gQ INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



PERSPECTIVE PHENOMENA 



Apparent stair-^tep ascent of clouds. — The stair-step appearance of 

 the echelon cloud is perhaps the simplest sky phenomenon due to 

 perspective. The clouds producing this effect on the eye of the ob- 

 server are more or less evenly spaced, flat-bottomed cumuli of the 

 same base elevation — flat-bottomed and of constant level because of 

 the approximately uniform horizontal distribution of moisture. 



Since the clouds are at a higher level than the observer, each suc- 

 cessive cumulus, as the distance increases, is seen at a lower angle 

 than its predecessor; and the dark bases of any two adjacent clouds 

 appear to be connected by the lighter side of the farther one, thus 

 forming the alternate "tread" and "riser" of the stair-step. 



Apparent arching of cloud hands. — The arching of narrow cloud 

 bands, in a curve resembling the conchoid, is an optical illusion due 

 entirely to the projection of the cloud (above the observer's level) 

 upon the sky. The amount of arching increases with the closeness 

 (and elevation) of the cloud. 



Apparent divergence and convergence of crepuscular rays., (sun- 

 heams). — Everyone is familiar with the beautiful phenomenon of the 

 "sun drawing water." in which sunbeams, finding their way through 

 rifts in the clouds, are rendered luminous by the dust in their courses, 

 in the same manner as when passing through a window into a room. 

 Equally familiar and equally beautiful are also those streaks and 

 bands of pearly light, (when the lower atmosphere is illuminated), 

 and azure shadows, (where only the upper atmosphere is illumi- 

 nated), that often at twilight and occasionally at dawn radiate far 

 out from the region of the sun, and at times even converge tow^ard 

 the opposite point of the horizon. These, too, are only beams of 

 sunlight and shadow bands caused by broken clouds or irregular 

 horizon. Coming, as they do, from the sun, some 93,000,000 miles 

 awa}', necessarily the beams are practically parallel. Their ap- 

 parent divergence, convergence, and arching are all illusions due to 

 l^erspective, just as in the case of the seeming convergence of rails on 

 a long straight track. 



Other phenomena due to perspective are the apparent divergence 

 of auroral streamers ; the apparent flattening of the dome of the sky 

 (more noticeable when the sky is covered with high cirrus clouds) ; 

 the change, with elevation, of the apparent size of sun and moon, and 

 a similar change in the apparent distance between neighboring stars. 



REFRACTION PHENOMENA 



Refraction phenomena are due to irregularities in the density of 

 the atmosphere. Some, such as the scintillation or twinkling of stars, 

 have been observed and studied since remote times, certainly since the 

 days of Aristotle. (384-822 B. C), who noted the fact that stars 

 twinkle, while planets shine with comparatively steady light. 



It is true that on account of their sensible disks the scintillation 

 of planets is much less than that of fixed stars, but under favorable 

 circumstances it is quite perceptible. Even the rims of the sun and 

 moon boil or "scintillate" while, of course, any fine marking on either 

 or on a planet is quite as unsteady as the image of a fixed star. 



