SCIENCE. 



[Vol. XVII. No. 413 



Dr. Hann's Studies on Cyclones and Anticyclones. 



Professor Fereel's letter in Science of Dec. 19, commenting 

 on mine of May 30, closes with the suggestion that I should make 

 further statement of the matter of Dr. Hann's studies, which I 

 do with pleasure. 



The best reasoned general account of the convectional theory 

 of cyclones and anticyclones (by the latter term I mean areas of 

 high pressure) that I know of is given in Professor Ferrel's 

 " Popular Treatise on the Winds." Of various statements in regard 

 to cyclones, the following may be quoted from the concluding 

 paragraph on their vertical circulation: "The greater tempera- 

 ture of the interior [of cyclones] causes an upward expansion of 

 the air and greater vertical distances between the isobaric sur- 

 faces here than in the exterior part where the temperature fs less" 

 (p. S41). In regard to anticyclones or areas of high pressure, of 

 the kind that Dr. Hann has investigated, the following explana- 

 tion may be quoted : " The principal cause of the large areas of 

 very high barometer which frequently occur in the higher lati- 

 tudes in winter is undoubtedly found in the clearness of the atmos- 

 phere over these areas and the intense coldness produced by the 

 radiation of heat at a time when little is received from solar radia- 

 tion. The density and pressure of the air are much increased 

 from this cause, and the areas are too large and irregular for this 

 disturbance to give rise to a cyclone with a cold centre " (p. 345). 

 The inversion of temperature accompanying such areas of 

 high pressure is referred to on the next page, but still with the 

 implication that the mass of air in the anticyclone is cooled be- 

 low the temperature of the surrounding atmosphere, and there- 

 fore that it descends and flows out at the base by gravitative con- 

 vection. 



These quotations might be further extended, but they suffice to 

 show that the essential of the generally accepted theory of the 

 areas of low and of high pressure which appear so frequently on 

 our weather-maps is that the first are relatively warm, and the 

 second are relatively cold, when compared with their surround- 

 ings. Cyclonic and anticyclonic areas are both of common occur- 

 rence, and therefore as a rule their temperatures should be re- 

 spectively above and below the normal temperatures of their time 

 and place. 



Records of temperature made on high mountain-peaks fm-nish 

 the best means of testing the convectional theory of cyclones; 

 for, even if all other tests were successfully borne, failure under 

 this test would be fatal to the theory. Dr. Hann's essay on the 

 anticyclone of November and the cyclone of October, 1889, as ob- 

 served in the Alps, furnishes the best means of applying this test 

 that has come to my knowledge. It is true that one example of 

 each of these phenomena is not sufficient for final determinations, 

 and it is very apparent that the results would be far more con- 

 vincing if they included records from mountain stations scattered 

 ' over a much larger area than that of the Alps. Surely no one 

 will be more careful to supplement these deficiencies, whenever 

 possible, than Dr. Hann himself. 



I do not see any reason for believing that the anticyclone that 

 stood over the Alps in November, 1889, was exceptional in its 

 nature or in its relation to the smTOundlng atmosphere. All of 

 its features except its mean temperature warrant the belief that 

 it was a typical example of the phenomena referred to under the 

 heading of " Areas of Higli Pressure " in Professor Ferrel's treatise. 

 Unless it can be shown to have been of exceptional nature, the 

 abnormally high temperature of its air mass is a direct contradic- 

 tion of the fundamental idea of the convectional theory of areas 

 of high pressure. It has not been claimed that the conditions of 

 a cyclone exist in this high-pressure area; but the explanation of 

 high-pressure areas as quoted above is a direct corollary of the 

 cyclonic theory. If the corollary is contradicted by facts, the 

 theory needs revision. The burden of proof in this case lies with 

 those who would maintain that the anticyclone in question was 

 of so exceptional a nature that it cannot be regarded as a repre- 

 sentative of its class. Its long duration does not show it to be a 

 thing of another kind from other areas of high pressure: the long 

 duration merely gave good opportunity for repeated observation 

 of its prevailingly high temperature. 



As to the cyclone of October that was examined by Dr. Hann, 

 it was certainly of moderate development ; but it was as good an 

 example, according to Dr. Hann, as he could find. The observa- 

 tions that he quotes show that' its general central temperatures 

 were below the normal of its time and place. The fact that the 

 temperatures were not determined in the free air, but at stations 

 on the surface of the ground, does not seem to me to invalidate 

 their use here; for on the peaks where the critical observations 

 were made the air is generally in motion, and the mass of the 

 mountain is small; and for both these reasons the control of the 

 temperature of the air by the ground is not great enough to ex- 

 plain the reported low temperatures. Over a broad surface of 

 a lowland, where the wind is weaker and the opportunity for 

 contact of air and ground is greater, the case is different. The 

 low temperature of the central part of this cyclone may fairly he 

 regarded as contradictory to the convectional theory of cyclones, 

 unless it can be shown that the example in question was sur- 

 rounded by air more abnormally cooled than its own, or unless it 

 is shown to have been an exph-ing cyclone, — one whose long cir- 

 culation had so thoroughly. exhausted its supply of warm, moist 

 air, and so successfully warmed the surrounding air, that it had 

 no further support, as Professor Ferrel has shown might some- 

 times be the case. It is true that Europe might offer more ex- 

 amples of self-exhausted cj clones than occur in this country, for 

 they ai'e there advancing from moister into dryer regions; but it 

 is difficult to believe that so considerable a deficiency of tempera- 

 ture as probably occurred in the case under consideration should 

 be produced before the cyclonic motions had stopped, if they de- 

 pended entirely on a convectional origin. It is not likely that so 

 exceptional a case as this must be, if it is to be explained by con- 

 vection, would have been the very case that Dr. Hann happened 

 to choose for his studies. It is still more unlikely that both the 

 cyclone and the anticyclone here referred to should have been 

 exceptional members of their classes, both departing from the 

 normal in a way that would contradict the convectional theory. 

 As these are the first examples of their kind to be carefully ex- 

 amined by means of regular observations at stations at so high a 

 level, the probability is strongly in favor of their being ordinary, 

 and not extraordinary, phenomena; and as such they did not 

 possess the peculiar temperatures that the convectional theory 

 would lead us to expect. Although mere probability of this kind 

 does not close a case, it seems to me that it may be fairly said to 

 open it. 



I do not see that there is any necessary contradiction in this 

 discussion. The theories under consideration are not mutually 

 exclusive. Both may be true. The liberation of latent heat from 

 condensed vapor is an aid to the circjlation in both cases. Cer- 

 tainly there is nothing in Dr. Hann's essay to make one think 

 that thunder-storms, tornadoes, and desert whirls are not convec- 

 tional phenomena. It is entirely possible that true convectional 

 cyclones might prevail in the tropics, while driven cyclones might 

 characterize the temperate zones. A cyclone begun chiefly by one 

 process might be continued chiefly by the other. Of course, this 

 is hypothetical: it was not my intention last May to regard it in 

 any other light. For that reason my letter closed with an "if." 

 Others besides Professor Ferrel, however, understood me to have 

 abandoned the older theory and taken up with the newer. I 

 tried to state Dr. Hann's point of view, and I do not regret hav- 

 ing stated it so fairly that it was taken for my own. That I had 

 not adopted it as fully as Professor Ferrel implies, may be in- 

 ferred from the close of my eighth paragraph and from the 

 middle of the ninth, as well as from the ending of the letter al- 

 ready referred to. But in making this explanation, I do not wish 

 to be understood as not welcoming the new theory. The abnor- 

 mal vi'armth of anticyclones had been in ray mind as a difficulty 

 in the way of convection, yet I had expected that cyclones would 

 be found to be still warmer; and it was not until reading Dr. 

 Hann's forcible statement that I perceived I had become too 

 strongly settled in favor of the prevailing theory. On recognizing 

 this partiality, I made all the more effort to give full and fair 

 consideration to the new one. It seemed to me nothing less than 

 a duty to announce the facts and Dr. Hann's interpretation of 

 them in the same journal that had published my outline render- 



