708 MR R. T. OMOND ON 



the mean hourly number of occurrences of each wind, the totals ranging from 90*0 to 

 92*0, according to the number of days in the three months included in each season. In 

 the body of each section of Table II. is the percentage difference of the number of 

 cases of each wind at each hour from the mean values at the foot. Thus in spring 

 there were on the average 16"3 cases of N. wind, but at l h there were 1 7*9, or 10 per 

 cent, above the mean ; at 2 h , 19'0, or 17 per cent, above, and so on, bold type indicating 

 an excess and italic type a defect. 



An examination of Table II. shows that the character of the hourly variations is 

 substantially the same in all the seasons, but that the changes are greatest in amount 

 and most regular in distribution in summer, and that in winter they are much smaller 

 and chance irregularities break into the periods of maximum and minimum frequency. 

 The values in spring and autumn are intermediate between those of summer and of 

 winter. 



Considering the summer values, it is evident that there is an excess of north wind 

 in the early morning, the greatest excess being 28 per cent, at 5 h ; that east winds have 

 a maximum about noon and another shorter-lived maximum at 18 h and 19 h ; while 

 north-east winds have two distinct maxima, one in the early morning a little before the 

 time of the north maximum, and one in the late afternoon at the time of the second 

 maximum of east wind. The minimum of north occurs just after noon, while that of 

 east extends over several hours before and after midnight, and the two minima of north- 

 east are just before midnight and at 13 h . South winds show less relative variation 

 than north, but are almost opposite to them in their times of greatest and least fre- 

 quency, there being a maximum about noon and a minimum near 6 h ; but there is also 

 a secondary maximum about midnight, and another minimum in the late afternoon. 

 South-east winds resemble south more than east in their hourly frequency. West 

 winds, like south, have double maxima, the chief one at 6 h , and a secondary one about 

 17'', the two minimum periods being ll h and 22 h . South-west winds have also two 

 maxima, at 4 h and 16 h , with a well-marked minimum at 20 h , and a secondary in- 

 determinate one in the forenoon. North-west winds are almost the same as north in 

 their hourly variations. Calms and variables are least frequent in the early morning, 

 and attain their maximum occurrence in the afternoon ; their variation is greater than 

 that of any individual wind, ranging in summer from 30 to 40 per cent, below and 

 above the mean. Thus all winds except north and north-west have two times of 

 maximum and minimum frequency in the day, which may be taken as an indication 

 that the variation in wind direction throughout the day is of a somewhat complex 

 character. 



This diurnal variation in direction is more clearly seen if Lambert's well-known 

 formula be applied to the numbers for each hour. This formula takes no account of 

 velocity, but only of the number of cases of each wind that occurs : it is 



E. - W. + cos 45° (N.E. + S.E. - S.W. - N.W .) _ 



N. - S. + cos 45° (N.E. + N.W. - S.E. - S.W.) ~ aD ' 



K 



