NO. n.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 597 



radiations prevails over the other, and whether the resulting effect is to 

 warm or to cool the surface of the earth, and to cause the temperature of 

 the air to rise or to fall, are questions dependent chiefly upon the altitude of 

 the sun. 



That the radiation from the ttvilight has an appreciable heating power 

 seems to be distinctly proved by the observations from the Fram. We have 

 found the annual minimum temperature of the air to occur as early as the 

 end of January or the beginning of February (p. 483, and note, p. 484), though 

 the sun appears above the horizon as late as about the beginning of March. 

 In the mean time, the twilight is rapidly increasing in duration and extent 

 (Table, pp. 594, 595), and seems to act as a heat-twilight (Warme-Diimme- 

 rung), as Dove has called it. 



The diurnal period of the velocity of the wind (pp. 292 and 293, PI. I) 

 comes out very distinctly in the different seasons, with a minimum in the 

 night and a maximum a little past noon, in accordance with the general rule 

 found elsewhere in the lower strata of the air. The range is small, only a 

 fi'action of one metre per second. This phenomenon is generally explained 

 by the Espy-Koppen theory. 

 This theory requires 

 1^* that the velocity of the wind increases from the surface of the earth 



upwards, 

 S'^'y the presence of ascensional and descensional currents in the atmosphere, 



and 

 3'1'y that these currents be more strongly developed in the day-time than 

 in the night. 

 We know that the diurnal period of the wind's velocity is generally well 

 developed on land stations in latitudes where the sun shines during the day, 

 and feebly, or not at all, on the sea. In the former case we have the more 

 or less rugged surface of the earth acting as a resistance to the wind, and 

 causing the lower air to move more slowly than the higher layers, and present- 

 ing different conditions in adjacent patches of the surface for absorption of 



