WEATHER AND GLAOIATION REEDS 299 



of amplitudes which are hii«!;o enou'jjh, when combined in similar 

 phase, to nearly overpower the maximum sun-spot effect on the 

 solar-constant values. 



The li)05 to li)2i) monthly avera<4es of sun spots show little appar- 

 ent relation with that of solar radiation. The yearly means, how- 

 ever, 11)05-1921), exhibit a more evident comparison. A closer resem- 

 blance between the two is secured when the U to 4 year oscillations in 

 solar radiation, which are not appreciable in sun spots, are smoothed 

 by means of four, and further smoothed by taking the means of each 

 consecutive two means, Figure 1, as was done by Clayton (1923). 



Accordin*^ to Abbot (1920), the intensity of solar radiation is ex- 

 })ecled to increase when sun spots are numerous. However, when an 

 individual sun spot crosses the central part of the sun a depression 

 in solar radiation is usually noticeable, which is attributed to a sort 

 of cloudiness over each sun-spot group. The resulting depressing 

 effect may exceed one of increased radiation. Thus it nuiy be said 

 that there is a real relation, though not a very close one, between 

 sun spots and solar radiation. 



EFFECTS OF ATMOSPHERIC OPACITY 



The radiation from the sun which reaches the earth is considerably 

 reduced by the opacity of the earth's atmosphere. According to 

 H. N. Russell (1926), only about 7U per cent of the initial solar radi- 

 ation gets through to sea level when the sun is at the zenith and the 

 air is free from dust and clouds. The opacity increases as the sun 

 approaches the horizon, due to the greater distance the rays have to 

 travel through the atmosphere and the loss of the shorter rays by 

 diffraction. The only satisfactory way of dealing with the?e varia- 

 tions is to measure with the spectrobolometer the energy received 

 when the sun is at various altitudes. Each different wave length 

 has to be measured separately to tletermine the depletion for each 

 and the various amounts summed up to find the total solar radiation 

 outside the atmosphere. The process is definite, but laborious. 



Normal opacity of the air may be much increased by the presence 

 of four by-products, which may be regarded as factors that affect 

 solar radiation — namely, water vapor, carbon dioxide, ozone, and 

 dust. Water vapor has the most thermostatic influence. In tropical 

 regions, where the humidity is high, nearly half of the sun's heat is 

 absorbed by a cloudless sky. A cloud surface may reflect more than 

 70 per cent of the sun's rays and absorb a large jiart of the remain- 

 der, while a canopy of dense clouds may permit onl}' a small ])art of 

 the sun's radiant energy to reach the earth. Since carbon dioxide 

 and ozone are present in the air in very small quantities, their influ- 

 ence is subordinate to that of water vapor. Ozone probably absorbs 



