410 



Mr. H. Blanford. The Diurnal Barometric 



" On the Kelations of the Diurnal Barometric Maxima to 

 certain critical Conditions of Temperature, Cloud, and 

 Rainfall." By Henry F. Blanford, F.E.S. Received 

 March 30 —Read May 3, 1888. 



It is not my purpose in this paper to discuss the general problem of 

 the diurnal barometric variation. It is certainly a very complex 

 phenomenon, and one o£ which no satisfactory analysis has yet 

 been made. The atmospheric stress (whatever be its nature) that 

 originates the oscillation, is followed by movements which alter both 

 the vertical and horizontal distribution of the gravitating mass, and 

 the striking differences that characterise the diurnal curve of 

 pressure on mountain peaks, plains, and valleys, and on the ocean as 

 compared with the land, are doubtless due in a large measure to these 

 resulting redistributions of the mass. 



Amid all the recorded variations of the oscillation as a whole, the 

 feature that displays the greatest constancy is the occurrence of a 

 maximum in some hour of the forenoon, and of a second maximum 

 one or two hours before midnight. The exceptional cases, in which 

 these two critical phases are much shifted from their normal positions, 

 are but few, and may probably all be explained by gravitation effects 

 being superadded to the normal semidiurnal oscillation. 



One of the most anomalous forms of the diurnal oscillation yet 

 recorded is that given by Professor Mohn, for the North Atlantic, 

 between latitudes 62° and 80°, in the summer months.* The general 

 form of this pressure curve is similar to that of the diurnal tempera- 

 ture curve. It falls to a minimum in the early morning hours, and 

 rises to a maximum between 1 h. and 3 h. 30 m. p.m. But of the three 

 curves for different years and latitudes given by Professor Mohn, two 

 show, as a subordinate feature, a small rise to a secondary maximum, 

 between 10 and 11 p.m., and two an irregularity in the morning rise, 

 such as would result from a small wave with a maximum about 7 or 

 8 a.m., in combination with the principal oscillation of twenty-four 

 hours' period. At Christiania and Upsala the phases of the single 

 period oscillation are reversed, the maximum being in the night, the 

 minimum in the day, but the semidiurnal element exhibits characters 

 similar to those of the North Atlantic curve. 



This comparative constancy of the semidiurnal element of the 

 oscillation, which was originally pointed out by Lamont,t seems to 

 indicate that it depends more directly on the action of the sun than 



* ' No/ske Nordhays Expedition,' 1876-1878. 



f ' Sitzungsber. d. Bayerisch. Akademie,' 1862, vol. 1, p. 89. 



