Chapter 15 — METEOROLOGICAL ELEMENTS 



However, most snow falls as parts of crystals, as 

 individual crystals, or more commonly as clus- 

 ters and combinations of these. Snow occurs 

 in meteorological conditions similar to those in 

 which rain occurs, except that with snow the ini- 

 tial temperatures must be at or below freezing. 

 Snow falling from convective clouds is termed 

 snow showers. 



Snow Pellets 



Snow pellets are white, opaque, round (or 

 occasionally conical) kernels of snowlike consis- 

 tency, 0.08 to 0.2 inch in diameter. They are 

 crisp and easily compressible. They may rebound 

 or burst when striking hard surfaces. They 

 occur almost exclusively in showers. 



Snow Grains 



Snow grains consist of precipitation of very 

 small, white, opaque grains of ice similar in 

 structure to snow crystals. When the grains hit 

 hard ground, they do not bounce or shatter. 

 They usually fall in small quantities, mostly 

 from stratus, and never as showers. 



Ice Pellets 



masses. They are composed either of clear Ice 

 or of alternating clear and opaque snowflake 

 layers. Hail forms in cumulonimbus clouds and 

 is associated withthundershower activity. Surface 

 temperatures are usually above freezing when hail 

 occurs. Determination of size is based on the 

 diameter, in inches, of normally shaped hail- 

 stones. 



Ice Prisms (Ice Crystals) 



A fall of unbranched ice crystals in the form 

 of needles, columns, or plates. These are often 

 so tiny that they seem to be suspended in 

 the air. They may fall from a cloud or from 

 clear air. In an Aviation Observation they are 

 called Ice Crystals. They are visible mainly 

 when they glitter in the sunlight or other bright 

 light; they may even produce a luminous pillar 

 or other optical phenomena. This hydrometeor 

 which is frequent in polar regions, occurs only 

 at very low temperatures in stable air masses. 



Rime 



Rime is a deposit of ice composed of grains 

 more or less separated by air. Sometimes 

 it is adorned with crystalline branches. It forms 

 when supercooled water droplets strike an ob- 

 ject at a temperature below freezing. 



Ice pellets are transparent or translucent 

 pellets of Ice which are round or irregular, 

 rarely conical, and which have a diameter of 

 0.2 inch or less. They usually rebound when 

 striking hard ground and make a sound on im- 

 pact. There are two main types of ice pellets. 

 One is composed of hard grains of ice consisting 

 of frozen raindrops or largely melted and re- 

 frozen snowflakes (formerly "sleet"); this type 

 falls as continuous precipitation. The other type 

 is composed of pellets of snow encased in a thin 

 layer of ice which has formed from the freezing 

 of either droplets intercepted by the pellets or 

 of water resulting from the partial melting of 

 the pellets; this type falls as showery pre- 

 cipitation. 



Hail 



Ice balls or stones, ranging in diameter from 

 that of a medium-size raindrop to an inch or 

 more, are referred to as hail. They may fall 

 detached or frozen together into irregular, lumpy 



Glaze (Clear Ice) 



A coating of ice, generally clear and smooth, 

 but with some air pockets. It is formed on ex- 

 posed objects at temperatures below or slightly 

 above 32°F by freezing of supercooled drizzle 

 or rain drops. Glaze is denser, harder, and 

 more transparent than rime. 



Drifting and Blowing Snow 



The result of snow particles being raised 

 from the ground by a strong turbulent wind. 

 Drifting snow consists of particles being lifted 

 to small heights above the ground, and visibility 

 is not less than 7 miles at eye level although 

 obstructions below this level may be veiled 

 or hidden by the particles moving nearly hori- 

 zontal to the ground. Blowing snow, however, 

 consists of particles raised and stirred violently 

 to moderate or great heights. Visibility is poor, 

 less than 6 miles , and the sky may become ob- 

 scured when the particles are raised to the 

 greater heights. 



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