398 Table 116 



SIZES OF ATMOSPHERIC PARTICLES 



The following data are taken from an article and accompanying chart prepared by 

 Simpson. 1 For spherical particles, diameters are given. 



Rain drops > 2 X 10" 2 cm. 



Limit of unaided vision 5 X 10 -8 cm. 



Cloud particles 2 X 10" 3 - 4 X lO -1 cm. 



Wright's salt nuclei at 80% rel. hum 1 X 10" 4 cm. 



Wave length of visible light 7 X 10" 5 - 4 X 10" 6 cm. 



Smoke particles 1 X 10"* - 1 X 10" 5 cm. 



Lower limit of microscopic vision 2 X 10" 5 cm. 



Ultra-large ions 2 X 10-" - 1 X 10" 6 cm. 



Mean free path of molecules in air at N. T. P. 9 X 10"* cm. 



Langevin ions 1 X 10" 6 - 5 X 10" 6 cm. 



Large medium ions 5 X 10"* — 1.5 X 10~ e cm. 



Small medium ions 1.5 X lO"" - 7 X 10" T cm. 



Lower limit of ultra-microscope 5 X 10"' cm. 



Small ions IX 10" 7 cm. 



Molecules (2 atoms) 3 X 10" 8 cm. 



Findeisen 2 has shown that there is a physical difference between a cloud particle and 

 a rain drop which fixes the boundary with a fair amount of precision, namely, a rain 

 drop must be able to reach the ground without completely evaporating. Findeisen derives 

 a formula which shows that the distance traveled in an atmosphere not completely 

 saturated varies as r 1 and gives the following table : 



Distance of fall before evaporation 

 Pressure 900 mb., temperature 5 °C, relative humidity 90 percent. 



Radius of drop, r Distance of fall before evaporation 



cm. 



lO" 2 150 m. | 



10" 1 42 km. frain drops 



2.5 X 10" 1 280 km. 



1 Simpson, G. C, Quart. Journ. Roy. Meteorol. Soc, vol. 67, p. 99, 1941. 



2 Findeisen, W., Meteorol. Zeitschr., vol. 56, p. 453, 1939. 



Table 117 



EVAPORATION OF FREELY FALLING WATER DROPS 1 



The mass evaporation rate dM/dt is the product of two factors that may be ex- 

 tracted from table 117 A and table 117 B. 



iM. = [ 4na ( 1 + ^)] [D(p*- Pb )] g.sec." 1 



where a is the radius of the drop, .s is the equivalent thickness of the transition shell 

 outside the drop, F is a dimensionless factor, D is the coefficient of diffusion, p a is the 

 saturated vapor density at the surface of the drop and p& is the vapor density of the 

 environment. 



Table 117 A gives the first factor when entered with the droplet diameter 2a and the 

 ambient temperature. 



Table 117 B gives the second factor when entered with the ambient temperature and 

 relative humidity. 



Probable uncertainty, based on precision of measurements, is less than 4 percent. 



1 Kinzer, G. D., and Gunn, Ross, Journ. Meteorol., vol. 8, p. 71, 1951. 



{continued) 



SMITHSONIAN METEOROLOGICAL TABLES 



