OPERATION AND RESULTS OF ‘PROJECT PLUVIUS’ 15% 
within a small and shallow cloud cap there is no 
possibility of getting warm-cloud precipitation 
release, because the lifetime of the droplets will 
be too small. Therefore, I suggest the following 
explanation in such cases as this. The cloud base 
was either low from the very beginning, or the 
air below the Nimbostratus base was moistened 
through these 100 hours of rain. Then, orographic 
cloud caps with more or less intense condensa- 
tion, colloidally stable in themselves, formed over 
those regions that were higher and clad with 
forest, because of the great increase in friction 
and its stemming effect on the flow. At last, the 
general rain from the frontal cloud system swept 
down through the cloud caps and ascertained a 
very good release of precipitation within them. 
For the next project, July—-October 1954, we 
selected an area between the great Lake Vinern 
and Lake Vittern in southwestern Sweden partly 
shown in Figures 4 and 5, representing an inter- 
esting orography (plains, dislocations, forests, 
shore lines, ete.). Here we had 750 gages; in ad- 
dition in 1955 we had 150 stations in the central 
part of the region measuring the wind direction 
and velocity (the latter with a cheap pressure- 
tube hand anemometer). The two rainfall maps 
RAINFALL mm. One day: 27. Vil. 06-18z, 1954. 
ou 
1954 (cold- 
front) and for August 7, 1954 (warm-front), both 
with a general current from south-southeast. In 
the cold-front case, obviously, orography has had 
very little effect on the distribution; the expla- 
nation evidently being that the cloud base was 
high, since it was summer, and the stratification 
chosen as examples are for July 27, 
was unstable. Thus, no low feeder clouds would 
form above the low hills or plateaus of the area, 
and there were, instead, convective cells moving 
with the direction of the gradient wind, giving 
streaks of precipitation parallel with that di- 
rection. 
Now I may remind you of the Figure 23 of 
my first lecture (see T. Bergeron, ‘Problems and 
Methods of Rainfall Investigation,” p. 28, this 
volume, 1960), where I underlined that two 
main patterns may be superimposed on most 
precipitation distributions, owing to two main 
mechanisms, the convective cell mechanism, and 
the stationary lee-wave mechanism. Figure 4 1s 
an example of the Case 1, and Figure 5 of 
Case 2 with alternating maxima and minima. 
There is even a minimum occurring where you 
would expect a maximum because of the passage 
of the air over the table mountain and so on. 
e Rainfall station 
=== Shore -line 
“133m alt. (400 ft.) 
VATTERN 
SROs 
N 
7B. 1954 
Fic. 4—Instability pattern of rainfall (cold-front with SSE wind aloft) shown by a meso-scale network 
of Pluvius gages in a region with plains and plateau hills between the two great lakes of southern Sweden 
