DISCUSSION 25 
17 show a convergence along the southern edge 
of the extended rain area, between the N flow 
descending from the divide near 63°N and the 
WSW flow to the south of the convergence line. 
The air ascending at this line is carried towards 
NW by the predominant SE flow aloft (see Fig. 
19). This may explain why the main part of the 
rain area lies to the north of the convergence of 
the flow in the lowest layers. The dynamic 
trough normally forming to the lee of a moun- 
tain range will favor a frictional inflow at the 
Earth’s surface in this region. Usually, however, 
this inflow will not be powerful enough to cause 
and sustain any large-scale lifting of this very 
dry ‘fohn’ air, since this air will never reach the 
condensation level, the lifting cooling thus being 
dry-adiabatic. In the case studied here, however, 
the air in the frictional layer had been moistened 
by the weak but steady rain from the overhang- 
ing Altostratus-Nimbostratus sheet behind the 
stationary front. When the condensation level 
within this postfrontal mP became low enough, 
the tendency to a general convergence at low 
levels would favor an orderly overturning within 
this conditionally unstable cold mass, leading to 
the formation of the marked convergence line 
and rain area in the trough. A scrutiny of the 
individual vorticity changes in this stationary 
trough confirms the assumed existence of a 
marked convergence within it. 
Figure 18 displays the ensuing precipitation 
distribution with up to SO millimeters in the 
region of the downward current at the Earth’s 
surface. The area of great precipitation extends 
even over eastern Norway. This case is only 
partly analogous to the Ohio case, but it shows 
that there are precipitation systems that have 
not yet been explained, and occurring where one 
would not expect them according to conventional 
theories. They may have been of orographic or 
frontal origin, but they are now simply condi- 
tioned by an orderly convergence and lifting, 
which is of extreme importance because it may 
give so much precipitation. A cross section from 
northern to southern Sweden (Fig. 19) shows the 
down-flow of the northerly wind, the converg- 
ence, and the resulting lifting, producing this 
very astonishing precipitation where there is no 
front and where one would expect a warm and 
dry fohn instead. 
Last, I want to show that I think that there 
are two main precipitative effects superimposed 
on all different kinds of precipitation: those of 
the wavy kind and those of the convective kind, 
or stationary lee-wave precipitation and con- 
vective streaks of precipitation. Figure 20 shows 
the precipitation distribution in Holland meas- 
ured in the morning of October 26, 1945, after 
24 hr with a west southwest gradient wind. It 
shows a succession of rainfall maxima and min- 
ima from W to E over Holland. Excepting the 
dunes, northwestern Holland is absolutely flat, 
lying between one and five meters below sea 
level. Thus, it must be the difference of friction 
between sea and land that causes the first maxi- 
mum, Just above the dunes, and I presume that 
then a series of stationary lee waves is set up at 
the level of low clouds, as shown by Figure 21. 
One day later, on October 27 (Fig. 22), when 
the gradient wind had hardly changed at all, but 
the static stability had changed into instabil- 
ity, the stationary lee-wave system broke down. 
Instead, there were convective streaks of pre- 
cipitation more or less parallel with the wind, 
again reaching great amounts. In the first case 
a warm front passed Holland; the next day a 
back-bent occlusion passed over the country. 
Figure 23 shows the change that took place in 
the upper air during these two days. Figure 23a 
is the 500-mb map at the middle of the first pre- 
cipitation day, with WSW wind aloft and —18 
to —20°C at this level over Holland. The next 
morning (Fig. 23b), still with the same wind, the 
temperature at 500-mb had sunk to —28°C, 
this being the reason why the stability pattern 
had broken down. I presume that these two pat- 
terns will be more or less superimposed on all 
other precipitation mechanisms. 
Discussion 
Dr. Helmut Weickmann—Thank you, Doctor. 
It has been a very interesting lecture, which felt 
like a cool breeze after the heat of many years 
in which it was tried to increase rain by increas- 
ing the number of nuclei. It showed very con- 
clusively that the updraft is of prime importance 
in the generation of rain. 
Dr. Horace R. Byers—In your discussion of 
the ascent of air in tropical hurricanes, you said 
something about the augmentation of the tem- 
