AIR CURRENTS IN RELATION TO AVIATION 353 



applicable in practice for the daytime. At night time we have 

 a new set of conditions, because the wind is subject to a marked 

 diurnal variation. Unless the meteorological conditions are 

 undergoing large changes the wind falls off at sundown and 

 only begins to rise again after sunrise. 



The following figures for the average wind velocity during 

 July and August as recorded at different observatories in con- 

 nection with the Meteorological Office illustrate this general 

 statement : 



Table of average Wind Velocities in July and August 



The difference between the maximum and minimum is most 

 marked at the more inland stations. It is to be noticed also that 

 the percentage differences between the maxima for the different 

 places are less than those for the minima. 



The diurnal variation of wind and its variation with height 

 are effectively shown by the two diagrams of fig. 4, the first of 

 which shows the records of an anemometer on the roof of the 

 Bureau Central Meteorologique, Paris, the second those of an 

 anemometer at the top of the Eiffel Tower, 1,000 ft. high and 

 close to the Bureau. The diagrams give the average values of 

 the wind velocity for each hour of the day in each month of the 

 year; the lines which are drawn connecting the figures of 

 equal value show that at the foot the highest average velocity 

 of something over 3 metres per second is in the middle of the 

 day in January, February and March, the minimum being in the 

 night with an extreme of only 1 metre per second at 5 a.m. in 

 September. At the top of the Tower the minimum is from 8 to 

 10 a.m. in June but it is not less than twice the maximum at the 

 low level, whilst the high level maximum is as much as 1 1 metres 

 per second at midnight in January. 



On account of the diurnal variation, the evening and early 



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