E. Loomis — Areas of low pressure. 



9 



years 1873-1880. These maps show a considerable number of 

 other cases of like kind, some of which have been omitted be- 

 cause the depression of the barometer was small ; and others 

 because the position of the low center was not very sharply de- 

 fined, or was situated near the margin of the weather map. 



Among these twenty -four cases, there are only three in which 

 the paths of the two low centers were not inclined to each 

 other at an angle as great as 45°; in half of the cases the two 

 paths were inclined at an angle considerably greater than 45° ; 

 in eight or nine of the cases the angle was nearly as great as 

 90° ; and in three of the cases the angle was greater than 90°. 



It sometimes happens that within an area of low pressure, 

 having but a single center, a second low center is developed. 

 The following list shows twenty-four such cases, selected from 

 the Signal Service maps for 1873-1880. The maps show a 

 large number of other similar cases, but in the cases here cited 

 the depression of the barometer was generally considerable, and 

 the position of the low centers was distinctly indicated. 



Cases in which a second low center is developed within an area of 



low pressure. 



1873. 



Feb. 18.1-18.2 



1875. 



Jan. 30.3-31.2 



1878. 



Jan. 13.2-13.3 





Feb. 20.1-20.2 





May 4.2- 4.3 





Jan. 30.2-31.1 





March 28.2-29.1 





Nov. 3.1- 3.2 





March 12.3-13.1 



1874. 



April 25.1-25.2 



1876. 



March 5.3- 6.1 





Nov. 23.3-24.1 





Aug. 29.1-29.2 





March 25.2-25.3 



1879. 



March 29.1-29.2 





Aug. 30.1-30.2 





May 7.1- 7.2 



1880. 



Jan. 21.3-22.1 





Nov. 23.1-23.3 





May 7.2- 7.3 





Feb. 12.2-12.3 



1875. 



Jan. 24.2-24.3 



1877. 



Dec. 29.2-29.3 





April 17.1-17.2 



In a majority of these cases, the two low centers appear to 

 have subsequently coalesced ; but in several of them, the two 

 low centers moved off in directions inclined to each other at an 

 angle of 90° or more, and with unequal velocities. 



Over the Atlantic Ocean and Europe, cases similar to the 

 preceding are of much more frequent occurrence than in the 

 United States ; the depression of the barometer is generally 

 much greater ; and the low areas have a much greater geographi- 

 cal extent. By consulting Hoffmeyer's Weather Maps, we may 

 easily find examples in which two low centers move toward 

 each other from nearly opposite directions and coalesce ; and 

 we may also find frequent cases in which a great area of low 

 pressure, with but one center, undergoes a change by which two 

 low centers are developed, and these new low centers recede 

 from each other. Sometimes there is a further change by 

 which three or lour or even more low centers are formed, and 

 these low centers have a progressive movement in different 

 directions, and with unequal velocities. 



On the contrary, within a large area of low pressure, showing 



