140 3 (20 ra sie} 100 eo (o) x | . / ZO WIA Petes Height above ground (10° ft) cz Clo WZ 0 5 25 10 15 20 Time to fall to ground (hr) Fig. 5. Time Required for Spherical Particles Having a 2.5 Density and a Certain Diameter to Fall from a Given Altitude to the Ground.