produced only in thunderstorms, but may be ejected from the top or sides of a thunderstorm to fall and strike the ground without a cumulonimbus cloud directly overhead. Ice  crystals—Tiny unbranched crystals of ice in the form of needles, hexagonal columns, or plates. They are often so small that they may be suspended in air and are sometimes referred to as diamond dust. Ice crystals are visible mainly when they glitter in the sunlight or in spotlights at night. Although common in polar regions, this phenomenon occurs only during very cold temperatures in stable air masses. Ice crystals may fall from any type of cloud or from clear air. As moist air cools below -40°F, the water vapor may sublime directly to form ice crystals, and precipitate, without ever forming a cloud. PRECIPITATION INTENSITY.—Precipita- tion intensity is an approximation of the rate of fall or the  rate  of  accumulation  of  precipitation.  During  an observation,  intensity  for  each  type  of  precipitation (other than hail and ice crystals) must be determined. NAVMETOCCOMINST  3141.2  and  NAVMET- OCCOMINST 3144.1 provide valuable information on determining intensity by visibility, accumulation rate, size of the rain drops, sound on the roof, height of splashes, and the rate at which puddles form. The primary indicator for snow and drizzle is visibility. Table 1-4 summarizes the indicators to aid in your understanding of the term precipitation intensity. Direct   observation   is   the   best   method   of determining  the  type  of  precipitation  occurring. However, in chapter 2, you will study observation equipment   that   provides   valuable   indicators   of precipitation  intensity. PRECIPITATION CHARACTER.—Precipita- tion  character  is  a  term  used  to  describe  how precipitation  falls.  Three  terms  are  used  to  describe character:  continuous, intermittent,  and  showery. The term  continuous  precipitation  means  that  the precipitation falls for a long period of time over a specific   area.   When   the   system   producing   the precipitation is moving, use of the term implies that the area covered by the rain is extensive. Continuous precipitation  falls  from  stratiform  clouds,  especially nimbostratus. Continuous  precipitation  changes intensity only slowly, and may be of light or moderate intensity,  rarely  heavy.  When  used  alone,  the  terms rain, drizzle, and snow refer to either continuous or intermittent  precipitation. The term  intermittent precipitation  is used to describe precipitation that occurs for brief periods of time  (lasting  less  than  1  hour).  Intermittent precipitation changes intensity slowly, and is usually light. Although the overall area affected by intermittent precipitation is usually very large, at any given time only  a  portion  of  the  area  is  actually  receiving precipitation.   Like   continuous   precipitation, intermittent precipitation usually falls from stratiform clouds,  especially  nimbostratus. Sudden  starting  or  stopping  ofprecipitation  or  rapid changes  in  the  intensity  of  precipitation  indicate showery  precipitation. Showery precipitation, or showers,  fall  from  cumuliform  clouds,  especially cumulonimbus. Showers cover only a relatively small area at a given time, and, unless the cumuliform cloud is stationary, showers last only a brief time before moving on. Rain falling from cumuliform clouds is called a "rain shower," and a cumuliform shower of snow is called a "snow shower." The public popularly calls a very light snow shower "snow flurries." PRECIPITATION  THEORY.—Several   valid theories have been formulated in regard to the growth of raindrops. The theories most widely accepted today are treated here in a combined form. It is believed that most precipitation in the mid- and high-latitudes starts as ice crystals. The crystals melt and fall as liquid precipitation only when it passes through an above-freezing stratum of air. Due to the low freezing level in these regions, the abundance of water vapor in the atmosphere is found at, near, or below freezing temperatures. In clouds below freezing temperatures, water coexists in all three states: solid, liquid, and gas. Both  solid  and  liquid  particles  are present within most clouds. The higher vapor pressure for the liquid droplets compared to the low vapor pressure for the solid ice crystals tends to cause a net evaporation of gaseous water from the liquid droplets. In turn, there is a corresponding net sublimation of the gaseous vapor on to the solid ice crystals. This tends to retard the growth of the liquid drops while aiding the growth of the crystals. When the ice crystals become too large (too heavy) to remain suspended in the atmosphere, they fall as precipitation. In the low-latitudes (tropics) and much of the mid- latitudes during the warmer months, the freezing level in  the  atmosphere  is  generally  much  higher.  The abundance of moisture in the above freezing portions of the  lower  atmosphere  allows  the  majority  of  the precipitation to form initially as liquid water droplets. 1-34