NO. 8 BAROMETRIC PRESSURE — DUELL AND DUELL 27 



namely, that the number of calcium flocculi increases during and 

 shortly after bright chromospheric eruptions, agrees with our present 

 knowledge of solar physics. Analogous to the behavior of the calcium 

 numbers before pressure increases, 3 to 5 days before sca-level 

 pressure decreases the calcium flocculi character numbers likewise 

 decrease, after having been on the average particularly high 6 days 

 before the key dates. On the whole it can be seen from figure 20 

 that it is not quite hopeless to use certain other solar indices instead 

 of direct observations of ultraviolet eruptions, if reliable observa- 

 tional data about the bright chromospheric eruptions are not available. 

 One fact results rather clearly from figures 17 to 20, namely, that 

 in contrast to the solar particle invasions, the influence of the solar 

 ultraviolet invasions upon sea-level pressure seems to exist not only 

 in winter and in years with low sun activity, but also in summer and 

 in years with high sun activity. 



C. Attempt at a Physical E.xplanation of the Observ'ed 

 Relationship 



In the following chapter an attempt will be made to give a 

 schematic description of the manner in which the influence of short- 

 duration eruptions of extreme sliort-wave ultraviolet solar radiation 

 is transmitted to the troposphere. As some of the physical possibil- 

 ities which must be considered in that connection have already been 

 mentioned, when the possible etTects of particle invasions were dis- 

 cussed, the discussion can be confined here to a few facts of special 

 interest. Even on this occasion it cannot be stressed too strongly that 

 our description is rather hypothetical and doubtless will undergo 

 modifications if further light is thrown on these problems by other 

 investigators. 



The origin of these ultraviolet eruptions is confined almost exclu- 

 sively to a solar zone which lies between 40° N. latitude and 40° 

 S. latitude. There they appear mostly in the near vicinity of sunspot 

 groups which are found in a certain phase of development: Nr. IV 

 and V of the Brunner classification (31). Attendant solar phenomena 

 are the bright chromospheric eruptions which are observable by means 

 of a spectrohelioscope or a spectroheliograph, because of the simul- 

 taneous excitement of lines in the visii^le part of the spectrum, and 

 furthermore certain kinds of prominences. Known geophysical con- 

 secutive reactions are: the "Bay-disturbances" of the earth-magnetic 

 elements and of the electric earth current ; an abnormal U-layer, 

 the appearance of which is connected with a short-duration "fade-out" 



