NO. 17.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 603 



the clouds seem to intercept the radiation in a higher degree than in the 

 other seasons. 



The relatively small amount of the diurnal range of the velocity of the 

 wind is perhaps an indication of the relatively small height to which the whole 

 phenomenon reaches. Future polar expeditions, with a ship drifting like the 

 Fram, could solve the question by instituting regular anemometric observa- 

 tions from the crow's nest. 



The Table on p. 471 and PI. Ill, shows that the diurnal period of the 

 temperature of the air varies greatly in the different months. 



In the dark season the ordinary period is reversed, and we have the day 

 colder than the night. We have found (p. 496) the explanation of this pheno- 

 menon by means of the thermal wind-roses, the day-winds at this season 

 being generally colder than the night-winds. 



Meanwhile the Table on p. 477 and PI. IV, as also the Tables on p. 481 

 and PI. V, show that we have the ordinary period with minimum at night 

 and maximum near, or some hours after, noon in the winter and in the dark 

 season, with an overcast sky and with stronger winds. With clear weather 

 and feeble wind, the radiation from the earth has a tendency to produce an 

 inversion of temperature (pp. 497, 498). But with an overcast sky and stronger 

 winds, the lower and higher layers of the air are mingled together, and this 

 brings the temperature of the lower layers to rise. The greater the amount 

 of cloud, the less is the tendency to inversion; and the greater the velocity 

 the wind, the more thorough is the mingling. This is the case in the day 

 hours in a higher proportion than at night. In the winter and the dark 

 season the cloudiness is greater (pp. 512, 513, and PI. VII) and the wind 

 stronger (p. 293, and PL I) at day than at night. On the other hand the 

 average amount of cloud (p. 518, and PI. VII) and the average velocity of the 

 wind (p. 309, and PI. I) is rather small in these seasons, and the resulting 

 diurnal period comes out in accordance with that dependent on clear weather 

 and weak winds. 



In the months during which the sun is above the horizon, the diurnal 

 period of the temperature of the air has the ordinary run, with minimum in 



