February i, 1889.] 



SCIENCE. 



^3 



SOMETHING ABOUT TORNADOES.' 



Where atmospheric equilibrium is violently discomposed and 

 the agitation covers but a very limited area, the centripetal force 

 becomes much greater than in the case of cyclones. The gyrations 

 are exceedingly rapid and very near the centre ; in fact, the vio- 

 lence is greatest at the centre, and diminishes rapidly to the e.xter- 

 nal parts of the disturbance, where the gyrations cease altogether. 

 This form of disturbance is called a tornado. In order that condi- 

 tions may become favorable for tornadic development, the atmos- 

 phere must assume the unstable state. This state will be engen- 

 dered whenever the rate of decrease of temperature is greater in 

 the surrounding quiet air than it is in the mass of ascending -air. - 

 The adverse or counter m.ovement of northerly and southerly winds 

 induces the unstable state, because it makes the rate of decrease 

 of temperature greater in the quiet air than in the column of as- 

 cending air ; that is, the upper strata of the air will be made colder, 

 and the lower strata warmer. 



What are known as local storms, under this class of motions of 

 the atmosphere, are not affected by the rotation of the earth upon 

 its axis, because the area of disturbance is too small. The pro- 

 gressive motion of tornadoes to the north-east arises from the fact, 

 that, as they always form in the south-east quadrant of an area of 

 low barometer, they must come within and under the influence of 

 the general drift of the atmosphere on that side of the " low,' 

 which, according to the law of atmospheric circulation about the 

 centre of an area of low pressure, is always to the north-east. 



The condition of tornado-formation in regard to heat is simply 

 that of unstable equilibrium for saturated air at the existing tem- 

 perature, the other condition being that the air shall have a gyra- 

 tory motion relative to some central point, arising from any cause 

 whatever. In the unstable state, the lower strata of the atmos- 

 phere are liable to burst up through the upper layers at any point 

 where there may chance to exist some slight predetermining 

 cause, which is never wanting, arising from local conditions of tem- 

 perature and moisture. 



When an upper motion of the atmosphere is started at any point, 

 the air thus engaged is kept warmer, and therefore rarer, than the 

 region immediately surrounding it. This continues as long as the 

 ascending current is supplied with air nearly or quite saturated, or 

 until, from an inversion of the air in the lower strata, the state of 

 unstable equilibrium is changed. The violent whirling motion of 

 the air which characterizes the tornado is dependent upon a pre- 

 existing disturbed and gyrating state of the atmosphere. The case 

 is somewhat similar to that of water in a shallow basin running 

 out through a hole in the centre. If the initial state of the water is 

 that of perfect rest, it flows directly toward the centre with a very 

 slow velocity ; but, if there is the least initial disturbance of a gyra- 

 tory character when the water first begins to flow, it soon runs into 

 rapid gyrations around that centre. 



As we have shown, therefore, there are two principal conditions 

 upon which the occurrence of tornadoes depends, and in the ab- 

 sence of either of which they cannot take place. The one is the 

 state of unstable equilibrium of the air, and the other a circulation 

 motion with reference to any centre of disturbance. It is not im- 

 perative that the central area shall be stationary, but simply that 

 the motion of the air around it shall be such that when the latter 

 is drawn in toward the centre it will take a gyration around it. 

 When these two principal conditions are present, there is scarcely 

 ever wanting the secondary condition, which, through the effect of 

 some slight initial disturbance, gives rise to an upward burst of the 

 air through the overlying strata. The places and time most favor- 

 able to the development of unstable equilibrium and a gyratory mo- 

 tion of the atmosphere are those in which tornadoes are most likely 

 to occur. Of these two conditions, the unstable equilibrium is 

 the most important, since it more rarely occurs than the other, 

 which is scarcely ever so entirely absent as not to give some slight 

 gyratory motion, which becomes violent very near the centre. The 

 question naturally occurs. Where are the places on the earth's 

 surface, and what is the time. when conditions- are most- favorable 

 for the development of tornadoes ? 



^ Portion of a paper read before the National Geographic Society of Washington, 

 Nov. 16, i883, by J. .P. Finley. 



First, as to places. These are found where, in the general mo- 

 tions of the atmosphere as affected by continents and mountain- 

 ranges, currents of the air at the earth's surface, which come from 

 a warmer latitude, or at sea from a much warmer continent, are 

 caused to flow under the colder upper strata, where the normal 

 motion is nearly eastward, and where consequently the temperature 

 is the normal one, not affected by such motions as take place in the 

 lower strata: in other words, tornadoes are most likely to occur in 

 regions where warm, moist air flows underneath a colder and dryer 

 upper strata, coming from another direction. Such regions are 

 found particularly in the Mississippi, Missouri, and Ohio valleys, 

 and in Alabama, Georgia, and the Carolinas. The other condition 

 of- the atmosphere indispensable to the formation of tornadoes — 

 viz., a relative gyratory motion with regard to any point — is found 

 to an unusual extent in the regions above named, especially in the 

 winter season, which fact accounts for the frequent occurrence of 

 tornadoes in the Southern States during the winter and spring, and 

 occasionally in the Mississippi and the Ohio valleys. 



Second, as to time. The summer season is the most favorable 

 for tornadoes, when the interior of the continent is warmed up, and 

 the air of the lower strata is drawn from lower latitudes far up into 

 the northern portions of the country on the eastern side of the 

 Rocky Mountains, and the isothermal curve is deflected very de- 

 cidedly toward the north. From this cause the temperature of the 

 lower strata of this region becomes much higher than that of the 

 superdominant strata; and, if this condition does not of itself in- 

 duce the unstable state, it is readily accomplished by the addition- 

 of any small effect from some other cause, as from extremely warm 

 weather, in which the earth's surface and the lower air strata be- 

 come abnormally heated. 



The great moisture of the air in the southerly winds is also favor- 

 able to the induction of the unstable state, since such a change is 

 more readily brought about in air nearly or quite saturated. The 

 southerly currents curving eastward from the Rocky Mountain and 

 Appalachian ranges give rise to a general air movement of consid- 

 erable force toward the Atlantic Ocean, as the result of which, cold 

 counter currents pass southward to Texas, east of the Rocky 

 Mountains, and to Florida over the Appalachian range, somewhat 

 after the manner of the Arctic currents which flow southward to 

 Florida, between the Gulf Stream and the coast of the United 

 States. 



In the summer season this flow of cold air southward is confined, 

 to a comparatively narrow belt east of the Rocky Mountains, for at 

 that time the warm, moist currents of the Gulf are drawn very far 

 to the north and west. At this season the northern part of Texas 

 has the same mean temperature as Minnesota, the isotherms being 

 nearly north and south in direction ; and the temperature gra- 

 dient between the warm southerly winds on one side, and the cold 

 northerly currents on the other, is similar to that of the cold wall 

 between the Gulf and Arctic currents. 



In the winter and spring the flow of cold air southward from the 

 higher latitudes extends to the Appalachian range, where it over- 

 flows the warm, moist southerly currents from the East Gulf and 

 South Atlantic coast. It is this tendency of the air to flow in con- 

 trary directions, where the conditions are most favorable to pro- 

 duce the unstable state of the atmosphere, that pronounces the re- 

 gions here indicated as the " battle-ground of tornadoes." The 

 word " low," as is quite well known, marks upon the weather-map 

 the centre of lowest pressure as indicated by the barometer and 

 direction of the wind. This is practically the centre of the general 

 storm or atmospheric disturbance. 



I. The conditions for the development of tornadoes are most 

 favorable in the south-east quadrant of a " low : " in fact, they are 

 not to be looked for in any other portion of the general disturbance. 

 2. Tornadoes very generally accompany a ■' low," for the reason 

 that the condition of unstable equilibrium necessary' in the forma- 

 tion of a tornado is also required in the " low," at least in the up- 

 per cloud-region. 3. The unstable state in a " low " very rarely- 

 extends down to the earth's surface, so thatjornadoes are not ne- 

 cessarily visible in every general storm. 4. There are frequently 

 secondary whirls, incipient tornadoes, in the cloud-region of a 

 "low," the effects of which do not reach down to the earth's sur- 

 face, and the only visible effect above is the formation of a local 



