AIR CURRENTS IN RELATION TO AVIATION 355 



Beyond the layer of the gradient velocity many things may 

 happen and for the present we can only classify the observed 

 instances into types (5). Let me enumerate some of them : 



1. The gradient velocity having been reached the wind may 

 be steady in direction and velocity as we go aloft — we get a 

 " solid " current. 



2. The gradient velocity may be surpassed and the velocity 

 in the higher levels become two or three times as great as the 

 gradient velocity. 



3. The direction may change continuously without much 

 change of speed. 



4. The speed may fall off nearly to zero and above a layer 



Fig. 5. — Variation of wind velocity with height by day and by night (hypothetical). 



of comparative calm we may find a current from an entirely 

 different direction, perhaps a complete reversal or a current at 

 right angles to that of the surface layers. 



Without going into detail, let me say that south-westerly, 

 westerly and northerly winds are capable of giving examples ol 

 the type of solid current, gradually veering strong winds and 

 continuous increase of wind with height. North-easterly, 

 easterly and south-easterly winds often have their place taken 

 aloft by winds from other quarters, generally with a calm 

 layer interposed between the two currents. A northerly 

 current above a southerly one sometimes occurs. 



There is a fifth class in which the wind in the lowest layer 

 is apparently not related to the gradient wind. Such cases are 

 of importance from a meteorological and dynamical point of 



