76 CONSTRUCTION OF ISOBARIC CHARTS 



on the jJr and F^-maps. By the foregoing method of procedure, however, no isobaric 

 charts at sealevel would be obtained for those regions where the stations are at con- 

 siderable altitudes above sealevel. 



V. The Dynamic Significance of the Charts of p,-, F^ ; E^; and IT J';. 



The following conclusions are deduced on the distinct assumption that the eai'th 

 does not rotate and that friction does not exist. I defer to a later paper the consider- 

 ation of the influence of the rotation of the earth and of friction upon the dynamic 

 processes of the atmosphere. In this section I shall consider only the primary cause 

 of all atmospheric movements, in other words the want of uniformity as to tempei'ature 

 and humidity. This is that which has the power to set up a movement in an 

 atmosj^here otherwise at rest relative to the earth, whereas the earth's rotation and the 

 friction do not possess such power. 



Significance of p^^-maps. — The dynamic significance of the ^;»r™fips, namely, 

 the maps of the isobars on the different level sui'faces of gravity, is already familiar 

 enough through the daily use of the maps of the isobars at sealevel. I would only 

 here call attention to the fact that in order to obtain the acceleration of the particles 

 of air the pressure-gradient must be divided by the appropriate density of the air. 

 Consequently, in the higher levels where the air has a less density, the same gradient 

 of pressure will produce a much greater velocity than it would at sealevel. 



Significance of Vp-maps. — The dynamic significance of the F^-maps (which may 



be called topographic charts of isobaric surfaces, or maps showing the intersections of 



an isobaric surface by successive level surfaces of equal values of gravity), is seen from 



the fact that an air-particle moving on such an isobaric surface experiences the same 



acceleration as if it were confined to that surface and subject only to the force of 



gravity. Therefore, if we assume that an air-particle moves from a to b on the 



F25.o-chart (see Fig. 11, page 69), and during this movement remains in the isobaric 



surface, p — 25.0, then the acceleration of the particle may be found by dividing the 



difference in gravity-potential at the points a and h by the length of the path of the 



particle or the distance between a and h. Now the gravity-potential at a equals 



mile mile" 



Fo = 74 000,- s, and at h equals F6 = 73 000,- „ so that the difference in 



hour'' ^ hour 



'1 2 



gravity-potential at the two points is F,, — Ft = 1 000 r -<i- The distance between 



a and h is approximately 140 miles, whence the acceleration of the particle of air is 



seen to be ^ ,-^ = 7.14 -, t,. It is easy to calculate the velocity %\ of the air- 



140 hour'' ^ '' 



