1913] on The Winds in the Free Air 719 



above the surface, and in these clays of aerial navigation it is im- 

 portant to know the law of this increase. It seems that at Ditcham, 

 the increase in velocity is at first linear or nearly so, and that the 

 line representing the linear increase, passes through zero velocity at 

 sea level. That is to say, if we plot the wind velocity at the surface 

 and draw through it a line from zero velocity at sea level, the wind 

 velocities at other heights, up to half a kilometre to one kilometre, 

 will lie very nearly on this line ; this approximately linear increase 

 has been found to agree with observations at several land stations, 

 but over the sea other conditions probably prevail. 



But there are occasions when this state of things does not apply 

 at all ; this is often the case in light breezes, and at times when the 

 surface wind is very shallow, giving place to an entirely different 

 wind regime in the first kilomstre of height. At such times it often 

 happens, that the wind velocity is greatest a very little way above 

 the surface. The fact that there are two separate conditions em- 

 phasizes the danger of taking means. By taking the mean value of 

 a number of separate observations we might get as a result that the 

 wind neither increased nor decreased in the first kilometre of height, 

 which in reality is only true on very rare occasions. As has been 

 truly said, " La methode des moyenunes c'est le seul moyen de ne 

 jamais connaitre le vrai ! " 



Another question of great importance to aviators is the effect of 

 hills upon the winds blowing over them. The balloons used in my 

 investigations ascend at the rate of 500 feet per minute, and in a few 

 minutes are carried beyond the reach of ground eddies ; in some 

 cases, however, I have found that a balloon rose with more than its 

 normal velocity when passing over hills if a strong wind was blowing, 

 and the effect is visible sometimes even when the Imlloon is more 

 than a kilometre above the surface ; on other occasions very little 

 effect has been observed. More light is being thrown on this ques- 

 tion by the observations of Mr. J. S. Dines on slowly ascending 

 balloons. 



The lower layers of the atmosphere up to one or two kilometres 

 are the most important to aviators. To meteorologists the higher 

 layers offer problems of greater interest. In considering the winds 

 in the free air it is convenient to have some datum to which to refer 

 them. The observed surface wind is not convenient for this purpose, 

 being too much affected by local conditions near the ground. A 

 better datum is what is known as the gradient wind. Under the 

 influence of the Ijarometric gradient the air is being pressed towards 

 the areas of low pressure, but the wind is actually blowing more or 

 less along the isobars at right angles to the force. In much the 

 same way, water in a basin, when allowed to escape through a hole in 

 the centre, and when given a slight movement of rotation, moves 

 round the basin at right angles to the forces which are pressing it 

 towards the centre. In the case of the atmosphere the turning move- 



