492 



METEOROLOGY. 



this character is more pronounced in tropical 

 than in extra-tropical cyclones. Squalls are 

 one of the most characteristic features of a 

 tropical cyclone, where they surround the cen- 

 ter on all sides ; whereas in Great Britain, 

 squalls are almost exclusively formed along 

 that portion of the line of the trough which is 

 south of the center and in the right rear of the 

 depression. The difference is probably due to 

 the different intensities of the two classes of 

 cyclones. A patch of blue sky, commonly 

 known as the " bull's-eye." is almost universal 

 in the tropics, and apparently unknown in 

 higher latitudes. There are reasons for sup- 

 posing that the formation is dependent on in- 

 tensity of rotation. The trough phenomena 

 such as a squall, a sudden shift of wind, and 

 change of cloud character and of temperature, 

 just as the barometer turns to rise, and even 

 far from the center, which are so prominent a 

 feature in British cyclones have not even 

 been noticed by meteorologists in the tropics ; 

 but there are slight indications of these phe- 

 nomena everywhere. Every cyclone has a 

 double symmetry. One set of phenomena, 

 such as the oval shape, the general rotation of 

 the wind, the cloud-ring, rain area, and central 

 blue space, are more or less related to a central 

 point. Another set, such as temperature, hu- 

 midity, the general character of the clouds, 

 certain shifts of wind, and a particular line of 

 squalls, are more or less, related to the front 

 and rear of the line of the trough of a cyclone. 

 The author's researches show that the first set 

 are strongly marked in the tropics, where the 

 circulating energy of the air is great, and the 

 velocity of propagation small ; while the sec- 

 ond set are most prominent in extra-tropical 

 cyclones, where the rotational energy is mod- 

 erate and the translational velocity is great. 

 The first set of characteristics may conveniently 

 be classed together as the rotational, the second 

 set as the translational, phenomena of a cy- 

 clone. Tropical and extra-tropical cyclones 

 are identical in character, but differ in certain 

 details due to latitude, surrounding pressure, 

 and the relative intensity of rotation or trans- 

 lation. 



Eeviewing the state of our knowledge re- 

 specting the manner in which thunder-storms 

 are formed, Prof. Hann has remarked that in 

 European latitudes they do not usually break 

 out after the sky has been cloudy for a consid- 

 erable time ; but occasionally the phenomena 

 are observed after a day of persistent rain, and 

 in the midst of dense fog. A thunder-storm 

 frequently marks the end of a longer or shorter 

 period of fair weather. We can then best ob- 

 serve the approach and formation of the storm- 

 clouds, especially if, in a mountainous region, 

 we stand a little above the horizon. The most 

 characteristic sign of the rising storm is the 

 band of cirrus. From the side of the horizon 

 which looks darkest before the appearance of 

 the storm itself, there advances slowly a thick 

 veil of cirrus with fringed edges, which rises 



to a great height in the sky before the tem- 

 pestuous masses that follow it are revealed. 

 These masses, the real seat of the electrical 

 phenomena, have characteristic shape and color. 

 Sometimes also the front part of them forms 

 by a perspective effect an arch of clouds well 

 detached from the rest, which is often the fore- 

 boding of a violent squall of hail and wind. 

 Under the black, cloudy arch is distinguish able 

 a clearer kind of veil, which is nothing else 

 than the sheet of rain falling from the clouds. 

 In some cases this sheet is covered by a sort 

 of mobile curtain of floating clouds, descending 

 like drapery from the dark, cloudy arch. It 

 also, according to M. Hann, is a certain sign of 

 a violent squall. The cumulus or cumulo- 

 stratus forming the mass of the tempest is usu- 

 ally of a grayish color, verging upon blue or 

 tawny yellow ; while the curtain just spoken 

 of takes on more lively hues white, yellow, 

 or rosy. Lightnings may still be arranged ac- 

 cording to Arago's classification. Photography 

 has proved that zigzag lightnings consist not 

 of a single luminous trace, but of several sepa- 

 rated by dark lines, the changes of direction 

 of which are made, not at sharp angles, but in 

 curves. Single, undivided traces of lightning 

 are the exception. Surface lightnings present 

 the aspect of a sea of fire visible through the 

 clouds. According to M. Hapke's spectroscopic 

 examinations, they are different from zigzag 

 lightnings. M. Plant6 has made a study of 

 lightning-balls, and has produced something 

 like them artificially, on a small scale ; but 

 much remains to be learned about them. In 

 sparkling lightning the whole trace is broken 

 up into a multitude of sparks, or a zigzag 

 trace terminates in air, when there is usually 

 a fall of a thunder-bolt. Heat-lightnings near 

 the horizon are probably the signs of storms 

 raging beyond them. Sometimes they are seen 

 near the zenith ; when they are possibly too 

 far off for the thunder to be heard. In color, 

 lightnings are usually of a dazzling white or 

 bluish violet. 



The question of the reality of the supposed 

 equinoctial storms has been examined by Dr. R. 

 Muller, with the aid of the hourly records of the 

 anemometer at Pola for the years 1876-1886, 

 and of observations made upon the German 

 coasts by the " Deutsche Seewarte " for the 

 years 1878-1883. The results of both series 

 are in harmony. The Pola records show that 

 for the Adriatic no important influence in gen- 

 erating gales can be attributed to the equinoc- 

 tial seasons ; and a similar inference is drawn 

 from the German observations. At Pola, 63 

 per cent., on the German coast, 80 per cent, of 

 the storms occurred during the winter season 

 (October to March). At Pola, after the rela- 

 tive quiet of June and July, a tolerably regular 

 increase in the number of days with stormy 

 winds took place till the end of January. A 

 decrease occurred in February ; and March 

 had the greatest number of stormy days. On 

 the German coast, the greatest number of 



