vanishes at the surface.

“The whole energy of the system by which the inertia of the air and the frictional resistance are overcome and the motions maintained, is in the greater interior temperature and the temperature gradients, by which the circulation is maintained. This being kept up, the deflections and gyrations are merely the result of the modifying influence of the earth’s rotation, which is not a real force, since it does not give rise to kinetic energy, but merely to changes of direction.

“It must be borne in mind that the preceding is a representation of the motions and pressures of a cyclone resulting from perfectly regular conditions, in an atmosphere otherwise undisturbed, and having a uniform temperature, except so far as it is affected by the temperature disturbance arising from the cyclonic conditions. Accordingly results so regular are not to be found in Nature, but generally only rough approximations to them.

“Since the wind inclines less and less toward the center of the cyclone below the neutral plane and declines from the center above it, the upper currents above this plane in a cyclone are always from a direction, in the northern hemisphere, a little to the right of that of the lower currents, when not affected by abnormal circumstances.”

Observation of cyclones in Nature very well confirms the leading features set forth on theoretical grounds. If the vortex pass centrally over an observatory there is noted first a high barometer and calm air, attended perhaps by scurrying cirrus clouds; next a rapidly falling pressure and increasing wind, with dark clouds and precipitation, commonly accompanied by thunder and lightning; then the hushing of the storm to a dead calm, and low barometer and thinning or clearing of the clouds overhead; then a rising barometer with renewed winds in the reverse direction, and finally subsiding winds, rising barometer and clearing weather. These phenomena are the more definitely presented if the whirl is strong while its travel along the earth is slow. But owing to their progressive easterly motions, cyclones in the north have their moist hot southern masses elevated, chilled and precipitated on their eastern fronts and beyond, while their rear experiences the opposite action and is called the clearing side. Conversely in the tropics the westerly moving cyclones have cloudy and wet rears, because the easterly drift on high carries the precipitating masses toward the rear. The general hygrometric appearance of a centrally passing cyclone in middle latitude is thus described by Ferrel, §207:

“In the regular progression of a cyclone in the middle latitudes somewhat centrally over a place, the cloud and rain area of the front part, extending far toward the east, first passes over, occupying a half-day, or a day and more, and then the front part of the ring of dense cloud with a heavy shower of rainfall. After this there are indications of a clearing up, and even the sun may break through the cloud for an hour or two; but presently there is an apparent gathering and thickening of the cloud and a second shower. This is at the time of the passage of the rear side of the ring of denser cloud. After this there is the final clearing up.”

Except for special conditions, cyclones are never stationary, but drift along with the general march of the atmosphere, like dimpling eddies in a stately flowing river. In general, therefore, their trend is westward in lower latitudes, eastward in middle and higher latitudes, with a pace slow or swift according to the prevailing current. Notably also they have a poleward trend. Thus, if the path extends from tropic to temperate clime, it is frequently concave toward the east and sensibly parabolic in form. This is markedly true of those swift-whirling, small cyclones called hurricanes,[67] and particularly those vigorous ones blowing past the West Indies and the Philippines, and those that vex the Indian Ocean.

As to the speed of travel of cyclones, that may be judged, at least for northern latitudes, from the accompanying table, taken from Loomis,[68] and showing the average monthly rate of progression in miles per hour, of cyclone centers over the United States, the Atlantic Ocean and Europe. In general, beyond the tropics tall cyclones travel faster than short ones, owing to the faster drift of the higher strata.

Month.United States.Atlantic Ocean
Middle Latitudes.		Europe.
January33.817.417.4
February34.219.518.0
March31.519.717.5
April27.519.416.2
May25.516.614.7
June24.417.515.8
July24.615.814.2
August22.616.314.0
September24.717.217.3
October27.618.719.0
November29.920.018.6
December33.418.317.9
Year28.418.016.7

To find the actual speed of the wind at a place, of course, the linear velocities of whirl and of translation must be combined; or, vice versa, if one of these be known it can be graphically subtracted from the observed wind velocity to find the other. This combination of two wind components to find their resultant, or, vice versa, can easily be done by laying off on paper, arrows of suitable length and direction to represent the two known velocities, placing the head of one arrow to the tail of the other, then completing the triangle, and taking its third side to represent the required wind velocity, in magnitude and direction. Obviously if the cyclone moves eastward, whirling oppositely to the hands of a watch, the swiftest wind is on its right side, which consequently is known as the dangerous side. In the northern hemisphere, therefore, the rule for dodging a great whirlwind is to run north, if that be practicable.

Stationary cyclones occur under favorable conditions. At least that name has been applied to columns of hot air streaming up from a fixed base, more or less circular. Every island in the ocean generates such a vortex on a clear, hot summer day, since its temperature far exceeds that of the surrounding water. All day long this uprush continues whatever be the humidity. And if the soil slopes upward steeply, the vortex is so much the stronger, particularly if the island be in a calm region. Above such a tract the gulls and vultures, and possibly even man, might soar all day without motive power. This condition and its interesting possibility deserve investigation.

Cyclones may occur at any season, but in general they are most abundant when the greatest temperature disturbances occur. The relative frequency of tropical cyclones for various localities and for the twelve months of the year is seen in the following table[69]:

The Yearly Periods of Cyclone Frequency in Several Seas

Arabian Sea.Bay of Bengal.S. Indian Ocean.Java Sea.China Sea.Havana.
No. of years23413940...85363
No. of cyclones701155312214355
Authority.Chambers.Blanford.Piddington
Thom and Reid.		Piddington
and Thom.		Schuck.Poey.
Jan.62172521
Feb.40254202
Mar.3219823
April13815823
May18167051
June2990053
July33001012
Aug.34001927
Sept.45202723
Oct.627201617
Nov.14167085
Dec.3861732

The tornado is a slender cyclone or hurricane. It is usually but a few yards or rods in diameter, and seldom exceeds one mile across its active column, whereas a cyclone may cover an area of any size from fifty to one or two thousand miles in diameter. Moreover, the cyclone requires for its inception an extensive pressure gradient marked by closed isobars, and once generated may last several days. A tornado per contra may spring into action where the lateral pressure is uniform, spend its force in a few moments, and leave a uniform barometric field in its wake. In shape the tornado is usually of greater height than width. The cyclone is far-flung laterally, but in height may not exceed the narrow tornado, since both must terminate beneath the isothermal layer, and commonly do not extend so high. Both vortices are caused by the ascensional force of hot air. In both the air spirals in and upward at the bottom, out and upward at the top, constantly cooling by expansion, and finally descends on the outside to complete the closed circulation. In general the tornado is the more violent and destructive, though limited to a brief and narrow path. More aptly, perhaps, the tornado may be called a slender hurricane of brief duration; both of them being small cyclones, or aërial vortices, of minor size and concentrated intensity. The relation of the tornado and cyclone has been defined as follows, by Professor Moore:

“The cyclone is a horizontally revolving disk of air of probably 1,000 miles in diameter, while the tornado is a revolving mass of air of only about 1,000 yards in diameter, and is simply an incident of the cyclone, nearly always occurring in its southeast quadrant. The cyclone may cause moderate or high winds through a vast expanse of territory, while the tornado, with a vortical motion almost unmeasurable, always leaves a trail of destruction in an area infinitesimal in comparison with the area covered by the cyclone.”

Two initial conditions seem essential to the genesis of a substantial tornado. In the first place, the atmosphere of its immediate locality must have appreciable gyration. Of course, in all extra equatorial regions the air has some incipient whirl due to the earth’s rotation, and this whirl is magnified as the fluid is sucked into the vortex. But the magnification may be slight owing to the brief lateral displacement of the air feeding the tornado. If, however, the fluid be drawn from a considerable distance, and have from local conditions some additional whirl superadded to that due to the earth’s rotation, the gyratory flow in the medium near the vortical axis may be very swift. On the other hand, the additional whirl, due to local conditions, may tend to neutralize that due to the earth’s component, thereby leaving a very feeble gyration, if any. But in general the rotation of tornadoes is observed to be in the direction of the earth’s component; to the left north of the equator, to the right south of it. This observation is doubtless the more striking because when the accidental local spin conspires with the permanent terrestrial one, the resultant whirl is intensified, while in the opposite case it is so enfeebled as to attract scant, if any attention.

In the second place, the genesis of a tornado requires unstable equilibrium in the local atmosphere. This instability, as in cyclones, may arise from abnormal temperature gradation. Thus, if along any vertical the temperature falls more than six degrees Centigrade for one thousand meters ascent, a mass of air started upward will continue to rise, since it cools less rapidly than the environing medium. In this way there will ensue a continuous uprush of air so long as the unstable state endures; and the action may be very vigorous if a large stratum of air is greatly heated before it disrupts into the cold upper layers. In general, the loftier the tornado the more violent it is, just as the taller flue generates the stronger draft with the same temperature gradient.

Dynamically, the tornado may be treated as a rotating pillar of air in which each mass of fluid fairly retains its angular momentum. This means that for any mass of the whirling air the radius of its path, multiplied by its circular speed, remains a constant product; in other words, the velocity of whirl varies inversely as the radius. Accordingly, the circular velocity is exceedingly rapid where the radius is very small. Now, when any mass runs round a circle its centrifugal force is known to be directly as the square of the speed of its centroid and inversely as the radius. But by the above assumption the speed itself is inversely as the radius. Hence, the centrifugal force varies inversely as the cube of the radius of the inflowing mass of air. This centrifugal force, acting on the inner layers of air of the rotating column, must be supported by the pressure against them exerted by the outer layers as they pass inward. Thus there is a strong barometric gradient from the remote still air toward the swiftly whirling parts of the vortex.

It follows from the above argument that inside a tornado the barometric pressure may be much below the normal; and it is easy to see that if a barometer, starting from some point on the tornado base, be moved vertically upward it must show a declining pressure, but if moved upward and outward it may be made to show a constant pressure all the way to the upper portion of the vortex. The instrument would thus travel along an isobaric, bell-shaped surface opening upward. On a series, therefore, of concentric circles on the base of a tornado, we may erect a family of coaxial bell-shaped surfaces to mark the points of equal pressure, and thus map out the isobars of the vortex. Inside these coaxial surfaces reaching to earth, others of still lower pressure may be drawn tapering downward to a rounded point and terminating at various places on the axis. In an actual tornado one of these infinitely numerous funnel-shaped isobaric surfaces may become distinctly outlined and visible, if the air has sufficient moisture to start precipitation when it reaches a surface of suitably low pressure. This quite usually occurs in Nature, the funnel sometimes reaching to earth, sometimes only part way, according to the pressure at which precipitation begins, this pressure depending, of course, on the percentage of humidity of the uprushing air.

The form of the funnel-like cloud ere it reaches the earth is interesting. Being an isobaric surface, it would support in static equilibrium a free particle resting on it and sharing its rotatory motion. The lower rounded part of the funnel is parabolic, the upper outer part hyperbolic; the two together delineating the well-known Rankine double vortex of hydrodynamics. Students of hydrostatics know that when a glass of water is spun round its axis at a fixed velocity, the dimple observed is of parabolic form, and if frozen will sustain in repose a small shot resting on its surface and whirling with it. Similarly the lower part of the funnel is parabolic because in it the air rotates, as one solid body, while the broader part of the funnel is hyperbolic because in it the air has a speed inversely proportional to its radius of motion.

If everywhere in a tornado the circular velocity of the inflowing air were inversely proportional to the radius, as above assumed, the speed near the axis would be indefinitely great. This cannot be admitted. Practically, the inflow ceases when the centrifugal force of the gyrating stratum equals the pressure urging it toward the axis. Within this stratum is a column of air rotating everywhere with constant angular velocity about the vortical axis, and thus having quite calm air at its center. Outside this solidly rotating core the air spirals radially inward and upward. Some idea of the stream lines in such spiral flow may be obtained from Fig. 50 if a rapid circular motion be added to the inward and upward velocity represented by the arrows.

In the foregoing discussion no account of friction was taken. Near the earth’s surface this dampens the whirl and centrifugal force, so that the air flows more directly into the vortex, while farther aloft the centrifugal force near the axis so effectually checks the inflow as to allow the central core of air to rush up nearly unimpeded, as in a walled flue, taking its draught mostly from the lower part. As a consequence, the upward speed of the heated air in the tornado tube may be enormous, supporting in its stream objects of considerable mass.

Morey

Fig. 50.—Funnel-like Cloud
Sometimes Observed in a Tornado.

The true horizontal speed anywhere in a tornado is compounded of the velocities of gyration and of translation, as in the cyclone. Hence the advancing side may be considerably the swifter and more destructive, particularly more destructive since the impact of air increases as the square of the velocity. If the vortex were stationary it would be equally dangerous on all sides, standing erect and symmetrical; but it drifts with the whole mass of air, sometimes quite swiftly and often with varying speed of travel at different levels; thus, in its slenderest forms, appearing bent and not infrequently twisted, as it advances writhing serpentlike through the sky. Furthermore, the intensity of whirl may fluctuate momentarily, with consequent shifting of the isobaric surface, including that one whose form is visible by reason of incipient condensation; and thus the funnel-like misty tongue appears to dart earthward as a foggy downshoot from the cloud above, whereas its parts are really rushing upward at all times very swiftly, whether visible or not. This agile protrusion of the nimbus, now a tongue, now a dark and mighty tower, is the strenuous part of the storm, the abominated “twister” which the Kansan farmer sedulously shuns, or peeps at from a hole in the ground. Unwelcome, indeed, are its visitations, when, with mickle and multitudinous roar, it claps his house in sudden darkness, hurls it aloft and sows its sacred relics over all the adjoining township, “that with the hurly-burly hell itself awakes.”

Theory, as well as experience, accredits the tornado with vast energy and power. For, suppose a surface stratum of air one mile in area and one thousand feet thick to increase in absolute temperature one per cent, thus uplifting the superincumbent atmosphere ten feet. The total energy stored in this way equals the weight lifted multiplied by its upward displacement. The weight is a ton per square foot and the displacement is ten feet; hence the stored energy is ten-foot tons per square foot of the heated tract, or about 280,000,000 foot tons for the square mile of heated air. This is equivalent to the work of one million horses for over a quarter of an hour. A goodly percentage of this stored work may be converted into kinetic energy in the active part of the dry tornado. It is the energy of a vast reservoir suddenly gushing through a tall penstock. It is a colossal upward cataract, an aërial Niagara, a Johnstown flood suddenly liberated and quickly spent.

A vortex of that description possesses enormous devastating power, for it is endowed with four destructive elements: rapid onset for razing, violent spin for distorting, swift uprush for lifting, low pressure for disrupting. These four grim powers may operate at once and in accord. When, for example, they assault a house, the horizontal blasts push and wrench it on the foundation, the cellar air suddenly expanding puffs it aloft, the internal air bursts its walls or windows, the uprush carries its members on high and scatters them wantonly to the four winds. These powers are abundantly attested by authentic reports from many localities.

When the tornado appears as a misty column it is familiarly called a “waterspout,” particularly if it appears over a sea or lake. As already explained, the visible and cloudy portion of the column is due to condensation of the aqueous vapor in the air, as it rushes expanding and cooling into the low pressure part of the vortex. From the lashed and rippling sea surface, where it upcones into the base of the spout, some water is carried aloft as spray mingling with the mist of the chilled vapor, but not necessarily in very large proportion, and never rising in solid body to the cloud, as popularly supposed. On the contrary, waterspouts, however massive and formidable looking, are very tenuous, and may occur on land or water indifferently. Doubtless they are better defined, more regular and more familiar over water, and hence their name; but essentially they are vapor spouts, though mingled at times with dust or spray. Owing to rapid precipitation of the uprushing aqueous vapor, there may be heavy rainfall on all sides of the waterspout, so that at sea it may be difficult for the observer to ascertain how much of the downpour is salt water and how much is fresh. On land the downpour is sometimes mingled with débris, and even with live fish and frogs caught up from neighboring bodies Of water. Copious hail also may fall with the rain, if the vortex be a lofty one.

Fig. 51.—Vertical Section of the St. Louis, Mo., Tornado of May 27, 1896, Showing the Vortex Tubes in a Theoretical, Truncated, Dumbbell-shaped Vortex.

Fig. 52.—Horizontal Section of St. Louis Tornado of May 27, 1896.

The following description and analysis of a representative spout is due to Professor Bigelow of the U. S. Weather Bureau:[70]

“The tornado may be illustrated by the St. Louis storm of May 27, 1896. It is a truncated dumbbell vortex out off at the ground on the plane where the inflowing angle is about 30°. This vortex is much smaller than the hurricane, although of the same type. It is about 1,200 meters high and about 2,000 meters in diameter on the surface. The vortex tubes are shown in Figs. 51 and 52. In these figures can be seen the vortex tubes, geometrically spaced, through each of which the same amount of air rises. The rotating velocity is greatest about 300 meters above the ground, but the dimensions are such as to produce enormous velocities in the lower levels. The radius in the outer tube is taken to be 960 meters, and the inner tube 55 meters. The radial inward velocity on the outer tube is—8 meters per second; on the outer tube the tangential velocity is 13 meters per second, and on the inner 224 meters per second; on the outer tube the vertical velocity is 0.27, and on the inner tube it is 80 meters per second. On the outer tube the total velocity is 15 meters per second, and on the inner tube 270 meters per second. The volume of air ascending in each tube is 774,500 cubic meters per second. On account of the distortion of the theoretical vortex, due to the cutting of the lower portion by the truncated plane, and to the progressive motion of the whole system that constitutes the tornado, there is difficulty in computing the pressure to fit these observed velocities and radii.

“Tornadoes occur in the southern and southeastern quadrants of areas of low pressure, along the borders of the cold and the warm masses which entered into the structure of the cyclone. When a cold mass is superposed upon a warm mass, as was the case at St. Louis, a tornado will occur if the difference in specific gravity be sufficient to inaugurate a violent mixing, and the rotation be about a vertical axis, instead of about a horizontal axis, as in the case of thunderstorms.”

Morey

Fig. 53.—Vertical Section of Short Tornado.

The size and form of waterspouts alter greatly with the state of the atmosphere. As Ferrel observes, they may vary “from that of a cloud brought down over a large area of the earth’s surface in a tornado where the air is nearly saturated with vapor and the general base of the clouds very low, somewhat as represented in Fig. 53, to that which occurs when the air is very dry, and when the tornadic action is barely able to bring the cloud down from a great height into a slender spout of small diameter, somewhat as represented in Fig. 54. Horner says that their diameters range from 2 to 200 feet, and their heights from 30 to 1,500 feet. Dr. Reye states that their diameters on land, at base, are sometimes more than 1,000 feet. Oersted puts the usual height of waterspouts from 1,500 feet to 2,000 feet, but states that in some rare cases they cannot be much less than 5,000 or 6,000 feet. On the 14th of August, 1847, Professor Loomis observed a waterspout on Lake Erie, the height of which, by a rough estimate, was a half mile, and the diameter about 10 rods at the base and 20 rods above.

“Judge Williams, in speaking of the tornado of Lee’s Summit, where he saw it, says: ‘It seemed to be about the size of a man’s body where it touched the clouds above, and then tapered down to the size of a mere rod.’”

Morey

Fig. 54.—Vertical Section of a Tall Tornado.

When the tornado vortex is so tall and strong as to carry raindrops up to freezing strata it is commonly known as a hailstorm. The congealing occurs usually in those isobaric surfaces which dip down in the center of the vortex, but reach only part way to the earth. As indicated in Fig. 55, the clear aqueous vapor near the earth is condensed to cloud on crossing an isobaric surface of sufficiently low pressure and temperature; then it proceeds as mingled cloud and rain till it crosses the freezing isobar into the region of snow and hail formation; thence finally curves outwardly to stiller air and descends as a cloud of mingled vapor, rain and frozen parts. Of this frozen shower one part may come to earth as hail or rain, the snow and sleet melting on the way; while another part may be redrawn into the swift uprush, and carried aloft till its frozen drops, or pellets, have grown so large by accretion as to plunge to earth by sheer bulk, even though they must traverse a furious ascending wind. A good illustration from Nature of this cycle in the center of a hailstorm is presented in the following by Mr. John Wise, America’s adventurous pioneer balloonist:

“This storm originated over the town of Carlisle, Pa., on the 17th of June, 1843. I entered it just as it was forming. The nucleus cloud was just spreading out as I entered the vortex unsuspectingly. I was hurled into it so quickly that I had no opportunity of viewing the surroundings outside, and must therefore confine this relation to its internal action. On entering it the motions of the air swung the balloon to and fro and around in a circle, and a dismal, howling noise accompanied the unpleasant and sickening motion, and in a few minutes thereafter was heard the falling of heavy rain below, resembling in sound a cataract. The color of the cloud internally was of a milky hue, somewhat like a dense body of steam in the open air, and the cold was so sharp that my beard became bushy with hoar frost. As there were no electric explosions in this storm during my incarceration, it might have been borne comfortably enough but for the seasickness occasioned by the agitated air-storm. Still, I could hear and see, and even smell, everything close by and around. Little pellets of snow (with an icy nucleus when broken) were pattering profusely around me in promiscuous and confused disorder, and slight blasts of wind seemed occasionally to penetrate this cloud laterally, notwithstanding there was an upmoving column of wind all the while. This upmoving stream would carry the balloon up to a point in the upper clouds, where its force was expended by the outspreading of its vapor, whence the balloon would be thrown outward, fall down some distance, then be drawn into the vortex, again be carried upward to perform the same revolution, until I had gone through the cold furnace seven or eight times; and all this time the smell of sulphur, or what is now termed ozone, was perceptible, and I was sweating profusely from some cause unknown to me, unless it was from undue excitement. The last time of descent in this cloud brought the balloon through its base, where, instead of pellets of snow, there was encountered a drenching rain, with which I came into a clear field, and the storm passed on.”

As might be expected the hailstones vary much in form, size and quantity. If by chance any stones become slightly flattened they ride level in the ascending current, and hence by aggelation grow most rapidly on the periphery which is a line of diminished pressure. At times they are more or less oval, and again they appear as fragments of considerable masses of ice, broken perhaps by collision in the violent parts of the tornado tube. Their great variety in shape and bulk may be appreciated from the following extracts taken from the records of the Signal Service:

Fig. 55.—Vertical Section of a Hail Tornado.

In Professional Paper of the Signal Service No. 4, describing the tornadoes of May 29th and 30th, 1879, in Kansas, Nebraska, Missouri, and Iowa, this passage occurs relative to a tornado at Delphos, Mo.:

“On the farm of Mr. Peter Bock, in the adjoining township of Fountain, about 4 miles W. of the storm’s centre, and during the hailstorm that preceded the tornado, masses of ice fell as large as a man’s head, breaking in pieces as they struck the earth. One measured 13 inches in circumference, another 15, and a hole made by one that fell near the place of Mr. J. H. Kams measured 7 inches across one way and 8 the other. This immense fragment of aërial ice broke into small pieces, so that its exact size could not be determined.”

The following description is given of the tornado that visited Lincoln County, Neb., at that time:

“At first the hailstones were about the size of marbles, but they rapidly increased in diameter until they were as large as hens’ eggs and very uniform in shape. After the precipitation had continued about fifteen minutes, the wind ceased and the small hail nearly stopped, when there commenced to fall perpendicularly large bodies of frozen snow and ice, some round and smooth and as large as a pint bowl, others inclined to be flat, with scalloped edges, and others resembled rough sea-shells. One of the latter, after being exposed an hour to the sun, measured fourteen inches in circumference.”

The following was reported by the Signal Service observer at Fort Elliott, Tex., 1888:

“A thunder-storm began at 4.10 p.m. and ended at 7.40 p.m., moving from southwest to northwest. Hail began at 5.18 p.m. and ended at 5.26 p.m., the hailstones being spheroidal in shape and about two inches in diameter; formation, solid snow. The ‘break’ (hills) at the foot of the plains several miles northwest of station were absolutely white with hailstones for three hours after the storm. This was observed by everybody at the station; on the morning of the 26th I walked down to the Sweetwater Creek, three fourths of a mile distant, and saw great banks of hailstones which had been washed down during the night. The bottoms along the Sweetwater were literally covered with banks of hailstones from six to eight feet in depth. It was estimated that there was enough hail to cover ten acres to a depth of six feet. The hailstones killed five horses which were out on the prairie on a ranch six miles north of station. The Sweetwater Creek was higher than ever known before, the freshet destroying nearly the entire post garden. The high water is supposed to have been caused by a ‘cloud-burst’ at or near the foot of the plains, where the Sweetwater has its source; there was only 0.36 inch of rainfall at the station. On Sunday, May 27th, hailstones were collected on the banks of the Sweetwater, which had been washed down and lay in drifts 6 feet deep, actual measurement by the observer.”

When, after imprisonment and long sustention in a powerful tornadic vortex, the accumulated rain or hail finally breaks through and pours down to earth, in solid cataract, the phenomenon is commonly called a cloud-burst. The foregoing example is a partial illustration. The following is quoted from Espy, describing a cloud-burst near Hollidaysburg, Penn., in which the water seems to have poured down nearly in a solid stream:

“On examining the northern side of this ridge, large masses of gravel and rocks and trees and earth, to the number of 22, were found lying at the base on the plain below, having been washed down from the side of the ridge by running water. The places from which these masses started could easily be seen from the base, being only about 30 yards up the side. On going to the head of these washes they were found to be nearly round basins from 1 to 6 feet deep, without any drains leading into them from above. The old leaves of last year’s growth, and other light materials, were lying undisturbed above, within an inch of the rim of these basins, which were generally cut down nearly perpendicularly on the upper side, and washed out clean on the lower. The greater part of these basins were nearly of the same diameter, about 20 feet, and the trees that stood in their places were all washed out. Those below the basin were generally standing, and showed by the leaves and grass drifted on their upper side how high the water was in running down the side of the ridge; on some it was as high as three feet. It probably, however, dashed up on the trees above its general level.”

Dry whirlwinds of moderate size, but sometimes of considerable violence, frequently occur in clear weather when the percentage of humidity is small and when the vertical temperature gradient is unusually pronounced. In this case there may be strong agitation of the air, rendered visible at the earth’s surface by light débris on land, or boiling of the water at sea; but the main body of the tube is invisible and free from mist except high up where precipitation begins, capped by a growing patch of white cloud in a clear sky, and which may gradually broaden and condense sufficiently to cause a shower of rain. On land the dry whirlwind may be delineated as a tall column, by whirling dust or sand. In this case, if the gyration is violent, the central core may appear clean and clear owing to the centrifugal force which keeps the grains out where they are balanced by the pressure of the inrushing air. In such vortices the sand spout may appear to be hollow as in the case of waterspouts whose interior cores are free from cloud or condensed vapor. On the other hand, myriads of mild transparent whirlwinds unmarked, except by down or humanly invisible dust, or dim aërial refractions, may frisk and play in the boundless sky unnoticed by the blunt eyes of men, yet constantly engaged in generating or marshaling the clouds and in buoying upward the ponderous eagles, the vultures and the whole brood of passive flyers whom we have not yet learned to emulate. Thus when we remember that an upward trend of air of scarcely one yard per second, and too feeble to support a falling hair, is yet sufficient to carry the condor and albatross without wing beat, it seems important to explore these minor vortices and to ascertain their availability and practical usefulness for human soaring.