E. Loomis — Areas of low pressure. 7 



nearly with the direction of the storm's progress, than does the 

 average wind for the same season of the year ; but there is no 

 evidence that there is an exact agreement between these two 

 directions. If we claim that the progressive movement of a 

 storm area is due to the progressive movement of the general 

 mass of the atmosphere in which it is formed, it seems necessary 

 to admit that a mass of the atmosphere, of considerably greater 

 extent than the storm area, is advancing in the same direction 

 and at the same rate as the storm advances. In order to decide 

 whether such is the fact, we need only to consult a well-con- 

 structed weather map, of sufficient dimensions to include not 

 merely a storm area, but a considerable margin beyond it. 

 The storm maps which have accompanied, my former papers are 

 too limited to furnish the required information in a form which 

 is entirely satisfactory, and it is desirable to have similar maps 

 for several successive days. The Signal Service maps afford 

 abundant materials for this purpose, and Hoffmeyer's maps are 

 still better, since they include a much larger portion of the 

 earth's surface. If we open a volume of these maps anywhere 

 at random, we shall not find the general mass of the atmosphere 

 surrounding a great storm moving forward in the same direc- 

 tion as that in which the storm advances. 



If we follow the progressive movement of a great storm from 

 day to day, by means of maps representing the phenomena at 

 intervals not greater than eight hours, we shall find that in 

 front of the storm the air appears to be drawn in toward the 

 center, by which means the pressure on the front side of the 

 storm is diminished. The air, thus drawn in toward the center, 

 rises to a considerable elevation above the surface of the earth 

 and its vapor is condensed. In the rear of the storm, the ex- 

 terior air rushes in and restores the pressure on that side ; and 

 as the result of this double process, the point of least baromet- 

 ric pressure is carried forward. This movement of the exterior 

 air in the rear of a storm, is not necessarily in the same direc- 

 tion as that in which the storm center advances. In the 

 United States, storms almost invariably advance eastward, and 

 generally toward a point a little north of east ; but the wind 

 which presses upon the rear generally comes from the north or 

 northwest, which direction is often at right angles, or nearly at 

 right angles, with the direction in which the storm center ad- 

 vances. This movement of the air, by which the center of 

 least pressure is carried forward, bears some analogy to the 

 movements which cause the advance of a wave upon the sur- 

 face of the ocean, and hence we may with propriety say that the 

 progressive movement of a storm area, is the movement of a 

 great atmospheric wave. 



Besides these general considerations, there are various special 



