NO. 17.] THE METEOROLOGICAL PERIODS IN THE ARCTIC SEA. 599 



patches of the surface become differently heated, and we have as a result 

 convectional ascending and descending currents. A higher temperature is 

 followed by a greater evaporation, and the greater lightness of the air produced 

 by the aqueous vapours adds to the ascending power of the warmer parts 

 of the air. The radiation upwards from the surface may also be locally 

 different and create differences of cooling. But the general cooling of the 

 surface by radiation tends to make the air above it heavier, sinking and 

 stagnant. The calms are more frequent in all seasons in the night than in 

 the day (pp. 307, 308; PI. I). 



(2) When there is a wind, the obstructions presented by the rugged 

 surface of the ice will force the air-current to rise on the windward side, and 

 to descend more or less directly on the leeward side. The stronger the wind, 

 the greater will be the effect of the ruggedness in creating ascensional and 

 descensional motions of the lower air. 



All kinds of radiation are generally checked by clouds covering the sky, 

 and this check increases with the amount of cloud. When the sun is above 

 the horizon, detached clouds may sometimes increase the radiation of heat 

 from the sky. 



The radiation from the sun and the atmosphere has a diurnal period 

 with a maximum at noon and a minimum at midnight or during the night. 

 The convectional currents in the lower air caused by this radiation must have 

 the same diurnal period, and the result is the diurnal period of the velocity of 

 the wind. The descensional currents bring the upper air with its greater 

 velocity down, and the ascensional currents carry the more slowly moving 

 lower air up from the surface of the earth. The higher the sun, the more 

 rapid is the convection; and the fresher the wind, the more does the effect 

 of the obstructions increase the force of the ascensional and descensional 

 currents. The lower the sun, the more does the radiation from the surface 

 of the earth tend to make the lower layers of the air quiet. 



The Tables on pp. 292 and 293, and PI. I, show that the diurnal period 

 of the velocity of the wind comes out as on land-stations with a maximum 

 near or a little after noon, and a minimum in the early morning hours. The 

 period is the same as that of the radiation from the sun and sky. The range 

 (max.— min.) has an annual period. It is lowest in December, rises to a 

 maximum in May, falls to a second minimum in July, and rises again to a 



