WEATHER 41 



5400. FOG AND RIME 



5410. FOG. Minute droplets suspended in the atmosphere. These 

 droplets have no visible downward motion. Fog differs from clouds in that 

 the base of fog is at the surface, while the base of clouds is above the sur- 

 face. It is easily distinguished from haze by its dampness and grey color. 

 It is unusual for fog to form when the difference between the air tempera- 

 ture and the temperature of the dew point is greater than about 2.2°C (4°F). 



5411. Shallow Fog. Low- lying fog that does not reduce horizontal 

 visibility at a level 33 feet (10 meters) or more above the surface. 



5412. Ice Fog. Suspended particles in the form of ice crystals. It 

 occurs at low temperatures, and usually in clear, calm weather in high 

 latitudes. The sun is usually visible and may cause halo phenomena. 



5420. RIME. Rime is classified as soft or hard. 



5420.1. Soft rime consists of white layers of ice crystals deposited 

 chiefly on vertical surfaces- -especially on points and edges of objects- - 

 generally in super- cooled fog. On the windward side of the rigging, soft 

 rime may grow to very thick layers, long feathery cones, or needles pointing 

 into the wind and having a structure similar to that of frost. 



5420.2. Hard rime is an opaque, granular mass of ice deposited chiefly 

 on vertical surfaces in wet fog at temperatures below 0°C (32°F). It is 

 more compact and shapeless than soft rime, and may build out into the wind 

 as glazed cones or feathers. 



5500. HAZE, SMOKE, AND DUST 



5501. Haze, smoke, and dust may be observed near land. Haze re- 

 sembles a uniform veil with a bluish tinge when viewed against the sun or 

 clouds at the horizon. When smoke is present the sun has a reddish tinge. 

 Dust imparts a tannish or grayish hue to distant objects, and the sun appears 

 pale with a yellowish tinge. 



5600. MISCELLANEOUS 



5601. In the column headed "Remarks" of Form 615-5, enter halos 

 (solar or lunar), coronas (solar or lunar), rainbows, fog bows, and auroras. 



5700. PRESENT WEATHER (ww) 



5710. GENERAL. The term "present weather" refers to the state of 

 the weather occurring at the time of the observation or within the 1-hour 

 period prior to the observation. Code Table 6, "Present Weather" (in tables 



