NO. n.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 607 



and the windward side of the wind-rose (p. 285) indicates that such south- 

 erly to south-westerly winds are most prevalent in July. The temperatures 

 above zero are imported by the winds. 



The diurnal period of the tension of vapour is very distinctly developed, 

 even in the winter and the dark season (p. 501 and PI. VI). The range 

 generally follows that of the temperature. It is very small in the dark 

 season, and rises, even in spring when it is greatest, only to 0'2 mm. The 

 period has only one maximum, after noon, as sea-board stations generally have 

 in temperate latitudes. The diurnal ascending currents are too weak to carry 

 the vapours upwards at a rate sufficient to produce a secondary minimum at 

 the warmest time of the day. At night the vapours are occluded by the 

 snowy surface. 



The annual period of the tension of vapour (p. 502 and PI. Ill) generally 

 follows the corresponding period of the temperature of the air. It has its 

 maximum in July, like that of the air; but the minimum of the tension falls 

 in January, while that of the temperature falls later. The whole range is 

 only 4'3 mm. 



The diurnal period of the relative humidity (pp. 505 to 507, and PL VII) 

 does not present any particular features. It is very inconsiderable in the 

 winter months and the dark season, and its range rises in summer only to 

 2'5 per cent. 



The annual period of the relative humidity has its maximum in July 

 (p. 508, and PL VII) and minimum in October. Its march is not very regular. 

 The great humidity in the summer months is most prominent. 



The diurnal period of the amount of cloud (pp. 512 to 513, and PL VII) 

 shows a greater cloudiness in the day than in the night, in all seasons, and, 

 with some exceptions, with strong than with feeble winds (p. 517, and 

 PL VIII). 



In winter there is no period with feeble winds, but a rather well- 

 developed period with fresh winds. These winds being cyclonic (p. 304), they 



