INSTRUCTIONS TO MARINE METEOROLOGICAL OBSERVERS 43 



If no rain is falling at the time of observation, but lias fallen 

 during the preceding hour, one of the numbers in decade 2 should 

 be used. If, in addition to the ordinary or large rain drops, there 

 are also considerable amounts of very small drops, one of the num- 

 bers 58 or 59 of the preceding decade should be used. 



(f) Decade 7 (numbers TO-79) should be used only in the case of 

 ordinary snow, grains of snow, or ice particles; that is, fairly con- 

 tinuous precipitation in solid form from a continuous cloud sheet. 

 The sky in this case is generally covered with a layer of snow 

 clouds forming from an altostratus system, or with a uniformly 

 gray but relatively high canopy of clouds, generally with formless 

 masses of cloud below (which may even be present in such quanti- 

 ties that the ujjper cloucls are completely hidden). Sometimes, how- 

 ever, as when isolated snow crystals or small snowflakes fall, the 

 sky may be covered with a layer of stratus or stratocumulus. 



Numbers 71 or 72 should be used only when isolated snow crys- 

 tals fall without causing the horizontal visibility to become less than 

 4. kilometers (2i/2 miles). Numbers 73 to 77 should be used in the 

 case of ordinary snowfall. The number 78 should be reserved for 

 granular snow (see Glossary). 



The number 79 should be used for ice crystals or for frozen rain- 

 drops (grains of ice) . These two phenomena, which are rarely, if 

 ever, observed at sea, are quite unlike in character. Furthermore, 

 they do not occur simultaneously. The recipient of the message 

 will be able to judge which one is meant. 



If no snow is falling at the time of observation, but has falleji 

 during the preceding hour, one of the numbers in decade 2 should 

 be used. If, in addition to the precipitation in solid form, ordinary 

 rain also falls, the numbers 68 or 69 of the preceding decade should 

 be used. 



(g) Decade 8 (numbers 80-89) should be used only in the case 

 of precipitation of a shoiuery character, but no thunderstorm, at the 

 time of observation. The showery character is distinguished not 

 only by the rapid beginning and cessation of the precipitation, and 

 by its widely varying intensity, but also, and primarily, by the ap- 

 pearance of the sky — rapid alternation between dark threatening- 

 showier clouds and short bright periods, often w^ith deep blue sky. 

 Sometimes there is no definite bright period between showers, which 

 is due either to a layer of high cloud (often the forerunner of a 

 new rain area) or to the spaces between the shower clouds being- 

 filled up with low but lighter clouds. It may also happen that 

 the precipitation never completely ceases between the showers; the 

 arrival of a shower cloud is then manifested by a sudden darkening 

 of the sky and by a sudden increase in the intensity of the precipi- 

 tation. 



(h) Decade 9 (numbers 90-99) should be used only in ths case 

 of a thunderstorm; that is, precipitation at the time of observa- 

 tion with thunder at the same time or during tlie preceding hour. 



(i) When precipitation has been going on for a long time, the 

 observer has no difficulty in deciding whether it is of the shower 

 type, continuous type, or intermittent type, but when the precipita- 

 tion has only recently begun the observer may often find it diffi- 

 cult to decide to which type the precipitation belongs. 



5672° — 38 — —4 



