DAILY VARIATION. isi 



Witli the exception of July 1902-03 each of these sub-divisions shows the reduced phase, 

 the jTreatest reduction talcing place in Jiuie — ^the midwinter mouth. 



If we now examine the curve for the winter in figure 59, we see that it varies from the 

 curves for the other aeries in one important particular. The fall from the evening maximum 

 to the morning minimum is irregular. After commencing to fall from 22 hours to 24 hours 

 the fall is arrested and it rises shghtly from to 2 hours and then falls to a minimum at 

 G hours. On the analogy of the other curves the fall shou'd have been something like that 

 shown by the dotted line in the figure. The whole effect has the appearance as if an increase 

 of pressure had been imposed on the normal daily variation between midnight and 6 a.m. 



If we examine the curves for May, June and July, this same abnormality is clearly 

 visible on each. It is also showm on the curves for each of these months in each of the 

 separate periods 1902-03 and 1911-12. There can be no doubt that this abnormal rise of 

 pressure between midnight and 6 a.m. is a real effect during the winter months. If we 

 eliminate the effect to some extent by correcting the curve as shown by the dotted line in 

 figure 59 the value of A2 for the winter season becomes 136°, instead of 116°, i.e., the 

 phase has become much more nearly equal to the phase in other months. 



It appears reasonable to onclude that the half daily period has the same phase 146° 

 throughout the year, but in the winter, the abnormal rise in pressure between hours and 

 6 A.M. alters the apparent phase by nearly 30°. 



This abnormal rise in the pressure is exceedingly interesting. As stated above, an examin- 

 ation of the curves of the individual months leaves little doubt of the reality of the pheno 

 menon. In our discussion of the daily variation of the temperature a similar abnormality 

 of the temperature at 4 a.m. was noted. It is impossible not to connect the two phenomena. 

 It was shown that the temperature effect was visible in the records of Snow Hill and the 

 Gauss Station. The pressure effect is shown in the Snow Hill records, but not in those for 

 the Gauss Station. 



No simple explanation of either of these two effects is obvious, and they are worthy 

 of a much fuller investigation than it has been possible to give them here. 



Cause of the twelve-lwvrly haromeler oscillation. — In low latitudes the twelve-hourly baro- 

 meter oscillation is very strongly marked and at all places within 50° latitude of the equator 

 the maxima and minima occur at every place at approximately the same local time. The 

 conclusion that this oscillation is due to a wave travelling around the world parallel to the 

 equator is therefore irresistible. 



In 1882 Lord Kelvin * suggested that this wave was due to a natural oscillation of the 

 atmosphere. Since then a great deal of research has been undertaken to calculate the ampli- 

 tude and phase of this oscillation by applying mechanical laws to the known constants of 

 the atmosphere. Theory shows that such a wave should have a constant phase at all places 

 on the same latitude when local time is used and that the amplitude should be largest at 

 the equafor and zero at the poles. 



When the results of meteorological observations made in north polar regions came to be 

 examined it was found that there the oscillation conformed with neither of these conditions. 

 In the first place the amplitude of the oscillation was found to be much larger than it 

 ought to have been according to the theory and that the phase was nothing like constant 

 in local time. 



When discussing the results of the Lady Franklin Bay Expedition, 1881-83, Greely f 

 remarked that in high latitudes the phase of the half daily barometer oscillation was much 

 more nearly constant if Greenwich time was used than if local time was used. In other 



Thomsou. p. R. S. Ediu. 11, 1882. f See Haun. Met. Zoit. 7, p. 8, 1800. 



