HANS-WALTER GEORGII 
400 
350 
300 
250) 
200 
150 
100 
1956 
— Monthlyaverage 
© radiation days 
7 days with showers 
| 
. days with continous rain 
= ground fog in evaporation 
= days with high wind speed 
in 700 mb level 
1957 
Fic. 1—Influence of certain weather-conditions on the freezing nucleus concentration at Frankfurt 
am Main 
active above —30°C. Figure 1 shows the aver- 
age monthly freezing nucleus concentration com- 
pared with the concentration on such days when 
a certain weather situation prevailed. It can be 
seen in Figure 1 that the concentration of freez- 
ing nuclei is above average: (a) During and af- 
ter showers as well as at the time of a strong 
vertical mixing. The shower activity causes a 
down draft of a great number of highly active 
freezing nuclei into the air layers close to the 
ground, especially of such particles which have 
already participated in the formation of the 
shower in higher levels of the atmosphere. Some- 
times, such ice particles still preserve an ice-like 
structure on their surface which enables them to 
become effective once again at temperatures only 
slightly below the freezing point (preactivated 
or ‘trained’ nuclei). (b) During evaporation of 
fog droplets. Very high values of freezing nuclei 
were always found on days with ground fog, es- 
pecially when the fog was in a state of evapora- 
tion. The activation of the nuclei may be ex- 
plamed in connection with drying of their 
surface. Possibly electric phenomena at the sur- 
face of the nuclei occur in connection with the 
evaporation which may also be partly responsi- 
ble for the increased freezing activity [M/viihlei- 
sen, 1958]. (c) When there are high wind speeds 
in high altitudes. The strong upwinds are mostly 
connected with high wind velocities near the 
ground thus removing the air pollution in the 
Rhein-Main basin. 
The concentration of freezing nuclei in the 
ground-layer of the atmosphere is remarkably 
low: (a) On summer days with intensive solar 
radiation. The convection causes an up-current 
of the aerosol particles while the air sinking 
down by continuity reasons has a small aerosol 
content. (b) On days with continuous precipita- 
tion leading to a wash-out of freezing nuclei. 
The influence of the different weather ele- 
ments on the concentration of freezing nuclei 
active above —30°C is summarized in Figure 2. 
The annual average value of the 308 measure- 
ments carried out from May 1956 to May 1957 
at Frankfurt was used as 100%. The other aver- 
ages refer to this value subject to the above de- 
scribed meteorological phenomena taking place. 
It can be recognized that on days with evaporat- 
ing fog, showers, and advection of fresh air 
masses in higher levels the concentration of 
freezing nuclei was higher than normal and 
amounted respectively to 138, 127 and 109% of 
the annual average. On the other hand the con- 
centration on days with rain and on days with 
calm radiation weather reaches respectively only 
