Diurnal Variations of the Pressure of the Wind. 



xcvu 



Table 92. — Diurnal Variation of the Maximum Pressure of the Wind within 10™ at the Observation 

 Hours, for each Astronomical Season and for the Year, deduced from the Observations of the Years 

 1843-6. 



Mak. 



Nov., 



Feb., 



May, 



August, 





Mak. 



Nov., 



Feb., 



May, 



August, 





Mean 



Dec, 



March, 



June, 



Sept., 



Year. 



Mean 



Dec, 



March, 



June, 



Sept., 



Year. 



Time. 



Jao. 



April. 



July. 



Oct. 





Time. 



Jan. 



April. 



July. 



Oct. 





h. m. 



lb. 



lb. 



lb. 



lb. 



lb. 



h. 



m. 



lb. 



lb. 



lb. 



lb. 



lb. 



12 10 



-007 



-0-21 



-0-20 



-0-12 



-0-15 







10 



+ 0-13 



+ 0-25 



+ 0-23 



+ 0-17 



+ 019 



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 



-Oil 



4 



10 



-0-03 



+ 0-14 



+ 0-18 



+ 0-11 



+ 0-10 



17 10 



-0-02 



-0-12 



-0-15 



-0.09 



-0-09 



5 



10 



+ 0-02 



+ 0-03 



+ 0-14 



+ 0-02 



+ 0-05 



18 10 



-005 



-0-08 



-0-11 



-0-09 



-0-08 



6 



10 



-0-03 



-005 



+ 0-03 



-0-05 



-002 



19 10 



-0-05 



-0-10 



-0-03 



-0-10 



-0-07 



7 



10 



-0-01 



-045 



-0-06 



-0-07 



-0-07 



20 10 



-0-03 



+ 0-03 



+ 0-13 



-0-04 



+ 0-02 



8 



10 



+ 0-02 



-0-19 



-0-13 



-0-10 



-0-10 



21 10 



+ 004 



+ 0.11 



+ 0-17 



+ 0-07 



+ 0-10 



9 



10 



-0-02 



-0-14 



-019 



-0-10 



-0-11 



22 10 



+ 0-07 



+ 0-21 



+ 019 



+ 0-15 



+ 0-15 



10 



10 



+ 0-02 



-016 



-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 irregularities produced by the large atmospheric disturb- 

 ances ; the following, however, are the approximate epochs of maximum and minimum : — 



Winter, 

 Spring, 

 Summer, 

 Autumn, 



Nov., Dec, Jan., 

 Feb., March, April, 

 May, June, July, 

 Aug., Sept., Oct., 



Year, 



Minimum. 



2^ 10m A.M. 



llh 40m P.M. 



9'^ P.M. 4^^ A.M. 



8^^ P.M. 7^ A.M. 



Mean. 

 &" 36m A.M. 



1"^ se"' A.M. 



7^ 21m A.M. 

 8^ 32m A.M. 



Maximum. 

 Qi^ IQm P.M. 

 I'' 55m P.M. 

 Ih 45m P.M. 

 1^' Om P.M. 



V A.M. 7*^ 57" A.M. 1^ 30m P.M. 



Mean. 



a^^ P.M.— iih P.M. 



5^^ 32m P.M. 

 61' 30m P.M. 

 511 27m P.M. 



5I' 53m 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 of 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 diffei'ence is observable in each quarter ; this 

 ■will be seen most satisfactorily in Plate IX., where the dotted curves of Avind 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 hours 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 I'' p.m., and in the night portion of the 

 curve, from l^* a.m. : the pressure of the wind, therefore, is related more directly to the position of the 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. 



