CHAPTER II. 



THE OBSERVATIONS OF AIR AND SEA MOTIONS. 



97. The Common Wind-Observations. Taking up the subject of the kine- 

 matics of atmosphere and hydrosphere, we have first to discuss the observations to 

 be used as the basis of the kinematic diagnosis. We shall begin by considering the 

 observations of wind. 



Even a rough examination shows the wind to be very irregular, its direction 

 and intensity changing rapidly in varying limits. By using finer methods of obser- 

 vation smaller irregular air-movements will be discovered which would otherwise 

 escape our attention. Directions and intensities of wind noted at meterological 

 stations are therefore always averages, the smaller irregularities not being discovered 

 and the greater ones being smoothed out by the personal estimate of the observer 

 or by a regular treatment of the records of the self-recording instruments. 



It is therefore only certain average air-motions which can be submitted to a 

 kinematic analysis. Neglecting the small irregularities in the large-scale meteor- 

 ology, we make a similar approximation as when in laboratory experiments on 

 fluid motion we neglect the irregular molecular motions existing according to the 

 kinetic theory. But in both cases indirect effects of the small motion arise in the 

 form of an apparent increase of frictional resistance. The question of this resistance 

 will be taken up in the dynamic part of this book. 



For our kinematic investigations we have to mention these irregularities only 

 on account of the uncertainty which they cause in the noted average direction and 

 intensity of the wind. When quantitative use is to be made of the wind-observa- 

 tions, it will be important to use rational methods both for taking the observations 

 and for smoothing out the irregularities. Especially it will be important that the 

 same method should be used for these purposes at all cooperating stations. The 

 best results will be obtained by self-recording instruments, the averages being taken 

 from the values registered during an interval of time extended equally long before 

 and after the epoch of observation. The average should be formed by vector -addition 

 and registering instruments should allow an easy determination of this average. 

 The vector formed by taking the separate averages of the recorded directions and 

 of the recorded intensities will not be the true vector-average ; but it may be used 

 approximately instead of the true vector-average if the variations of direction and 

 intensity have not been too strong during the interval for which the average is 

 formed. As meteorological wind-observations have not been organized in view of 

 our quantitative applications, they are very imperfect from our point of view. In 

 Europe, besides the fundamental imperfection that the principle of simultaneity is 

 not carried through, all sorts of wind-observations are used, from personal estimates 



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