10 E. Loornis — Contributions to Meteorology. 



several low centers, a low center may disappear from simple 

 changes of pressure. In like manner a second low center may 

 disappear and so on. In many cases the changes in the posi- 

 tion and magnitude of the low centers are so rapid, that in com- 

 paring two weather maps for successive days, we frequently 

 find it impossible to identify a low center on one of the maps, 

 with, its corresponding low center on the other map. Examples 

 may be easily found to illustrate all of these different cases, so 

 that it seems to be unneccessary to present a selected list. In 

 cases like these, it surely will not be claimed that the move- 

 ment of the low centers can be ascribed to a simple drifting of 

 the general mass of the atmosphere in which the low areas were 

 formed. 



If we reject the drift theory, it will doubtless be asked how 

 can we explain the fact that in the middle latitudes, storms 

 almost invariably advance toward the east, and the opposite 

 movement only occurs occasionally, and seldom continues longer 

 than one or two days. This fact seems to result from the pre- 

 valent movement of the wind towards the east. The result 

 however is not due to a general drifting of the mass of the 

 atmosphere within which the low area is formed ; but to the 

 fact that the pressure on the west side of the low area is more 

 steady and persistent than that on the east side. The character- 

 istic features of a great storm movement are a motion of the 

 air from all sides spirally inward, together with an upward 

 movement resulting in the condensation of vapor at various 

 places within the low area. Now if the air pressed in with 

 equal force on all sides of the low center, and if there was an 

 equal precipitation of vapor on all sides, there does not appear 

 to be any reason why the low center should advance at all. It 

 sometimes happens that the pressure on the west side is very 

 small, while there is considerable pressure on the east side, and 

 in such cases the low center moves toward the west. Many 

 examples of this kind are shown by the Signal Service maps, 

 and also by Hoffmeyer's charts. But this movement toward 

 the west cannot be long maintained. In the middle latitudes, 

 the east winds are exceptional, and result mainly from disturb- 

 ances caused by storms. On the contrary, the west winds result 

 from general causes which are permanent in their character, and 

 are independent of storms ; and if there were no storms the 

 west winds would rarely be interrupted. During the prevalence 

 of an east wind, the causes which produce west winds are not 

 destroyed; their influence is only temporarily suspended ; and 

 they soon return with a force, not impaired, but rather aug- 

 mented by their temporary suspension. The pressure on the 

 west side of storm areas is thus a strong and persistent one, 

 while that on the east side results from temporary causes, and 



