December 4, 1914] 



SCIENCE 



823 



and perhaps also because of sound reflection 

 from the upper limit of the fog. Local inter- 

 ference resulting from such reflection may ex- 

 plain the silent regions so commonly observed 

 in connection with fog signals. 



In a similar way, the boundaries between 

 air bodies of different temperatures and hu- 

 midities in thunderstorms are important in 

 prolonging thunder' and in preventing the 

 sound from traveling great distances.* 



THE THUNDEESTOEM AND ITS PHENOMENA 



Peofessoe W. J. Humphreys under the 

 above title has published^ a careful summary 

 of the present knowledge relative to thunder- 

 storm physics and has added many new 

 points. Professor Humphreys recognizes five 

 types of thunderstorms, which occur as fol- 

 lows : (1) in regions of high temperature and 

 nearly uniform pressure (heat thunderstorms) ; 

 (2) in the southeast quadrant of an almost 

 circular cyclone; (3) between the branches of 

 a V-shaped cyclone; (4) in the trough be- 

 tween two anticyclones; (5) on the boundary 

 between warm and cold waves. All but the 

 first are produced essentially by the over- and 

 under-running of winds of different tempera- 

 tures, which in some way cause moist air 

 masses to rise. Each of these types is illus- 

 trated with a set of three successive weather 

 maps at 12-hour intervals. 



The squall-wind associated with thunder- 

 storms is thought to be the outward flow from 

 a cataract of air which is cooled and kept cold 

 in its descent by the rain or hail falling 

 through it. The sudden rise in air pressure 

 at the onset of a strong thunderstorm is as- 

 cribed to the combined eifect of the downward 

 thrust, greater density and relatively small 

 absolute humidity of this cold wind, and to 

 the interference which the thunderstorm 

 offers to the free flow of the general winds. 



The splitting of raindrops in falling 

 through the ascending air currents character- 



3 W. Sehmidt, Meteorologische Zeiisdhrift, Jan- 

 uary, 1914, pp. 33-37. 



i'W. J. Humpkreys, Monthly Weather Beview, 

 June, 1914, p. 379. 



5 lUd., pp. 348-380, 28 figs. 



istic of thunderstorms is, according to 6. C. 

 Simpson, the source of the lightning. The 

 small drops with negative charges go up with 

 the wind while the larger ones with positive 

 charges stay below. Thus in a thunderstorm 

 there is usually a region of positive electric- 

 ity between the negative earth and the nega- 

 tive upper portion of the cloud. When the 

 charge becomes sufficient to ionize a path 

 through the air, a series of direct current dis- 

 charges usually take place along approxi- 

 mately the same line. A progressive lengthen- 

 ing of a lightning streak in its successive six 

 discharges is shown in a picture taken with a 

 rotating camera (Pig. 26). Professor Hum- 

 phreys is to be congratulated on his thorough 

 and simple presentation of such a difficult 

 subject. 



EAINDROP VELOCITIES^ 



J. LiZNAE in deriving a new mathematical 

 formula for determining the velocities of rain- 

 drops has included the effect of the viscosity 

 of the air and friction on the horizontal ex- 

 panse of the drop. This formula is 



V = 602.6v'"r-J- 57.013 — ?3.291r=, 

 where v is in cm. per sec, r in mm. and at- 

 mospheric pressure at 986.6 mb. (Y40 mm.). 

 Applied to hail this formula becomes 



■!; = 20.795(\/722.2r-|-l/?— 1/r). 

 The velocities of raindrops of the radii indi- 

 cated are roughly as follows : 



r 0.5 mm., v 400 cm. per see. 



r 1.0 mm., v 600 cm. per see. 



r 1.5 mm., v 700 cm. per see. 



r 2.7 mm., v 800 cm. per see. (maximum). 



Large drops are retarded by flattening in the 

 air. According to Wiesner's observations this 

 flattening of falling drops will cause those of 

 3.6 mm. radius to form rings and split into 

 smaller drops. 



NOTES 



The June number of the Monthly Weather 

 Beview, which appeared early in October, has 

 more than 100 pages (quarto), which are de- 

 voted mostly to meteorological articles. The 



8 Meteorologische Zeitschrift, July, 1914, pp. 

 389-347. 



