MONTHLY ANALYSIS OF WIND
Number of days in each month during which the mean velocity of the wind was respectively below the value mentioned hereunder.
Month | Year of least wind (No. 8) | Year of most wind (No. *8*) |
| 5 10 15 20 | 5 10 15 20 |
| m.p.h. m.p.h. m.p.h. m.p.h. | m.p.h. m.p.h. m.p.h. m.p.h. |
———+———-+——-+———-+———-+———-+———+———+———-+
Jan. 5 11 23 27 3 6 15 23
Feb. 5 19 23 28 0 2 8 16
Mar. 5 10 20 23 0 1 11 18
April 6 16 23 28 1 7 16 26
May 1 14 24 30 3 11 24 31
June 1 12 22 26 1 10 21 27
July 8 18 29 31 1 12 25 29
Aug. 2 9 23 30 1 9 18 30
Sept. 1 13 25 30 1 12 24 28
Oct. 5 17 21 26 0 4 16 29
Nov. 6 11 20 26 3 7 19 28
Dec. 1 5 19 24 2 7 23 29
———+———-+——-+———-+———-+———-+———+———+———-+
Total 46 155 272 329 16 88 220 314
During the year of least wind there were only eight separate occasions upon which the average hourly velocity of the wind was less than six miles per hour for two consecutive days, and on two occasions only was it less than six miles per hour on three consecutive days. It must be remembered, however, that this does not by any means imply that during such days the wind did not rise above six miles per hour, and the probability is that a mill which could be actuated by a six-mile wind would have been at work during part of the time. It will further be observed that the greatest differences between these two years occur in the figures relating to the light winds. The number of days upon which the mean hourly velocity of the wind exceeds twenty miles per hour remains fairly constant year after year.
As the greatest difficulty in connection with pumping sewage is the influx of storm water in times of rain, it will be useful to notice the rainfall at those times when the wind is at a minimum. From the following figures (Table No. 13) it will be seen that, generally speaking, when there is very little wind there is very little rain Taking the ten years enumerated in Table No. 11, we find that out of the 314 days on which the wind averaged less than six miles per hour only forty-eight of them were wet, and then the rainfall only averaged .l3 in on those days.
TABLE No. 13.
WIND LESS THAN 6 M.P.H.
—————-+——————-+——————+————+—————————————————
Ref. No. | Total No. | Days on | | Rainfall on each
from Table | of days in | which no | Rainy | rainy day in
No. 11. | each year. | rain fell. | days. | inches.
—————-+——————-+——————+————+—————————————————
1 | 16 | 14 | 2 | .63 and .245
2 | 15 | 13 | 2 | .02 and .02
3 | 39 | 34 | 5 | .025, .01, .26, .02 and .03
4 | 36 | 29 | 7 | / .02, .08, .135, .10, .345, .18
| | | | \ and .02
5 | 34 | 28 | 6 | .10, .43, .01, .07, .175 and .07
6 | 32 | 27 | 5 | .10, .11, .085, .04 and .135
7 | 33 | 21 | 2 | .415 and .70
8 | 46 | 40 | 6 | .07, .035, .02, .06, .13 and .02
9 | 26 | 20 | 6 | .145, .20, .33, .125, .015 & .075
10 | 37 | 30 | 7 | / .03, .23, .165, .02, .095
| | | | \ .045 and .02
—————-+——————-+——————+————+—————————————————
Total | 314 | 266 | 48 | Average rainfall on each of
| | | | the 48 days = .13 in
The greater the height of the tower which carries the mill the greater will be the amount of effective wind obtained to drive the mill, but at the same time there are practical considerations which limit the height. In America many towers are as much as 100 ft high, but ordinary workmen do not voluntarily climb to such a height, with the result that the mill is not properly oiled. About 40 ft is the usual height in this country, and 60 ft should be used as a maximum.
Mr. George Phelps, in a paper read by him in 1906 before the Association of Water Engineers, stated that it was safe to assume that on an average a fifteen miles per hour wind was available for eight hours per day, and from this he gave the following figures as representing the approximate average duty with, a lift of l00 ft, including friction:—