2. PRECIPITATION MODIFICATION 



Artificial Alteration of Natural Precipitation 



The scientific basis of all efforts to 

 modify precipitation artificially rests 

 on manipulating the rates of reaction 

 of natural precipitation mechanisms 

 Our qualitative understanding of nat- 

 ural precipitation mechanisms is in 

 rather good shape. (See Figure VI-2) 

 But our knowledge of the quantitative 



aspects of these processes is generally 

 quite poor. There are several reasons 

 for this state of affairs: 



1. The process rate coefficients are 

 inadequately known. 



2. Several of the processes are 

 competitive, so that small initial 



differences may give one of 

 them an ever widening advan- 

 tage. 



3. The initial and boundary condi- 

 tions are known to be important 

 but are poorly understood and 

 difficult to measure. 



Figure VI-2 — PRECIPITATION PROCESSES 



CONTINENTAL NUCLEI 

 WATER VAPOR 



nucleation 



condensation 



ICE NUCLEI 

 WATER VAPOR 



nucleation 

 deposition 



MARITIME NUCLEI 

 WATER VAPOR 



CIRRUS 

 SEEDING 



NARROW CLOUD SPECTRA 



slow broadening 

 by coalescence 



heterogeneous 

 freezing 



SECONDARY 

 ICE PARTICLES 



SECONDARY 

 ICE PARTICLES 



ICE CRYSTALS - 



nucleation 

 condensation 



BROAD CLOUD SPECTRA 



vapor 



deposition 



i 



SNOW CRYSTALS—- 



clumping 

 SNOWFLAKES 



heterogeneous 

 freezing "" 



_ FROZEN DROPS J X 

 ICE PELLETS 



coalescence 



t 



DRIZZLE 



riming 



clumping 



SECONDARY 

 ICE PARTICLES 



RIMED CRYSTALS 

 RIMED FLAKES 



continued 

 coalescence 



riming 



t 



GRAUPELS 



heterogeneous 

 freezing 



wet and dry 



riming with 



drops and 



crystals 



partial 

 melting 



I 



BRIGHT BAND 



RAIN SLEET 



(WARM) 



GRAUPELS 



HAIL 



SNOW 

 GRAINS 



melting 



RAIN 



RIMED FLAKES 

 SNOW PELLETS 

 GRAUPELS ] 



* 



^ partial 



melting 

 I 



BRIGHT BAND 



continued 

 coalescence 



SNOW 



■ melting ■ 



GRAUPELS RAIN 

 SNOW PELLETS 

 SMALL HAIL 



^ heterogeneous . 

 freezing 



SLEET 



RAIN 

 (WARM) 



In this flow chart, the precipitation process is seen to begin with water vapor and 

 one of several different types of nuclei. Through various processes, the nuclei 

 obtain vapor and grow. The final form of the precipitation depends on the environ- 

 ment through which the precipitation falls. The various forms of precipitation that 

 are observed in nature are listed at the bottom of the chart. By tracing their path 

 upward through the chart, it is possible to determine the conditions necessary for 

 their production. 



169 



