CHAPTER 5 ATMOSPHERIC PHENOMENA Atmospheric phenomena include all hydrometeors, lithometeors,  photo-meteors,  and  electrometeors  and their associated effects. As an observer, you have the opportunity   to   observe   and   record   some   of   these phenomena on a daily basis; however, as an analyst you must understand how and why these phenomena occur and  what  effects  they  can  have  on  naval  operations. Some phenomena have little effect on naval operations, but others such as extensive sea fogs and thunderstorm activity can delay or cancel operations. HYDROMETEORS LEARNING    OBJECTIVE: Identify the characteristics of hydrometeors (precipitation, clouds, fog, dew, frost, rime, glaze, drifting and blowing snow, spray, tornadoes, and waterspouts). Hydrometeors   consist   of   liquid   or   solid   water particles that are either falling through or suspended in the  atmosphere,  blown  from  the  surface  by  wind,  or deposited on objects. Hydrometeors comprise all forms of  precipitation,  such  as  rain,  drizzle,  snow,  and  hail, and such elements as clouds, fog, blowing snow, dew, frost tornadoes, and waterspouts. PRECIPITATION Precipitation includes all forms of moisture that fall to Earth’s surface, such as rain, drizzle, snow, and hail, etc.   Dew,   frost,   clouds,   fog,   rime,   glaze,   spray, tornadoes, and waterspouts are not forms of precipitation, although they are hydrometeors. Precipitation   is   classified   according   to   both   form (liquid, freezing, and solid) and size (rate of fall). The size of precipitation drops determines their rate of fall to a large extent. Rain Precipitation that reaches Earth’s surface as water droplets with a diameter of 0.02-inch (0.5 mm) or more is  classified  as  rain.  If  the  droplets  freeze  on  contact with  the  ground  or  other  objects,  the  precipitation  is classified as freezing rain. Rain falling from convective clouds  is  referred  to  as  rain  showers.  Showers  are usually  intermittent  in  character,  are  of  large  droplet size, and change rapidly in intensity. Drizzle Drizzle   consists   of   very   small   and   uniformly dispersed droplets that appear to float while following air  currents.  Sometimes  drizzle  is  referred  to  as  mist. Drizzle  usually  falls  from  low  stratus  clouds  and  is frequently accompanied by fog and reduced visibility. A  slow  rate  of  fall  and  the  small  size  of  the  droplets (less than 0.5-mm) distinguish drizzle from rain. When drizzle  freezes  on  contact  with  the  ground  or  other objects,  it  is  referred  to  as  freezing  drizzle.  Drizzle usually restricts visibility. Snow Snow consists of white or translucent ice crystals. In  their  pure  form,  ice  crystals  are  highly  complex hexagonal  branched  structures.  However,  most  snow falls as parts of crystals, as individual crystals, or more commonly as clusters and combinations of these. Snow occurs in meteorological conditions similar to those in which  rain  occurs,  except  that  with  snow  the  initial 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 snow-like consistency, 0.08  to  0.2  inch  in  diameter.  They  are  crisp,  easily compressible, and may rebound or burst when striking hard surfaces. Snow pellets occur almost exclusively in snow showers. Snow Grains Snow grains consist of precipitation of very small, white, opaque grains of ice similar in structure to snow crystals. They resemble snow pellets somewhat, but are more flattened and elongated. When the grains hit hard ground,  they  do  not  bounce  or  shatter.  Snow  grains usually  fall  in  small  quantities,  mostly  from  stratus clouds, and never as showers. 5-1