lyo HUMPHREYS: METEOROLOGICAL PARADOXES 



At the times of spot minima the solar atmosphere is clearest; 

 the extreme ultraviolet radiation presumably, therefore, at a 

 maximum ; the upper atmosphere richest in ozone, and the earth 

 most conservative of its heat, and, because of the minimum 

 (if that be the case) of cirrus, also most receptive of solar radia- 

 tion — so receptive and so conserv^ative, perhaps, as to gain slightly 

 in temperature despite the decrease in the heat supply. 



THE SUN RISES BEFORE IT IS UP 



This paradox about the sun rising before it is up is equally 

 true of the moon and the stars, and is also one of the best known 

 and easiest explained of all meteorological paradoxes. 



Everyone is familiar with the fact that as light passes slant- 

 ingly from one medium to another, as from air to glass, for in- 

 stance, it does not continue on in the same straight line, but 

 abruptly changes direction at the interface according to well- 

 known laws. And the same thing is true of the rays of Ught 

 that pass from space into and through the atmosphere of the 

 earth; except that, in this case, as the density of the atmosphere 

 gradually increases from zero at its outer boundary to a maxi- 

 mum at the surface of the earth, so too the change in direction 

 of the entering light is equally gradual. The total change of 

 direction by the time the surface of the earth is reached depends 

 upon the wave-length, or color, of the light ; the slope at which it 

 enters, or zenith distance of the luminous object; the tempera- 

 ture and barometric pressure at the place of observation; the 

 humidity; and several other minor factors. On the average, 

 however, light from a star for instance, that appears to be 90° 

 from the zenith, and, therefore, on the horizon — just rising, say^ — 

 has been bent out of its original course by about 34 '.5. That is, 

 it comes into view (rises) while actually more than half a degree 

 below the horizon. And as the angular diameter of the sun 

 and the moon are each less than this horizon refraction, it follows 

 that when the sky is sufficiently clear the whole of either lumi- 

 nary may be seen before even its topmost portion is up; that is, 

 before it is geometrically above the horizon, or actually within 

 90° or less of the zenith. 



