596 H. MOHN. METEOROLOGY. [NORW. POL. EXP. 



We see from this Table that the visible radiation from the sun reaching 

 the surface of the earth in the dark season, is only indirect, or brought about 

 by means of the sky or the twilight, and that it is inconsiderable, particu- 

 larly in December, even at noon; while there is no radiation from this source 

 reaching the surface of the earth at midnight. 



In the equinoctial months, at noon, we have both the direct radiation of 

 the sun, and that of the sky or twiligth, but at midnight only the radiation 

 from the latter. 



In the sunny season we have to do with both kinds of radiation, at mid- 

 night as well as at noon. 



The difference between the amount of the total radiation at noon and at 

 midnight, or the diurnal range of the radiation, on a unit of horizontal sur- 

 face, has an annual period with a minimum in December and a maximum in 

 the summer months. The fact that the maximum in our Table is to be 

 found in August and not earlier, is due to the relatively lower mean latitude 

 of the Fram in August. 



The diurnal variation of the radiation is the chief factor that determines 

 the diurnal periods of the various meteorological elements. 



The principal meteorological effect of the radiation from above, is the 

 heating of the atmosphere and the surface of the earth. This surface consists, 

 in the present case, during the whole year, chiefly of frozen water, snow and 

 ice, and only in the summer months partly of melted water. The energy of 

 the radiation is mainly exerted in warming and evaporating ice, and in sum- 

 mer in melting and evaporating ice, and partly warming and evaporating the 

 melted ice. 



On the other hand, radiation of heat from the surface of the earth into 

 space, the sky and the clouds, is always going on. 



The radiation from the sun and sky, and the radiation from the earth, 

 have the greatest effect when the sky is clear. 



The effect of both radiations upon the surface of the earth is checked 

 by clouds. With an overcast sky the radiation from above is screened off, 

 more or less, from the earth, and the radiation from below is radiated back 

 to the surface of the earth. 



The radiation from the sun tends to warm the surface of the earth, and 

 the radiation from the surface to lower its temperature. Which of these 



