April 26, 1900] 



NATURE 



611 



LETTERS TO THE EDITOR. 



[ The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 manuscripts intended for this or any other part of Nature. 

 No notice is taken of anonymous communications. '\ 



Recent Exploration in the Upper Air and its Bearing 

 on the Theory of Cyclones. 



Aboit ten years ago there was an interesting discussion in 

 regard to the theory of cyclones, by such leaders in meteorology 

 as Ferrel, Blanford, Hann and Davis (see Nature, vol. xliii. 

 p. 82 and p. 470). Since then, considerable new material has 

 lieen accumulated by research in the free air with kites and 

 balloons, and it seems appropriate to consider its bearing on 

 current theories. 



In America the work with kites, by Mr. Rotch, has resulted in 

 the discovery of the following facts : — 



(i) The atmosphere is separated by sharply marked inverted 

 vertical gradients of temperature into superposed strata, each 

 atratum potentially warmer than the one beneath. By potentially 

 warmer is meant that if any stratum were brought down, ii would 

 be heated by compression and become warmer than the stratum 

 it replaced. There are usually two, and sometimes three, strata 

 between the ground and the altitude of 3000 metres. The 

 boundaries between these strata are regions of sharp contrast in 

 a vertical direction of temperature, of humidity (both ab.solute 

 and relative), and sometimes of wind direction. These 

 boundaries are also regions of maxima in wind velocity, and 

 the regions where clouds are chiefly found. 



(2) In the changes of condition of the atmosphere from day to 

 day, the minima of temperature and humidity occur simulta- 

 neously at all levels, except that in a superficial stratum within 

 alxjut 300 metres of the ground, the minimum sometimes occurs 

 later as a result of surf.^,ce cooling. 



(3) The air column up to 3000 metres above barometric 

 minima at sea-level averages about 10° F. warmer than the 

 air column up to 30CO metres above barometric maxima at sea- 

 level. 



(4) All the conditions which characterise the surface cyclone 

 and anticyclone, such as the circulation of the wind around a 

 central area, the clouds and the rainfall, usually do not exceed 

 the height of 3000 metres. Above that height there is an en- 

 tirely different distribution of pressure and wind circulation from 

 that observed at the earth's surface. 



<5) In the areas of low pressure in the upper atmosphere the 

 air is cold, extremely dry and clear. In the areas of high pres- 

 sure in the upper atmosphere the air is warm and frequently 

 moist. 



In kite-flights made on November 24 and 25, 1898, at Blue 

 Hill, there were evidences of three distinct wind circulations. 

 The surface cyclone did not exceed 800 metres (or half a mile) 

 in thickness, and above this was a warm-centre cyclone with 

 dense clouds and precipitation about 2000 metres in thickness. 

 At the height of 3000 metres (or about two miles) the wind, on 

 November 24, was found blowing from the south and circulating 

 around an area of low pressure with a cold, dry central area ; 

 while at the same time at the surface of the earth the wind was 

 from the north and circulating around a warm-centre surface 

 cyclone. (See Bulletins No. i, 1899, and No. I, 1900, of the 

 Klue Hill Meteorological Observatoryi. 



In France, M. Teisserenc de Bort has made a study of the air 

 by means of ballons sondes launched at frequent intervals from 

 Trappes. His results show that the annual period in the tem- 

 perature of the air is well marked up to and exceeding ten kilo- 

 metres. They show further that during the irregular warm and 

 cold periods in the atmosphere the i.sotherms rise and fall 

 simultaneously at all heights up to at least ten kilometres. In 

 other words, the warm and cold waves aloft occur simultaneously 

 with those near the surface (see Comptes rcndus, August 21, 1899). 



Dr. llergesell, of Strassburg, has discussed the records of the 

 international balloon ascents, and derived a number of important 

 conclusions. Among these are : (i) In the highest strata of the 

 atmosphere attainable by balloons the temperature change from 

 day to day, and the temperature gradients in a horizontal direc- 

 tion, are very marked. Within distances of only a few hundred 

 kilometres are sometimes temperatures at the same level which 

 <iiffer as much from each other as 3o''-40'' C. (2) Such temper- 

 ature distributions as that which brought frosts in Europe on 



i 



NO. I59I, VOL. 61] 



May 13, 1897, are not local and confined to the earth's surface, 

 but meteorological phenomena of great extent and importance 

 which embrace the entire atmosphere above Europe. (3) By 

 computing and plotting the air-pressure for the heights of 5000 

 and 10,000 metres, it was found that the areas of low pressure 

 at these heights coincide approximately with the areas of low 

 temperature, and in most cases are many hundreds of kilometres 

 from the surface cyclone. Thus, on March 24, 1899, the surface 

 cyclone or area of low pressure was along the north coast of the 

 Mediterranean, near Italy, while the minimum pressure at 5000 

 and 10,000 metres was in Finland, or even further north {Meleo- 

 roloi^sche Zeitschrift, January 1900). 



To compare these facts with theory, I have looked up the 

 views expressed in modern text-books and recent literature. I 

 find a number of different opinions in regard to the causes of 

 cyclones, and have classified them as follows : — 



(i) Instability produced by a rapid decrease of temperature 

 with increase of height ; that is, by a vertical gradient equalling 

 or exceeding the adiabatic rate. This may be called the theory 

 of Vt'rtical instability. 



(2) Instability produced by differences of temperature in a 

 horizontal direction. In such a case, in con.sequence of the 

 the difference in density, there is established a convectional 

 interchange of air between areas of different temperatures, and 

 there result differences of pressure, and consequently cyclones 

 and anticyclones. This theory I call the convection theory. 



(3) If a current of damp air is deflected upward by any means, 

 mechanical or otherwise, it cools by expansion, and its moisture 

 begins to condense. This condensation retards further cooling, 

 so that the air in rising may cool at a rate less rapid than that 

 ordinarily existing in the atmosphere. In such a case, the air 

 would continue to rise, and the conditions would be favourable 

 for storm formation, as long as the supply of moisture lasted. 

 This has been called the condenmtion theory. 



(4) When bodies of water, moving in different directions or 

 with different velocities in the same direction, come in contact, 

 whirls and eddies are .set up between them. It is therefore con- 

 ceivable that the large masses of air moving between the equator 

 and the pole, may, at their places of meeting, produce similar 

 large eddies, such as the cyclones of the weather map. These 

 have been called dynamic cyclones and also driven cyclones. 



Probably no meteorologist believes that any one of these 

 causes acts entirely alone in cyclone formation. But a difference 

 of opinion arises as to which is the principal cause, and to what 

 extent the others are subsidiary causes. All theories agree in 

 ascribing the primary cause to differences of temperature, either 

 local or between equator and pole. 



Vertical instability can scarcely be considered the primary 

 cause of cyclones, because, as stated above, the atmosphere is 

 found normally separated into strata, each one potentially warmer 

 than the one beneath. The fatal objection to the condensation 

 theory, as pointed out by Dr. Hann, ,is that cyclones in temper- 

 ate latitudes are more violent in winter than in summer. Latent 

 heat is not so effective an aid to storm action in winter as in 

 summer, and yet it is in winter that our cyclones possess 

 their greatest violence. According to the theory of driven 

 cyclones, "The masses of air set in motion polewards by the 

 upper gradients are resolved, in part, into great whirls, the prin- 

 cipal progressive motion of which is controlled by the prevailing 

 west component of the former. The influence of the inequali- 

 ties of the earth's surface, the different heating and cooling of the 

 land and ocean, and the bringing in of aqueous vapour and its 

 condensation, come thus into account, as matters of secondary 

 importance," (Nature, vol xliii. p. 470). " If . . . cyclonic 

 and anticyclonic disturbances are produced by the irregular flow 

 of the general winds, it is probable that these disturbances would 

 originate in the higher regions of the atmosphere, where the 

 winds blow much faster than near the earth's surface. The 

 differences of pressure produced at high altitudes would be felt 

 down to sea-level ; and, a'sthe lower winds move with compara- 

 tive slowness, they would be governed by the gradients thus im- 

 posed on them by the irregular movements of the upper winds. 

 According to this theory, an area of high pressure would be 

 perceived at sea-level beneath a district where the upper currents 

 crowd together ; and an area of low pressure, or a cyclonic 

 storm, would be developed beneath a re>iion where the upper 

 currents are somewhat divergent." (Davis's " Elementary 

 Meteorology," 1894, p. 219.) 



As stated above, observation does not show areas of minimum 

 pressure in the upper air above areas of minimum pressure at 



