600 



H. MOHN. METEOROLOGY. 



[NORW. POL. EXP. 



maximum in September and October. This period is not in accordance with 

 the annual period of the radiation from the sun and sky. 



I would here insert the following remark. The observations from the 

 Fram embrace a comparatively short space of time, and the results drawn 

 from them regarding the periods of the meteorological elements may in several 

 instances be less trustworthy than if there had been a longer series of observa- 

 tions at our disposal. It will not do, therefore, to draw far-reaching conclu- 

 sions as to the connection between the different phenomena, and between 

 those phenomena and their probable causes. Regarding the velocity of the 

 wind it must be borne in mind as a fact in favour of the observations, that 

 they have always been made with the same anemometer, and are not estimated 

 wind-forces 1 . 



1 The diurnal period of the force of the wind resulting from the CHALLENGER observa- 

 tions (CHALLENGER Report, Vol. II, p. 25) is derived from the wind's force estimated 

 according to the Beaufort scale. Dr. BUCHAN says: "With respect to the open sea, 

 it is evident from the mean curve for the five oceans that the diurnal variation is 

 very small, there being apparently two indistinctly marked maxima about midday and 

 midnight respectively .... It seems probable that the line representing the tiue 

 diurnal variation in the velocity of the wind is practically a uniform straight line, 

 with the single exception of a small rise about midday, not quite amounting to a mile 

 per hour." 



The observations from the Norwegian North- Atlantic Expedition 1876 1878 (Vol. II. 

 Meteorology, pp. 125 to 128) which were made with hand- anemometer (in 1877 and 

 1878, the same instrument as that used on board the Fram) give as means for the 

 three summers 187678 (88 days) in the Norwegian Sea the following result, dv 

 being the difference from the diurnal mean (in metres per second, smoothed): 



The diurnal period comes out very distinctly with a maximum from noon to 4 p. m., 

 and a minimum from 4 to 5 a. m. This is the same period as that which we have 

 found from the Fram. It does not seem improbable that the resistance presented by 

 the waves may add to the small force of convectional currents in producing the period 

 found. Dr. JULIUS HANN remarks (Lehrbuch der Meteorologie, p. 390, note 7): "The 

 nocturnal maximum of the force of the wind found at sea by ships under canvas has 

 also been ascribed to the circumstance that the sails during the night get wet and 

 draw better." In the night an observer is certainly often inclined to note a higher 

 wind force with the same velocity, than during the day, the colder air being felt more 

 strongly at night than the warmer air is felt during the day. Results from measured 

 wind velocities may be considered much more reliable than results from estimated 

 wind-forces. 



