Precipitation  Forms Precipitation  includes  all  forms  of  moisture  that  fall to the earth’s surface, such as rain, drizzle, snow, and hail. Precipitation is observed and classified by form, type, intensity, and character. PRECIPITATION  FORM.—Precipitation  form is the state that the moisture is in: liquid, freezing, or frozen.  Liquid  precipitation  is any precipitation that falls as a liquid and remains liquid after striking an object, such as the earth’s surface or the skin of an aircraft. Rain and drizzle are the only two types of liquid  precipitation. Freezing precipitation is any precipitation that falls as a liquid and freezes upon contact with an object, such as freezing rain or freezing drizzle. In this form of precipitation, the liquid water may be a super-cooled liquid and freeze upon contact with an object, or the water droplet may have an above freezing temperature and freeze upon contact with an object that has a temperature below freezing. (Super-cooled liquids have  a  temperature  below  their  normal  freezing temperature, but still exist in the liquid state.) Small freezing  drizzle  particles  form  a  milky  white  ice coating, typically referred to as rime ice, especially on aircraft in flight. Larger freezing drizzle and freezing rain drops form a transparent ice coating known as clear ice on aircraft in flight or as glaze ice on the ground, power lines, or trees. Frozen precipitation is any precipitation of water that falls in its solid state, such as snow, hail, or ice pellets. Different forms of precipitation may occur together, such as mixed rain and snow; but such an occurrence is simply a mixture of forms, not a separate form of precipitation. PRECIPITATION   TYPE.—Precipitation type is the  term  used  to  identify  various  precipitation. Discussion  of  the  types  of  precipitation  follows: Rain—Liquid  precipitation  that  has  a  water droplet diameter of 0.02 inch (0.5 mm) or larger. If the water droplets freeze upon contact with a surface, the phenomenon is called freezing rain. Drizzle—Liquid precipitation that consists of very small and uniformly dispersed droplets of liquid water that appear to "float" while following air currents. Drizzle usually falls from low stratus clouds and is frequently accompanied by fog. A slow rate of fall and the small size of the droplets (less than 0.02 inch) 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—Precipitation  that  consists  of  white  or translucent ice crystals.    In their pure form, the ice crystals are highly complex, hexagonal, branched structures. Snow falls as a combination of individual crystals, fragments of crystals, or clusters of crystals. Warmer conditions tend to favor larger crystal sizes and clusters  of  crystals.    Snow  must  form  in  cloud temperatures below freezing, though it may fall through air at above freezing temperatures for a short period of time before melting. Snow Pellets/Small Hail—White, opaque, round (or  occasionally  conical)  kernels  of  snowlike consistency, 0.08 to 0.2 inch in diameter. They are crisp, easily compressible, and may rebound or burst upon striking a hard surface. Snow pellets occur almost exclusively in snow showers. Snow Grains—Very  small,  white,  opaque  grains of ice similar in crystal structure to snow. Whereas the crystal structure of snow has very fine, needlelike branches, the crystal structure of snow grains has thicker, denser elements, with the space between hexagonal branched commonly completely filled. Snow grains do not bounce or shatter on hard surfaces. They usually fall in small quantities, mostly from stratus clouds and never as showers. Ice Pellets—Transparent or translucent particles of ice that are either round or irregular (rarely conical) and have a diameter of 0.2 inch or less. They usually rebound upon striking hard surfaces and make a sound upon  impact.  The  term  ice  pellets  describes  two different types of similar looking solid precipitation. One type is composed of hard grains of ice formed from freezing rain or the refreezing of melted snowflakes. It falls as continuous precipitation and is sometimes referred to as sleet. Another type is composed of pellets of snow encased in a thin layer of ice.  It is formed from the freezing of water droplets intercepted by snow pellets or by the refreezing of a partially melted snow pellet. This type falls as showery precipitation and is usually  associated  with  thunderstorms. Hail—A clear to opaque ball of hard ice, ranging in diameter from 1/8 inch or so to 5 inches or larger. Hailstone size is measured and reported in inches, but hailstones  are  usually  compared  to  common  objects when reported to the public by television or radio, such as pea size, walnut size, golf-ball size, baseball size, or softball size. Hail  frequently  displays  a  layered appearance of alternate opaque and clear ice. It is 1-33