DAILY VARIATION— SEPTEMBER TO MARCH. 



61 



(3) the velocity of the wind in metres per second, v ; and (4) the diurnal amplitude 

 of the temperature of the air, A, for the months March to September. 



Table 25. 



' The radiation varies regularly from month to month, and is nearly constant from May 

 to August. 



The amount of cloud varies considei ably, and is highest from May to September. 



The velocity of tht; wind does not vary much. 



The diurnal amplitude of the temperature shows the largest variation during the months 

 from March to September. 



From March to April the amplitude of radiation increases rapidly, the amount of cloud 

 decreases to a minimum, as does also the velocity of the wind ; and all three 

 factors are working to raise the diurnal amplitude of the temperature. From April 

 to May the amplitude of radiation rises a little, but the amount of cloud is rapidly 

 increasing, and also the velocity of the wind. The two last-named factors bring the 

 amplitude of the temperature down notwithstanding the eiTecfc of the radiation. 



From May to June the amplitude of radiation increases very little, but the amount of 

 cloud increases one degree, and the temperature amplitude is lowered by the cloudi- 

 ness in spite of the increasing radiation and decreasing wind velocity. 



From June to July the same process goes on. 



From July to August the amplitude of radiation rises (the mean latitude of the Fram 

 being relatively lowered), and the amount of cloud and the velocity of the wind 

 decreases. The temperature amplitude rises, all three factors working together. 



From August to September the amplitude of radiation is going down rapidly, the amount 

 of c'.oud and the velocity of the wind are increasing, and all three factors cause the 

 amplitude of the temperature to decrease. . 



We see that the maxima of the diurnal amplitude of the temperature in both April 

 and August, correspond with a higher amplitude of the radiation, and minima of 

 cloud and wind velocity. 



The low amplitude of the temperatra-e in summer, particularly in July, corresponds to a 



high degree of cloudiness during a season which has hardly one clear day.' 



There can be little doubt that Professor Mohn's explanation contains part of the truth ; 



but if the observations are examined in another way it will be seen that it is not the whole. 



Professor Mohn attributes most of the irregularities in the temperature amplitude from 



month to month to the changes in the cloud and wind. That this cannot be so will be seen 



from the table given on page 604 of Mohn's work, part of which is reproduced here. 



