INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS gl 



It is well known that tlie atmosphere, generally, is so stratified that 

 with increase of elevation many more or less abrupt changes occur in 

 temperature, composition, density, and therefore, refrangibility. As 

 such layers glide over each other, billows are formed, and the adja- 

 cent layers thereby corrugated. The several layers frequently also 

 heat unequally, largely because of disproportionate vapor contents, 

 and thereby develop, both day and night, and at various levels, in- 

 numerable vertical convections; each moving mass differing, of 

 course, in density from the surrounding air, and by the changing 

 velocity being drawn out into dissolving filaments. Optically, tliere- 

 fore, the atmosphere is so heterogeneous that a sufficiently bright star 

 shining through it would produce on the earth a somewhat streaky 

 pattern of light and shade. 



Shadow hands. — A striking proof of the optical streakiness of 

 the atmosphere is seen in the well-known shadow bands that at the 

 time of a total solar eclipse appear immediately before the second, 

 and after the third, contact. 



Terrestrial scintillaMon. — ^A bright terrestrial light of small size, 

 such as an open electric arc, scintillates when seen at a great distance, 

 quite as distinctly as do the stars and for substantially the same rea- 

 son; that is, optical inequalities due to constant and innumerable 

 vertical convections or conflicting Avincls. 



Shimmenng .■ — ^The tremulous appearance of objects. The common 

 phenomenon of shimmering, seen through the atmosphere immedi- 

 ately over any heated surface, is another manifestation of atmosx^heric 

 refraction, and is due to the innumerable fibrous convections that 

 always occur over such an area. "^ 



Optical haze. — The frequent indistinctness of distant objects on 

 warm days when the atmosphere is comparatively free from dust, 

 and ascribed to optical haze, is due to the same thing, namely, optical 

 heterogeneity of the atmosphere, Avhich causes that unsteadiness 

 or dancing of star images that so often interferes with positional 

 and other exact work of the astronomer. Both are but provoking 

 manifestations of atmospheric refraction. 



An interesting and important result of astronomical refraction is 

 the fact that the sun, moon, and stars rise earlier and set later than 

 they otherwise woulcl. For places at sea level the amount of eleva- 

 tion of celestial objects on the horizon averages about 35', and there- 

 fore the entire solar and lunar disks may be seen before (on rising) , 

 and after (on setting), even their upper limbs would have appeared, 

 in the first case, or disappeared, in the second, if there had been 

 no refraction. This difference in time of rising, or setting, depends 

 on the angle or inclination of the path to the horizon. 



Green -flash. — As the upper limb of the sun disappears in a clear 

 sky below a distant horizon its last starlike point often is seen to 

 change rapidly from pale j-ellow or orange to green and finally blue, 

 or, at least, a bluish green. The vividness of the green, when the 

 sky is exceptionally clear, together with its almost instantaneous ap- 

 j^earance, has given rise to the name "green flash" for this phe- 

 nomenon. The same gamut of colors, only in reverse order, occa- 

 sionally is seen at sunrise. 



Terrestrial refraction. — The curving of rays of light is not confined 

 to those that come from some celestial object, but applies to those that 



