32 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



pass between any points within tlie atmosphere, whether at the same 

 or different levels. This latter phenomenon, known as terrestrial 

 refraction, causes all objects on the earth or in the atmosphere to 

 appear to be at greater altitudes than the}^ actually are, except 

 Avhen the surface air is so strongly heated as to cause an increase of 

 density with elevation. 



The distance to the horizon, corresponding to a given altitude, 

 therefore obviously depends upon the rate of vertical density de- 

 crease. 



Looming. — When there is an increase over the normal rate of verti- 

 cal density decrease, such as often happens over water in middle to 

 liigh latitudes, it gives rise to the phenomenon known as looming, 

 or the coming into sight of objects normally l)elow the horizon. 



Towering. — This phenomenon is similar to looming, and some- 

 times is so designated. It occurs, as occasionally happens, when the 

 inversion layer is so located that rays to the observer from the top 

 of an object are more curved than those from the bottom. The effect 

 is to make the top appear more elevated — it will tower and seem to 

 draw near. 



Sinking. — A phenomenon, exactly the reverse of looming, also fre- 

 quently observed at sea. It is caused by a decrease below the normal 

 in the rate of vertical density decrease of the atmosphere. 



Stooping. — The revei'se of towering. Occasionally rays from the 

 base of an object may be curved downward much more rapidly than 

 those from the top, with the obvious result of apparent vertical 

 contraction, and the production of effects quite as odd and grotesque 

 as those due to towering. 



REFRACTION BY WATER DROPS 



Rainbow. — The ordinary rainbow, seen on a sheet of water drops — 

 rain or spray — is a group of circular or nearly circular arcs of colors 

 whose common center is on the line connecting the observer's eye 

 with the exciting light, (sun, moon, electric arc, etc.), or rather, except 

 rarely, on that line extended in the direction of the observer's shadow. 

 A very great number of rainbows are theoretically possible and doubt- 

 less all actually occur, though only three, (not counting supernum- 

 eraries), certainly have been seen on sheets of rain. 



Rainbows are produced by a comi^licated process of refraction of 

 sunlight as it enters and passes out of the raindrops, internal reflec- 

 tion of the light within the drops and interference of the rays after 

 leaving the drops, (Davis). 



The records of close observation of rainbows soon show that not 

 even the colors are always the same ; neither is the band of any color 

 of constant angular width; nor the total breadth of the several colors 

 at all uniform; similarly the purity and brightness of the different 

 colors are subject to large variations. The gi-eatest contrast, per- 

 haps, is between the sharply-defined brillant rainbow of the retreat- 

 ing thunderstorm and that ill-defined, faintly tinged bow that some- 

 times appeal's in a fog — the '' white bow " or " fog bow." 



All these differences depend essentially upon the size of the drops, 

 and therefore inequalities often exist between even the several por- 

 tions, especially to}) and bottom of the same bow, or develop as the 



