The Structure of Minute Precipitation 
JOHANNES GRUNOW 
Deutscher Wetterdienst, Meteorological Observatory Hohenpeissenberg, Germany 
Abstract—Continuous measurements with a raindrop recorder over a period of four 
years were used to study the unknown range of diminutive and minute amounts of pre- 
cipitation. Nearly half of individual rainfalls belong to this class. Frequency distribu- 
tions of amount, droplet size, and intensity in logarithmic scales are presented. 
Introduction—The range of minute precipita- 
tion below 0.2 mm amount is still nearly un- 
known. With regard to climatological or hydro- 
logical applications this range is without interest. 
But in biological respects and above all in precipi- 
tation physics this range has a practical and the- 
oretical importance. The limiting value of 0.2 mm 
is used because such an amount of precipitation 
is necessary to wet the rain gage before the first 
drop is recovered out of the sampling pot. The 
measurement of minute precipitation is only pos- 
sible by continuous raindrop recordings as also 
suggested by Bowen and Davidson [1951] with 
a raindrop spectograph and, in intervals of some 
minutes, by Blanchard [1953, 1957] and Lamp 
[1958]. The Hohenpeissenberg device (Fig. 1) is 
of a similar principle: the raindrops fall through 
a2 10 = 20 em’ opening on a prepared paper 
band. An electric synchron-motor transports the 
band with a velocity of 2, 5, or 10 cm/min. 
After exposure the paper is dried by a resistance- 
wire heater below the table which guides the 
paper band. The role of paper has a length of 
100 m, ample for a continuous record of 3Y% 
days. Spot records of raindrops impart an in- 
structive insight into the structure of rain, both 
droplet size, intensity, duration, and amount. 
It is laborious to measure many thousands of 
spot-diameters but there is no other way to this 
day to record all these characteristics simulta- 
neously. For the range of minute precipitation it 
seems to be the only method. The evaluations 
related to this range cannot be measured with 
the normal methods of precipitation-measure- 
ment. 
Frequency distribution of precipitation 
amounts—Comparing the records of a normal 
rain recorder with the raindrop recorder more 
than 40% of each single rainfall was not meas- 
ured by the usual method. In addition to the 
frequency distribution of the amount of pre- 
104 
cipitation derived by the records of a Hellmann 
recorder, the spectrum is extended in the range 
of diminutive and minute amounts the maxi- 
mum of which is reduced to an amount of 0.03 
to 0.04 mm (Fig. 2). The dotted lines take the 
wetting and evaporating effect within the rain 
gage into account. The presentation of this fre- 
quency distribution is only possible by using a 
logarithmic scale of the frequency ranges as sug- 
gested for meteorological, particularly precipita- 
tion, analyses by Guss [1955], Schneider-Carius 
[1957] and Hssenwanger [1960]. 
The frequency distribution is made up by 
part collectives the exact analysis of which would 
be possible with the procedure of Hssenwanger 
{1954, 1957]. The part collective of Class 7 
(1.6-2.5 mm) for instance is found on the thun- 
derstorm showers in May and June. The insta- 
bility in August favors the part collective of the 
range of 0.3-0.6 mm. The assumption that the 
minute precipitation amounts are originated only 
by drizzle or fine rain is not right. The most evi- 
dent part collective of 0.03 mm results from in- 
stability showers in June, July, and August, while 
there are scarcely months of transition in this 
class. There are drop-showers coming out of con- 
vection cloudiness, but also the fringe areas of 
more heavy showers at nearby localities. 
Duration—The duration of precipitation (Fig. 
3) ascertained with a normal precipitation re- 
corder is found with the frequency maximum at 
50 to 200 min, those ascertained with raindrop 
recorder at 3 to 30 min. Therefore the frequency 
maximum of duration for all single rainfalls shifts 
to the range of 40-76 min. The relatively high 
quota of minute precipitation with a longer dura- 
tion originates in certain weather situations: 
on the edge of a high pressure area, where air 
masses of different characteristics are adjoining, 
stabilizing weather in the rear of a depression and 
drizzle within moist-warm air together with a 
shght upslope circulation along the Alps. 
