PROGRESS OF METEOROLOGY IN 1889. 253 



Thunder-storms. — Karl Proliaskalias collected and discussed ii mass of 

 tliuudei'-storui observations from about three hundred stations in Steier- 

 mark, Kiirten, and Oberkrainfor four years, li*85 to 1888. About niuc 

 thousand reports were received annually, making an average of thirty 

 reports from each station. 



The average duration of a thunder-storm was 1.4 hours, being 1.2 in 

 si)ring, 1.4 in suunner, and 1.0 in autumn. The average velocity of 

 passing was 30 kilometers per hour ; hence the extent of the thunder- 

 storm cloud was at the highest 43 kilometers ; but if it be remembered 

 that the above computed duration represents the mean time between 

 tirst aud last thunder, the average extent of the usual thunderstorm 

 cloud does not exceed about 37 kilometers. The velocity of propaga- 

 tion of the thunderstorms is materially less iu these districts of the 

 southern Alpine mountains than in southern Germany. Thus the after- 

 noon velocity is 10 kilometers per hour greater for the latter than for 

 the former. This is due to the large number of local storms, " wJirmege- 

 witter," with their slow rate of movement. In hot summer days in the 

 Alps, in spite of a high barometer, frequent local thunder-storms arise, 

 which seem to be almost un^jrogressive. On certain selected days in July, 

 1887, for the hours from noon to 6 P. m., there were 1,193 reports, and 218 

 for the remaining eighteen hours. Thunderstormsoccur most frequently 

 when the barometer is about nornuil; those from the north and south 

 have the smallest area of extension, those from the west the greatest. 



In addition to these statistical results, Prohaska undertakes to ex- 

 plain the occurrence of thunder-storms and rain with a rising air press- 

 ure. The basis of his theory rests on the assumed backward inclination 

 of the axes of cyclones. This assumption leads to the conclusion that 

 the rise of the barometer immediately following the passage of baro- 

 metric minima is occasioned by dense heavy air adjacent to the earth's 

 surface pressing into the region of low" pressure. Now, as heavy air 

 masses come into a region of lower pressure they experience a continu- 

 ally smaller compression, and consequently there is developed an up- 

 ward gradient, and a rise of the air strata lying thereon must ensue. 

 Dynamic cooling is thus brought into play, so that in higher air strata 

 a fall of temperature takes place whilst the barometer is still falling. 

 Thus there is a causal connection between rising air pressure and the 

 formation of precipitation and thunder-storms, inasmuch as the rising- 

 air pressure consists iu the formation and condensation of a cloud 

 swell advancing in Iront like a true wave movement. { Meteor ologische 

 Zeitsehri/t, 1880, vi, p. 220.) 



The report of the director of the Hong Kong Observatory for 1888 

 contains a special study of thunder-storms in the colony during the past 

 five years. Dr. Doberck states that they are most frequent iu May, and 

 that they have not occurred iu November, December, and January. 

 In diurnal period they are most frequent about 1 A. m., and least so at 

 about 8 A. M., in the proportion of about two to one. 



