INDUCED PRECIPITATION 295 
(a2) that the silver iodide is being emitted in an active form; (%) 
that it diffuses in such a way as to enter appropriate cloud sys- 
tems; (c) that it does not suffer a high decay rate on exposure to 
the atmosphere. 
Influence of Meteoric Dust on Rainfall 
During the latter part of 1954 and early 1955 some new results 
were obtained from an unexpected direction which may have a 
crucial bearing on the whole problem of artificial stimulation of 
rainfall. 
It has become almost an axiom of meteorology that if in any 
given month the values of a meteorological quantity are totaled 
for an adequate number of years and over an adequate area, they 
Be 
+- 
c 
§ 3 NANA 
€c 
2 
‘4 
¢ 
3 I 
cw) 
10 20 3) 
Janvary 
Figure 4. The cloudiness measured at 9 A.M. in Sydney for each day 
in January over the period 1900-53. 
will average out and give a mean which (apart from seasonal 
trends) does not vary substantially from one day to the next. A 
good example of this is the cloudiness of Sydney for the period 
1900-53 for each day in January which is shown in Figure 4. 
This has a mean value of 0.33 with variations about this value 
of not more than a few per cent. 
If the daily rainfall of Sydney for the month of January and 
the first few days of February is plotted in the same way, the 
day-to-day variations are very much greater and in some cases 
show departures of 2 to I on one day as compared with another. 
In Figure 5 is given the daily rainfall totals for Sydney for the 
period 1859-1952, showing distinct maxima on January 13, 22, 
and February 1. 
