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Meteorology. — ''On the diurnal variation of the wind and the 

 atmospheric pressure and their relation to the variation of the 

 gradient.'' By Dr. J. P. van der Stok. 



(Communicated in the meeting of April 28, 1911). 



1. The diurnal variation of the wind, a plienomenon weil known 

 near coast stations as hind- and Seabreeze, is also observed, and 

 generally in a well marked degree, at land stations. 



On account of the laborious work of calculating the diurnal 

 variation from observations made hourly or at fixed hours, this 

 phenomenon has but rarely been investigated and, after all, the 

 knowledge acquired is hardly proportionate to the labour. 



The influence of the earth's rotation and of friction gives rise to a 

 rather complicated relation l)etween cause and effect, i.e. the variation 

 of the pressure gradient on the one hand and that of the wind on 

 the other, so that it is not possible to arrive at well founded con- 

 clusions based on the results of observations ojdy, without the 

 help óf some theory. 



For many places, as e.g. Helder, situated at the top of a land- 

 tongue, it would he difficult to tell a priori how^ and to what 

 degree the gradient varies in the course of a day, and neither 

 would it be possible to formulate simple and |)robable assumptions 

 as to the gradient in the case of a land station as de Bilt, sur- 

 rounded by regions of very different heat-absorption and radiation. 



The variability of the periodic gradient in different seasons is a 

 still more difficult problem as, near coast stations, the seacnrrents 

 and the temperature of the seawater in the surroundings, and near 

 land stations, the difference of physical properties of the adjacent 

 regions play an important part. 



Only for a station situated in an extensive and homogeneous 

 region the simple assumption of a heat wave propagating from 

 East towards West, with the sun, would be permissible and only 

 in such a case wonld it be possible to deduce the variation of the 

 gradient from the diurnal variation of the barometric height, as 

 observed in loco, provided the law of vai-iation depending upon 

 geographical latitude were know^n as well. 



In most cases, however, the variation of the gradient will be 

 smallest where the change of pressure is greatest and conversely, the 

 mechanism thus rather corresponding to an interchange of two 

 stationary sources of periodic pressure variation, situated at a fixed 

 distance the one from the other, than to a propagating wa^'e passing 

 at constant velocity. 



