58 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



ditions, toward lower pressure. There will seldom be an absolute coincidence 

 between the points of convergence or of divergence with the points of minimum or 

 of maximum pressure, or between the neutral point and the saddle point on the iso- 

 baric surfaces. The draftsman will often find that the observations of the wind 

 give full evidence for the existence of kinematic singularities, especially of neutral 

 points and of lines of convergence and of divergence at places where the chart of 

 pressure does not show the expected peculiarities. Examples where the pressure for 

 theoretical reasons shows other peculiarities will be considered below. 



For thermodynamic reasons the kinematic singularities are in similar relation 

 to the distribution of precipitation, cloudiness, and blue sky, as to that of pressure. 

 Within an area of precipitation or of cloudiness there is, as a rule, ascending motion 

 and therefore a probability of the existence of a point or of a line of convergence. 

 In the same manner within areas of blue sky there will usually be descending 

 motion and therefore a probability for the existence of a point or a line of divergence. 

 The neutral point, which has no relation to vertical motion, will be indifferent in its 

 relation to precipitation and to blue sky. 



The charts of precipitation, of cloudiness, and of blue sky may therefore be used 

 precisely as those of pressure, to get additional evidence in cases where the observa- 

 tions of the wind are not sufficient. But as in the case of pressure, the conclusion 

 can not be reversed. Especially there will often be found lines of convergence caus- 

 ing no precipitation. Examining the relation of the kinematic singularities to pres- 

 sure and precipitation, cloudiness and blue sky, it will probably be possible to decide 

 whether the singularity is a local one, concerning only the lowest strata, or whether 

 it has any connection with the motion also at greater heights. 



135. Consequences of the Stability of Atmospheric or Hydrospheric Equilib- 

 rium. The different layers of the air or the sea as a rule rest upon each other in 

 stable equilibrium. A mass of air or of water will not leave its level except it be 

 forced to do so. The currents will therefore always prefer to some extent to go round 

 instead of going over obstacles. In other words, the lines of flow will have a certain 

 tendency to follow the level curves representing the topography of the bounding 

 surfaces. Many striking examples of this are seen on the accompanying maps of 

 the air-motion. This dependency of the wind-direction upon topography is so strong 

 that it can be recommended to draw the lines of flow on outline-maps containing a 

 simplified representation of the topography of the land. In many cases the apparent 

 irregularity in the distribution of arrows representing the observed wind-directions 

 will be understood at once, by a comparison with the level curves of this map. 



Sea-motions will depend upon the configuration of the bottom still more than 

 air-motions on the configuration of the ground. The remarkable correspondence 

 of lines of equal salinity, or equal temperature,* even at the surface of the sea, with 



*In his paper "Some oceanographic results of the expedition with the 'Michael Sars', 1900" (Nyt Magasin 

 for Naturvidenskab, T. 39, Christiania, 1 901), Professor Nansen says, p. 153: "If we consider the chart (Plate I) of the 

 surface-salinity and temperature it must strike one how almost exactly the most saline surface-water follows the deepest 

 channel of the Norwegian sea, and how the isotherms especially of io c C. and 9 C.seem to bedeflected in a waysimilar 

 to the isobaths." Further observations on this and allied subjects are found in the same author's " Oceanography 

 of the North Polar Basin," pp. 260 et seq. (The Norwegian North Polar Expedition, 1893-96, Scientific Results, Vol. 

 Ill, Christiania, 1 902), and in Helland-Hansen and Nansen: The Norwegian Sea, Chapter X,p. 311 (Christiania, 1909). 



