AIR CURRENTS IN RELATION TO AVIATION 371 



limit depends upon the locality and on the velocity of the 

 wind and, to go more into detail, on the direction of the wind 

 and the height above the ground. The diagrams suggest 15 

 to 20 miles per hour as the limit of safety in ordinary gustiness. 



Besides the disturbances due to ordinary gustiness the air- 

 man may have to deal with line-squalls and recurrent squalls 

 such as those represented in the anemograms for Scilly, 

 March 4, 1908; Holyhead, March 18-19, 1900; Aberdeen, 

 December 31, 1907, to January 1, 1908, shown in fig. 16 and in 

 the barogram for South Kensington for April 5, 191 1 (fig. 17). 

 We must not forget the local isolated squalls characteristic of 

 thunderstorms of which the one represented in fig. 18 for 

 June 1, 1908, is a very noteworthy example. 



It is to be feared that a line-squall or thunder-squall is 

 essentially unsafe for any air craft except a floating spherical 

 balloon with a very large reserve of lifting power and even 

 for that it furnishes an experience which is better avoided. 

 (See an account of such an experience in Blasius on " Storms.") 

 We do not know enough of recurrent squalls to make any 

 useful statement about them except this, that they occur especially 

 in the western section of a cyclonic depression ; the occasions 

 when the changes incidental to that part of a depression are 

 in progress are not suitable for trial trips, nor even for the 

 ascents of experienced airmen. 



Bibliography 



(1) The Life History of Surface Air Currents: a Study of the Trajectories of 



Moving Air. M. O. Publication, No. 174. 



(2) Barometric Gradient and Wind Force. M. O. Publication, No. 190. 



(3) Report of the Advisory Committee for Aeronautics, 1909-10. Memorandum 



on Wind Structure, etc., by W. N. Shaw, F.R.S. 



(4) Weekly Weather Report. Observations in the Upper Air. 



(5) Cave, C. J. P., The Structure of the Atmosphere in Clear Weather. Cam- 



bridge University Press. (In the press.) 



(6) Mill, H. R., and Lempfert, R. G. K., Q.J. Roy. Met. Soc. vol. 30, 1904. 



(7) Lempfert, R. G. K., The Line Squall of February 8, 1906, Q. J. Roy. Met. 



Soc. vol. 32, 1906. 



(8) and Corless, R., On Line Squalls and Associated Phenomena, Q.J. Roy. 



Met. Soc. vol. 36, 1 910. 



(9) Shaw, W. N., Forecasting Weather. Constable & Co., Ltd. 

 (10) Vector Diagrams, Q.J. Roy. Met. Soc, vol. 36, 1910. 



(11) Report of the Advisory Committee for Aeronautics, 1910-11. Reports by 

 J. S. Dines. 



