DiUENAL VaEIATIONS OF THE PeESSUEE OF THE WlND, 
xcvii 
Table 92. — Diurnal Variation of the Maximum Pressure of the "Wind within 10^ at the Observation 
Hours, for each Astronomical Season and for the Yeai*, deduced from the Observations of the Years 
1843-6. 
Mak. 
Mean 
Time. 
Nov., 
Dec, 
Jan. 
Feb., 
March, 
April. 
May, 
June, 
July. 
August, 
Sept., 
Oct. 
Year. 
Mak. 
Mean 
Time. 
Nov., 
Dec, 
Jan. 
Feb., 
March, 
April. 
May, 
June, 
July. 
August, 
Sept., 
Oct. 
Tear. 
li. m. 
lb. 
lb. 
lb. 
lb. 
lb. 
h. m. 
lb. 
lb. 
lb. 
lb. 
lb. 
12 10 
-0-07 
-0-21 
— 0-20 
— 0-12 
— 0-15 
0 10 
+ 0-13 
+ 0-25 
+ 0-23 
+ 0-17 
+ 0-19 
13 10 
-0-10 
-0-15 
-0-19 
-0-08 
-0-13 
1 10 
+ 0-11 
+ 0-29 
+ 0-27 
+ 0-22 
+ 0-22 
14 10 
-013 
-0-14 
-0-21 
-0-09 
-0-14 
2 10 
+ 0-06 
+ 0-32 
+ 0-28 
+ 0-19 
+ 0-21 
15 10 
-0-03 
-0-08 
-0-22 
-0-10 
-0-11 
3 10 
+ 0-02 
+ 0-26 
+ 0-25 
+ 0-14 
+ 0-17 
16 10 
-0-05 
-0-10 
-0-18 
-0-10 
-0-11 
4 10 
-0-03 
+ 0-14 
+ 0-18 
+ 0-11 
+ 0-10 
17 10 
-0-02 
-0-12 
-0-15 
-0-09 
-009 
5 10 
+ 0-02 
+ 0-03 
+ 0-14 
+ 0-02 
+ 0-05 
18 10 
-0-05 
-0-08 
-0-11 
-0-09 
-0-08 
6 10 
-0-03 
-0-05 
+ 0-03 
-0-05 
-0-02 
19 10 
-0-05 
-0-10 
-0-03 
-0-10 
-0-07 
7 10 
-001 
-015 
-0-06 
-0-07 
-0-07 
20 10 
-003 
+ 0-03 
+ 0-13 
-0-04 
+ 0-02 
8 10 
+ 0-02 
-0-19 
-0-13 
-0-10 
-O-IO 
21 10 
+ 0-04 
+ 0-11 
+ 0-17 
+ 0-07 
+ 0-10 
9 10 
-0-02 
-0-14 
-0-19 
-0-10 
-0-11 
22 10 
+ 0-07 
+ 0-21 
+ 019 
+ 0-15 
+ 0-15 
10 10 
+ 0-02 
-0-16 
-0-21 
-0-13 
-0-12 
23 10 
+ 0-11 
+ 0-24 
+ 0-23 
+ 0-20 
+ 0-19 
11 10 
0-00 
-0-21 
-0-19 
-0-08 
-0-12 
202. Diurnal Variation of the Mean Pressure of the Wind. — It is evident from the means in Table 92, 
that 4 years' observations are too few to destroy the uTegularities produced by the large atmospheric disturb- 
ances ; the following, however, are the approximate epochs of maximum and minimum : — 
Minimum. Mean. Maximum. Mean. 
Wmter, Nov., Dec, Jan., 2^ 10^ a.m. 8^ 36"i a.m. O'^ 10"^ p.m. 3'^ p.m.— IP p.m. 
Spring, Feb., March, April, ll'' 40™ p.m. 1^ 56°^ a.m. 1'^ 55"^ p.m. 6^' 32™ p.m. 
Summer, May, June, July, 9^ p.m. — 4:'^ a.m. 1^ 21™ a.m. l^^ 45™ p.m. 6^ 30™ p.m. 
Autumn, Aug., Sept., Oct., 8^ p.m. — T^^ a.m. 8^^ 32™ a.m. 1'^ 0™ p.m. 5'^ 27™ p.m. 
Year, 1^ a.m. 7^ 57™ a.m. 1^ 30™ p.m. 5'^ 53™ p.m. 
It will be seen that the variation of the pressure of the wind obeys a law analogous to that of the variation of 
temperature, while the sun is above the horizon ; it follows the ascent and descent cf the sun, however, more 
closely than the temperature : thus, in winter the mean pressure of the wind occurs almost exactly at sunrise 
and at sunset, and the maximum occurs immediately after mid-day, in all instances anticipating the correspond- 
ing epochs for the temperature by an hour or more. A similar diiference is observable in each quarter ; this 
will be seen most satisfactorily in Plate IX., where the dotted curves of wind pressure are projected on the same 
mean or zero lines ; as the curves for the temperature of the air : while the day portions of the curves are evi- 
dently connected with each other, this is not the case during the night ; the minimum pressure of wind has upon 
the whole the same relation to midnight that the maximum has to mid-day. In winter the pressure varies 
irregularly from hour to hour during the night ; in summer and autumn the pressure is nearly constant for 
some hotirs before and after midnight. When we consider the mean for the year, we find the ordinates of 
equal value in the day portion of the curve at times equidistant from 1^ p.m., and in the night portion of the 
curve, from 1^ a.m. : the pressure of the wind, therefore, is related more directly to the position of tJie sun than 
to the temperature of the place, especially during the night. 
The range of the mean diurnal variation is least in winter, and it is greatest in spring and summer — the 
ranges are — 
Winter = 0-26 lb. Spring 0-53 lb. Summer 0-50 lb. Autumn 0-35 lb. Year 0-37 lb. 
