12 DYNAMIC METEOROLOGY AND HYDROGRAPHY. 



angular velocity of the cloud by a nephoscope. For as the height of the cloud can be 

 estimated approximately by its form, the velocity can be calculated. But besides 

 the smaller errors caused by the use of the estimated heights, great errors may 

 arise on account of the difficulty of recognizing with perfect certainty the cloud- 

 forms. Even if finer methods be used, based upon the measurement of the parallax, 

 the diffuseness of the objects observed causes difficulties, and the process of forma- 

 tion and dilution of clouds going on simultaneously with their motion makes the 

 interpretation of the observations difficult. 



But the great drawback of the cloud-observations is that they only give spor- 

 adic information, depending upon where clouds happen to be. What is wanted are 

 continuous records of the air-motion taken along vertical or quasi-vertical lines, cor- 

 responding to the continuous records of pressure, temperature, and humidity, which 

 we have considered in Statics. Continuous records of air-motion may be obtained 

 by the same ascents which give records of the scalar meteorologic elements. Besides 

 the other instruments, a kite can lift an anemometer registering the wind-intensity, 

 while the direction can be estimated roughly by the direction of the kite-line. But 

 the best results are obtained by observing the motion of free balloons by theodolites. 

 If the height of the balloon can be found from other data, only one theodolite is 

 required. Thus if the balloon carries a registering barograph, the direction and 

 intensity of the wind may be found as function of the registered pressure or as func- 

 tion of the height calculated from this pressure. But an important simplification 

 has recently been introduced. A closed caoutchouc-balloon has been found to 

 mount with a practically constant velocity, which can be calculated by the dimen- 

 sions and buoyancy of the balloon. Thus the height is known simply by the time 

 elapsed since the moment of its launching. The air-motion can therefore be deter- 

 mined much more easily than all other meteorological elements in the free air, the 

 instruments required being simply a theodolite and a small pilot-balloon. Accord- 

 ing to Hergesell* this method gives better determinations of the air-motion in the 

 higher strata than our ordinary station-instruments can give for the layer near the 

 ground. 



Just as other instruments, the pilot-balloons give the air-motion with the small 

 irregularities to some degree smoothed out. Small oscillations of the balloon are 

 seen as long as the distance is not too great, but as the observations are taken at 

 intervals which are long compared to the period of these small irregularities, only 

 averages are obtained. 



It should be observed that these averages in reality are of a complex nature, 

 being averages simultaneously as regards intervals of time and of height, and further, 

 that the air-motions found by the same pilot-balloon in different heights are not 

 strictly simultaneous. We have mentioned already this general imperfection of 

 observations obtained by aerological ascents (section 93) and its relatively small 

 importance when the ascents are arranged so as to be made with sufficient ver- 

 tical velocity. 



*H. Hergesell: Die Bedeutung der Pilotballonaufstiege fur die praktische Aerologie. Sixieme Reunion de la 

 Commission Internationale pour l'Aerostation scientifique a Monaco, 1909. Strassbourg, 1910. 



