WEATHER — WOOLAKD 



227 



i;::i!ii"i;^n:;:Ti .,„ , 



FiGDRE 2. — Variations of tlie meteorological elements at Washiiigton, D. C, January 22-23, 

 1936, recorded by self-registering instruments. Note the simultaneous abrupt changes 

 in all the elements at about 8 p. m., January 22, 75 meridian time, as the boundary 

 between a weak southerly current and a strong northwesterly current passed over the 

 city; the total drop in temperature was nearly 50° F. Considerable property damage 

 from wiud occurred. See also figures 3 and 4. 



was found that a number of individual currents ordinarily could be 

 identified and followed from day to day; they differed more or less 

 from one another in direction of movement, speed, temperature, and 

 other characteristics; and often ended abruptly against one another, 

 with the clouds, precipitation, and other weather phenomena defi- 

 nitely related to these discontinuities. 



It may be shown from dynamical theory that two atmospheric cur- 

 rents with different temperatures and velocities may, on the rotating 

 earth, flow side by side in mutual dynamical equilibrium, provided 

 the surface of discontinuity between them is inclined at an appropri- 

 ate (in general, small) angle to the horizontal, with the colder cur- 

 rent lying in wedge form under the warmer (fig. 4). If the 

 conditions necessary for this dynamical equilibrium are not fulfilled, 

 however, a disturbance develops at the boundary between the two 

 currents, and may result in the formation of a traveling cyclone. 

 The warm current advances against, and flows up over, the cold 

 current, while in the rear of the disturbance the cold air sweeps in 

 under the warm current (figs. 5, 6). In the regions where warm 

 air is forced to ascend, the adiabatic cooling may lead to condensa- 

 tion and precipitation of the admixed water vapor. This general con- 

 ception of the cyclone is the outgrowth of ideas successively expressed 

 and elaborated during the past hundred years by Dove, Helmholtz, 

 Blasius, Margules, Shaw, J. Bjerknes (1919), Solberg, (Bjerknes and 



197855— 4( 



-16 



