CLOUD WORK OF THE WEATHER BUREAU 353 



state of the pressure, temperature, and vapor tension — that is, 

 the physics of the air itself. (2) A very important subject has 

 been the determination of the direction and velocities of the 

 horizontal motions of the air in each of the eight principal levels, 

 on all sides of the anti-cyclones and cyclones, high and low 

 areas of pressure, as they move over this country. These move- 

 ments have been separated into two components, the first be- 

 longing to the general or undisturbed motion of the atmosphere* 

 which is about eastward in this latitude, and the second to the 

 local motions, which are gyrator) 7 and especially concerned with 

 descending and ascending vortices or storms. These data give 

 us for the first time definite information regarding storm com- 

 ponents, and these enable us to look into the theories much 

 more closely than heretofore. (3) This analysis has been sup- 

 plemented by a compilation of cloud motions taking place in 

 the cumulus or the cirrus levels, as derived from the Weather 

 Bureau cloud charts collected during the past twenty years, the 

 object of which is to show how the average anti-cyclone and 

 cyclone are affected by the circulation of the air over different 

 parts of the United States — that is, by the Rocky mountains, the 

 Lake region, the Gulf of Mexico, and the Atlantic States — the 

 results being exhibited on a series of colored charts. 



These practical facts lead to the necessity of definite theoret- 

 ical studies in order to account for them, and this again to sev- 

 eral other lines of research : (1) The first thing was to prepare 

 a system of standard constants and formulas by a comparative 

 study of the papers of several authors, and by the addition of 

 such new demonstrations as seemed desirable, so that the work 

 of many men in their several branches may be read as one con- 

 sistent meteorological scheme. This standard system represents 

 the outcome of several years' study of the subject. These formulas 

 include most of the thermodynamic or hydrodynamic conditions 

 likely to arise on a rotating body surrounded by an atmosphere, 

 like the earth. (2) Next, a completely new set of working 

 tables, based upon these formulas, has been prepared for the 

 barometric reductions from one level to another ; for studying 

 with the greatest accuracy the exact conditions of pressure, tem- 

 perature, and vapor tension at the level where a cumulus cloud 

 base forms by the vertical convection, at the place where the hail 

 forms, and at the level where the snow is produced, and finally 

 for computing the dynamic forces and the gradients of motion 

 according to the observed velocities. These tables are perma- 



