226 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 9 



tribution, which prevailed from the earliest days of synoptic meteor- 

 ology, was naturally suggested by the daily maps. It is, moreover, 

 consistent with the fact of common experience that the "winds" are 

 one of the principal immediate factors in daily weather ; because it is 

 the field of pressure which directly determines the flow of the air 

 that constitutes the wind, while other weather phenomena are more 

 or less directly produced by, or are incident to, the motions of the 

 air. Great difficulty has been encountered in attempting to obtain 

 from observations of the winds a satisfactory comprehensive knowl- 

 edge of the atmospheric circulation over the globe, but for the pur- 

 poses of the present discussion it is sufficient to recognize that the 

 air motions which make up the circulation of the atmosphere as a 

 whole constitute a complex and ever-varying system of innumerable 

 different interacting currents by which great quantities of air are 

 often transported from place to place over long distances ; and during 

 recent years, attention has been directed more and more to these air 

 streams themselves — their motions, extent, and physical properties, 

 and the relations of weather phenomena thereto — instead of being, 

 as in the earlier days of synoptic meteorology, almost wholly concen- 

 trated on pressure formations as such. 



It has been more or less vaguely realized for more than a century 

 that on many occasions two atmospheric currents, markedly different 

 in velocity and temperature, and sharply separated, may flow ad- 

 jacent to each other and produce distinctive weather phenomena as 

 one succeeds the other at a given place ; this idea was clearly expressed 

 as early as 1828 by H. W. Dove, who ascribed the storms of temperate 

 latitudes to conflicts between polar currents and equatorial currents. 

 Only very limited observation is needed to reveal striking instances 

 in which all of the meteorological elements change simultaneously 

 and abruptly in a pronounced manner (fig. 2), as the boundary be- 

 tween two different currents sweeps over the observing station, the 

 transition from one to the other amounting practically to an actual 

 discontinuity. Such moving discontinuities, often hundreds of miles 

 in length, between adjacent currents, frequently are quite conspicuous 

 on synoptic charts (fig. 3) ; they have long been recognized by the 

 forecaster, and were the subject of extensive theoretical investigations 

 by Helmholtz and Margules. It was not until during the World 

 War, however, that an intensive effort was made to investigate the 

 separate currents in detail, explicitly as a basis for the interpretation 

 of the phenomena shown on the daily maps. The investigation was 

 initiated in Norway, after the Scandinavian countries had ceased to 

 receive meteorological data from other countries because of the war. 

 On very detailed surface charts, prepared from unusually compre- 

 hensive observations at an exceedingly dense network of stations, it 



