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DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



every scale, down to the smallest, which must be considered as local disturbances. 

 These local eddies in connection with the sheltering effect of mountains and the devi- 

 ating effect of valleys make the use of wind-observations from mountainous regions 

 difficult. For such regions it would be good to have special information as to the 

 peculiarities of each station, i. e., to know the relation of the observed local wind to 

 the general wind to be found higher up, where the influence of the obstacles is reduced 

 or has disappeared. Signs representing these peculiarities could be introduced on 

 the outline maps. The best method of investigating these peculiarities would be by 

 sending up simultaneously from all stations pilot-balloons, giving the motions in the 

 free air with which the local motions at the ground should be compared. 



137. Cold Wave, Warm Wave. Let us suppose a certain mass of air has been 

 cooled down below the temperature of other masses in the same level. Equilibrium 

 will then be disturbed, and in order to restore it the cool and heavy air will tend to 

 spread out along the ground, driving away the warmer and lighter masses previously 

 covering it. These will on the other hand go up, in order to fill the space from which 

 the heavy masses of air sink down. In this case there will appear at the ground a 

 line of convergence a little before the front of the advancing cold wave (fig. 53).* 



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Fig. 53. Cold wave. 



Fig. 54. Warm wave. 



Let us consider, on the other hand, a warm mass of air resting originally in 

 hydrostatic equilibrium upon a thin sheet of cooler air. This arrangement will be 

 stable as long as there is no motion or only a feeble motion. But if the upper layer 

 has a sufficiently strong motion, the arrangement will be kinetically unstable. 

 The warm air will then roll up and sweep away the thin layer of cool air. In this 

 case there will arise at the ground a line of convergence a little before the front of an 

 advancing warm wave (fig. 54) . 



In such cases there is no reason to expect a minimum of pressure along the line 

 of convergence. There may come a sudden change of pressure as the line passes, but 

 the most striking effect will be the sudden change of temperature along the line, and 

 such a discontinuity of temperatures may give additional evidence for the existence 

 of a line of convergence when the wind-observations themselves are insufficient. f 



138. Lines of Convergence at the Sea's Surface. While the observations of 

 the motions themselves are difficult at sea, the situation of a line of convergence will 

 under favorable circumstances be strikingly visible, for the reason that all sorts 



*Cf. Sandstrom's paper, quoted p. 54. 



fCf. R. G. K. Lempfert and Richard Corless: Line squalls and associated phenomena, 

 the Royal Meteorological Society. London, April, 1910. 



Quarterly Journal of 



