STRUCTURE OF THE WIND. 115 



It is itiloresting to see fnuii this (able thai southerly winds of over 10 miles an hour 

 were bhiwing for more than lialf of the whole time. Northerly winds of more than 30 miles 

 an hour were very rare occurring only during one per cent, of the time. 



Structure of the Wind. 



As stated above (i)agc 94), a Dines pressure tube anemometer was used at the Hut. The 

 exposure of tliis instrument left much to be desired as a recorder of the true velocity, but for 

 a study of the ' structure of the wind ' tlie traces are of unique value. 



This instrument, as is w6ll known, records the instantaneous wind velocity at each moment 

 and as the wind is never steady from moment to moment, the recording pen is constantly 

 moving up and down the record. Whenever there is a gust the pen moves up the sheet 

 to indicate a high velocity, but the ne.xt minute it may be low on the paper to record a lull. 



Three characteristic charts of winds from the south are shown on figure 38, and three 

 of winds from the north on figure 39. 



As the \vind charts were changed each morning just after 8 a.m., the times printed on 

 the chart were not convenient, also the clock did not revolve the drum at the exact rate 

 for which the charts had been printed, therefore the correct time is shown on a line about 

 half-way up each chart, the hours being numbered fi'om 1 to 24. The direction of the wind 

 during each hour has also been entered on the chart. 



Gusliness. — The most striking difference in the two sets of records is the great gustiness 

 of the wind from the south compared with that from the noi-th. In fact the record (A) of 

 figure 38 exhibits a gustiness which is remarkable : during the hour 20 hoiu's to 21 hours the 

 wind rose to 74 miles an hour in a gust and fell a few minutes later to 10 miles an hour 

 in a lull, i.e., a change of wind velocity of 64 miles an- hour within a very few minutes. 



The gustiness which is so marked on this record was a characteristic feature of the winds 

 at Cape Evans and will be discussed first. 



In order to get a numerical value for the gustiness, we proceed as follows : — 



The vertical lines on the chart represent an hour of time, taking each hour we tabulate 



(«) the velocity of the wind in the highest gust, 

 {h) the velocity of the wind in the lowest lull, 

 (c) the mean velocity during the hour. 



The latter (c) is obtained by estimating the position of a line which has as much of the 

 trace above it as below during the hour. 



The gustiness is then defined as ^^^. 



Thus, in the hour 20—21 on figure 38 (A) a=74, 6=10, c=32 .-. gustiness=^'^ = ^=2-00. 



In the following discussion the results of calculating gustiness in this way are used. It 

 should be mentioned, however, that the gustiness was only determined for hours during which 

 there was no sudden change in the mean wind velocity. That neglecting this nile would 

 lead to errors can be seen by considering the hour 10 — 11 on figure 38 (A). In this hour the 

 lowest lull occurs with quite a different mean velocity from that when the highest gust occurs. 

 If one uses such hours it is obvious that the gustiness is made to appear larger than it ought 

 to be. Similarly, if the wind suddenly rises from a calm and a gust of, say, 30 miles an hour 

 is recorded towards the end of the hour the difference between the maximum and minimum 

 velocity would be 30 and the mean velocity during the hour, very small, say, 5 miles an hour. 

 This would give a gustiness of 6 which would obviously be too high and quite misleading. 



The mean values of the gustiness ol^tained by the method used in this discussion will 

 tlierefore be smaller than values derived from all hours irre.spective of changes in the mean 

 velocity during the hour. 



