292 THE FUTURE OF ARID LANDS 
Distonce Downwind from Smoke Generotor (km) 
Figure 1. The concentration of active freezing nuclei downwind from 
a silver iodide ground generator. 
Until recently these points had never been investigated and it 
was therefore decided to study them experimentally. Tests have 
now been carried out in Australia by Smith and Heffernan (2) and 
by Smith, Heffernan and Seely (3) with the following results. When 
a silver iodide generator emitting 10'® nuclei per hour is run in 
flat terrain under typical convective conditions, the distribution 
of effective nuclei downwind from the generator is as shown in 
Figure 1. It is seen that the freezing nucleus concentration drops 
to a low level at a distance of 10 to 12 miles downwind and that 
appreciable concentrations do not extend above heights of 2,000 
or 3,000 feet. The rapid decrease in concentration cannot be 
accounted for simply by diffusion of the freezing nuclei and cal- 
culation shows that they must be subject to a rapid decay in 
activity. Figure 2 gives the decay rate for two typical burners, 
showing in one case a decay of 10% times in 30 minutes. 
Bolton and Qureshi (1) conducted a separate investigation to 
arrive at the physical reasons for this decay and found that the 
most important single factor controlling the decay rate is the 
ambient temperature of the atmosphere. The rate of decay is in 
