78 INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 



cumulonimbus and a nimbostratus is often rather difficult to make 

 out. If the cloud mass does not cover all the sky, or if even small 

 portions of the upper parts of the cumulonimbus appear, the dif- 

 ference is evident. If not, it can only be made out if the preceding 

 evolution of the clouds has been followed, or if precipitation occurs; 

 its character is violent and intermittent (showers) in the case of 

 cumulonimbus, as opposed to the relatively gentle and continuous 

 precipitation of a nimbostratus. 



The front of a thunder cloud of great extent is sometimes accom- 

 panied by a roll cloud of a dark color in the shape of an arch, of 

 a frayed-out appearance, and circumscribing a part of the sky of a 

 lighter grey. This cloud is named "arcus" and is nothing more or 

 less than a particular case of fractocumulus or fractostratus. 



DEFINITIONS OF IMPORTANT HYDROMETEORS 



[Adapted from the International Atlas of Clouds and of States of the Sky. Paris, 1932] 



Rain. — ^More or less continuous i)recipitation consisting of faii-ly 

 large drops falling from an unbroken deck of clouds. The sky is 

 covered either with a layer of so-called rain clouds that has formed 

 from an altostratus system, or with a uniformly gray, but relatively 

 high, canopy of clouds, generally with shapeless masses of cloud 

 below. The latter may even be ])resent in such quantities that the 

 upper clouds are completely hidden. 



Snoir.- — More or less continuous ])recipitation consisting of solid 

 particles mainly in the form of hexagonal crystals falling from an 

 unbroken deck of clouds. The appearance of the sky is the same as 

 in the case of rain. 



Soft hail. — ^White, opaque, round pellets of snow-like structure 

 and about 2-5 nun in diameter. The pellets are crisp and easily 

 compressible ; they rebound when falling on hard surfaces, and there- 

 by often burst. Soft hail occui's mainly at temperatures near the 

 freezing point and mostly on land, often before or together with 

 ordinary snow. 



Had. — Irregular lum})s of ice varying in size from that of peas to 

 that of a man's fist. They are either completely transparent or 

 formed of alternating clear and opaque, or snow-like, layers. Hail 

 falls almost exclusively during severe or protracted thunderstorms 

 and never with temperatures below the freezing point. 



Drizzle. — More or less uniform precipitation consisting of numer- 

 ous tiny droplets (diameter generally less than i/^ mm) wdiich seem 

 almost to hover in the air and share its slightest movements. Driz- 

 zle falls from an unbroken, dense, low layer of stratus clouds. Driz- 

 zle can sometimes yield considerable amounts of precipitation, even 

 as much as 20 mm (0.80 inches) in 24 hours, particularly along coast- 

 lines and on mountain ranges. 



Shower. — Of the above mentioned hydrometeors, rain, snow, soft 

 hail, and ordinary hail may fall in showers. A shower is character- 

 ized not only by the quick onset and cessation of the precipitation 

 or its rapid changes of intensity but also, and more especially, by 

 the appearance of the sky. Showery weather is characterized by a 

 rapid alternation of dark, threatening shower-clouds and bright 

 periods of short duration. Deep blue skies are often observed to 



