METEOROLOGY. O 17 



a little in front of the re^jion of (lie tliunder-storm, there exist peculiar 

 teniperatiiieand pressure iclatious, niunel.v, thepressuie as we approach 

 the storm from the front suddenly rises and the temperaluic falls. 



The front edge of the storm sharply separatevS an area of higli i)ress- 

 ure from one of low, and equally separates an area of low temperature 

 from one of high. 



By considering the wind directions at the moment of the outbreak 

 of a thunderstorm we find that on the front edge, in general, the winds 

 are i)erpendicular to the isobars, if the latter run north and south, or 

 tliat the wind blows direct from the high pressure to the low pressure, 

 constituting a departure from the law of Uuys-Ballot. 



The existence of this sharply-defined passage from high to low i)res- 

 sure and from high to low temperature is so uniform that it is sutticnent 

 to study these isobars and isotherms in order to ascertain wliere the 

 front of the thunder-storm is at any moment. {Z. 0. G. M., xviii, p. 

 281.) 



298. [For most of the thunder-storms that the present writer has 

 examined or observed personally in this country and Europe he is con- 

 vinced that the explanation of the outflowing cool wind immediately in 

 front of the storm was correctly given by Espy at least as early as 1.S38. 

 This wind is by him and Professor Heury explained as almost wholly 

 due to the air dragged down by the falling rain and also cooled and 

 rendered heavier, which, striking the ground, must be pushed out. But 

 as the storm is undoubtedly advancing, either with the front edge of 

 an extensive area of denser air, or else is being carried along within a 

 great current of air, it follows, in either case, that the wind due to the 

 down-rush of rain will compound with the wind due to the general motion 

 of the atmosphere; we therefore generally find strong outward-goiug 

 winds on one side of a thunder-storm and light outward winds or calms' 

 on the other side of the rain. Ferrel shows that no great barometer 

 fall can be caused by these straight-out winds; there must be a rota- 

 tion to produce a depression ; on the contrary, air pushed down against 

 the earth and outwards may cause a slight temi)orary rise of i)ressure, 

 so that gusts and a sudden barometrical rise occur simultaneously.] 



299. Dr. W. Koppeu has given anexhaustivestudy of the destructive 

 thunder-storm of August 9, 18S1. The chart accompanying his essay 

 shows that at 2 p. M. alow barometer was central just south of Norway 

 and east of Denmark, while in the southeast quadrant from this, from 

 Denmark to Bavaria, numerous thunder-storms occurred. Thfe study of 

 these storms seem to him to aflbrd a striking example by which to test 

 the conclusions of his previous investigation, well known as "Contribu- 

 tions to our Knowledge of Boen and Thunder-Storms," to which the 

 present study forms a second memoir. After a minute description of 

 the i)henon)ena, follows a com]>arison and discussion on the causes and 

 explanation of the thunder-storm from which Uaun makes the following 



