INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 79 



accompany the latter. If there is absence of definite clearing be- 

 tween the showers it is clue either to the presence of a layer of high 

 cloud (which is often the forerunner of further precipitation) or to 

 the fact that the spaces between the shower-clouds are filled with 

 low though lighter clouds. It may even happen that the precipita- 

 tion never completely stops, the arrival of a shower being marked 

 in such cases by a sudden darkening of the sky and by a sudden in- 

 crease in the intensity of the precipitation. 



Fog, — Microscopic droplets of water, which appear to float in the 

 air, and Avhich cause a sensation of cold and dampness. If watched 

 carefully the water droplets may, under certain circumstances, be 

 seen almost to pass in front of one's eyes. On the whole fog appears 

 white, except near industrial regions, where it becomes dirty gray 

 or yellow. During a true fog which is not dispersing the horizontal 

 visibility is less than 1 km (1,100 yds.). 



J/m'Z^.— Light fog, in which the horizontal visibility is greater than 

 1 km (1,100 yds.). It does not cause a raw^ or damp sensation 

 because the water droplets are too small and far apart. Mist often 

 has a grayish appearance in distinction from actual fog.'^ 



Haze. — Tiny particles of dust or salt, which are dry and so extraor- 

 dinarily small that they are neither felt nor perceived by the naked 

 eye, but which give the air a characteristically smoky appearance. 

 Haze casts a completely uniform veil over the landscape ancl dulls its 

 colors. Against a dark background this veil has a bluish color but 

 against a light background (such as clouds on the horizon, snow-cov- 

 ered mountain tops, or the sun) it has a dirty or reddish yellow 

 color. Haze may be distinguished by this characteristic from grayish 

 mist, which it sometimes equals in intensity.- 



PART VI. OPTICAL PHENOMENA 



The material appearing under this heading has been taken, with 

 some modifications, from the work, Physics of the Air, by W. J. 

 Humphreys, of the Weather Bureau. It is, of necessity, very much 

 abridged from the original and consists principally of definitions 

 and descriptions. 



Many curious and beautiful phenomena, of which the mirage, the 

 rainbow, the halo, the azured sky, and the twilight glow are some 

 of the more conspicuous, are due to the optical properties of the air 

 and the foreign substances suspended in or falling through it. All 

 or nearly all, of them have been the objects of innumerable obser- 

 vations and many careful studies, the results of which, fortunately, 

 have been summarized and discussed by various authors, of whom 

 perhaps the best known are Pernter and Exner. 



Most optical phenomena attract the attention of observers, some 

 by their beauty, others on account of the rarity of occurrence. 

 Many of them are more or less closely connected with the weather. 

 All are of importance for one reason or another and call for careful 

 observance and accurate description. 



^ In North America the term "mist" Is commonly applied to precipitation of small 

 intensity ; hence it is used synonymously with drizzle or fine rain. 



- In North America the term "haze" refers to a lack of transparency in the atmosphere, 

 the cause of which is not specified. 



