Table 1-4.—Precipitation Intensity Indicators TYPE Rain, Snow Freezing rain Drizzle Rain, Snow Freezing rain Ice pellets Drizzle, Snow grains, Snow pellets and snow Rain Rain Rain LIGHT MODERATE HEAVY trace to 0.10" 0.11" to 0.30" >.30" ACCUMULATION trace to 2.5 mm 2.8 mm to 7.6 mm >7.6 mm PER HOUR trace to 0.01" >0.01" to 0.02" >0.02” trace to 0.3 mm >0.3 mm to 0.5 mm >0.5 mm trace to 0.01" >0.01" to 0.03" >0.03” ACCUMULATION trace to 0.3 mm >0.3 mm to 0.8 mm >0.8 mm PER 6 MINUTES Little/none Slow Rapid ACCUMULATION 2 5/8 mi 5/16 to £ 1/2 mi £ 1/4 mi VISIBILITY 2 0.55 nm 0.25 to 0.50 nm £ 0.20 nm 2 1,000 m 500 to 1000 m £ 400 m Easily seen Not easily seen Unidentifiable DROPLET Rain in sheets IDENTIFICATION Hardly noticeable Noticeable Heavy, several SPRAY OVER inches high HARD SURFACES Forms slowly Forms rapidly Forms very rapidly PUDDLES After rain droplets and/or ice crystals form, growth in size is aided by the process of accretion—the fusing together of small droplets that collide. Droplets may also collide with ice crystals and freeze upon contact to make a larger crystal. Turbulence within a cloud may increase  the  rate  of  accretion,  while  strong  updrafts within  cumulus  clouds  may  keep  the  crystals  or droplets,  which  are  continuously  increasing  in  size, suspended for longer periods of time and allow the growth of very large drops or crystals.  Chapter 9 of the text  Meteorology  Today  contains  more  detailed information on precipitation processes. Wind-blown  Forms A  few  reportable  hydrometeors  are  simply  moisture picked up from the ground or ocean surface and carried by the wind. Blowing and drifting snow, dust, sand, and blowing spray are hydrometeors of this group. BLOWING   SNOW/DUST/SAND.—The hydrometeor blowing snow, dust, or sand exists only when strong winds lift snow, dust or sand from the surface to a height of 6 feet or greater, and the snow, dust, or sand reduces the visibility to less than 7 miles. LOW DRIFTING SNOW/DUST/SAND.—This phenomena exists only when strong winds lift snow, dust, or sand from the surface to less than 6 feet, and the snow, dust, or sand does not reduce the visibility below 7 miles. BLOWING  SPRAY.—A hydrometeor that occurs only in very high winds, where water is lifted from the ocean by the wind and reduces visibility at eye level to 6 nautical miles or less. ELECTROMETEORS Our discussion includes lightning and auroras, which are the only significant electrometeors. Lightning Lightning  is  the  most  frequently  observed electrometeor.  This  massive  electrical  discharge  from rapidly  growing  cumuliform  clouds  is  a  very  dangerous phenomenon that kills an average of 85 people per year, injures hundreds of people per year, and causes property damage in the millions of dollars. For example, in 1989 over $72 million in civilian property damage was caused directly by lightning. 1-35