3^4 



SCIENCE PROGRESS 



tropopause, if not actually at that point, the initial falling off 

 being very rapid. The effect is much less marked in the case 

 of moderate winds, and does not exist in the case of light winds. 

 The average results were shown in a diagram which is here 

 reproduced (Fig. i ). When the direction of the wind was treated 

 in the same manner, it was found that in nearly every case there 

 was no change on entering the stratosphere, the direction being 



+er 



+4 



+2 



s 



5: 



-e 



5 10 13 20 25 30 35 40 



Velocity in Metres per Second 

 Fig. I. — Mean variation of wind velocity with height. 

 (a) Less than 13 metres/sec, in highest 2 kilometres of Troposphere. 

 (*) Less than 13 to 19 metres/sec. in highest 2 kilometres of Troposphere, 

 (c) Greater than 19 metres/sec. in highest a kilometres of Troposphere. 



very steady from a height of 2 kilometres to the highest point 

 reached. 



Passing on to the question of horizontal pressure and tem- 

 perature gradients, the former was found on the average to 

 increase slightly between the ground and the tropopause, and 

 then to diminish rapidly within the stratosphere. The hori- 

 zontal temperature gradient showed a complete reversal at the 

 tropopause : in the troposphere temperature of course decreases 

 horizontally in the direction of the cyclone, whereas in the 

 stratosphere the reverse is true. The suddenness of the 



