Water droplets of varying sizes, released through an atomizer, were collected on glass slides coated with uniform layers of magnesium oxide or carbon soot and silicone oil. Assuming that the droplets retain their original shapes in the oil film, calibrations were obtained for their spreading on oxide and soot layers of known thickness. The calibrations have been further applied to evaluate droplet size distributions of ground-based clouds

Kapoor, RK

Paul, SK

Sharma, SK

Ministry of Earth Sciences, Government of India

Jour. Ind. Tnst Sc. 61 (B), June 1979, pp. 135-141 
@ Printed in India 
Short Communication 
Calibration for measurements of droplet size distributions of ground- 
based clouds-A laboratory investigation 
S. K. PAUL, S. K.  SHARMA AND R. K.  KAPOOR 
Indian Institute of Tropical Meteorology, Ramdurg House, University Road, Poona 411 005, India 
Received on August 21, 1978; Revised on February 19, 1979 
Absh'act 
Water droplets of varying sizes, released through an atomizer, were collected on glass slides coated with 
uniform layers of magnesium oxide or carbon soot and silicone oil. Assuming that the droplets retain 
their original shapes in the oil film, calibrations were obtained for their spreading on oxide and soot 
layers of known thickness. The calibrations have been further applied to evaluate droplet size dislri- 
butions of ground-based clouds. 
Key words : Calibration, cloud droplet, atomizer, magnesium oxide, soot, silicone oil 
1. Introduction 
A study of cloud microphysics is important for a cloud physicist for enhancing krowicd~r  
in the field. Besides, such a s t u d j  is equally valuable in weather modification for impiov- 
ing seeding technology. 
The techniques being emplayed for collecting droplets consist of exposing slides coated 
with suitable materials to  clouds for definite periods. From craters fafolmed by the 
impinging droplets, actual sizes are evaluated using calibrations obtained in the labo- 
ratory. May1 obtained a nearly constant ratio, 0.86, of water droplet to crater size in 
a magnesium oxide layer for droplets up to 200 ,urn diameter. He did not carry out 
calibration below IO,,um. May's calibration has been extensively used for studying the 
microphysics of fogs and clouds (Frith2 ; Squiresand Gillespie3: Srivastava and Kapoor' ; 
Gathmans ; Kapoar et UP). The calibration, as obtained by Squires', in a thin carbon 
film has been extensively used a t  aircraft speeds (Warner8," G ~ o d m a n ~ ~ ) .  
The authors made an attempt to secure calibration, for use a t  low impact velocities 
and applicable to  cover droplets below IOpm also, using uniform layers of oxide and 
soot of kn0n.n thickness. They also studied the effect of coating thickness on crater 
diameter corresponding to a given droplet. Droplet size distributions of ground-based 
clozds, s~mpled  a t  a hill station, are presented using the calibrations obtained by the 
authors. 
2. Investigations by the authors 
Water droplet spray released by flame photometer in the laboratory, a t  9 litres per 
minute, was collected on glass slides kept normal to the stream of droplets. Each slide 
was hllf-costed with a leyer of either magnesium oxide or carbon soot of known thick- 
ness. The other hslf wzn cozted with a thin layer of silicone oil which served as the 
control (amm~ing that there is no spreading in the oil). Slides were exposed a t  distances 
S, 45 and 90 cm away from the atomizer nozzle where the mean flow rates were 1.5, 1.0 
and 0 - 5  metres per second respectively. Calibrations were made for thin, moderate and 
flick layers of oxide and also for a moderate layer of soot. For magnesium oxide, the 
moderate layer was about twice as thick as the thin layer while the thick layer was about 
thrice the thin layer. The thin laytr corresponded to an  average thickness of aboa: 
9 p n .  
Droplets in the oil film as well as  craters in the oxide/soot layer were scanned using 
a higk power optical microscope a t  400 x magnification. The droplets and the craters 
were cl.tegorised into seven classes depending upon minimum and maximum sizes obtained 
in e x h  cste (Tables I and 11). From the fifth class, the number of dropletsjcraters was 
quite less and so wider intervals were chosen for the last three classes. The intervzls 
within fifth to seventh classes are 1.75 times those within first to  fifth classes. 
(i) Magnesium oxide layer 
The combined results a t  the mean flow rate of 1 metre per second for different layers 
are tabulated (Table I). Curves showing the average ratio value of droplet to  crater 
size in different layers against the corresponding crater diameter are presented in Fig. I. 
The ratio values in all classes but the first one are nearly constant. As we proceed from 
thin to thick through moderate layer, this ratio value increases in all  the categories, 
suggesting thrreby that the spreading factor is quite sensitive t o  the layer thicknes? : the 
thicker the layer, the less is the spreading. Also, the ratia values are slightly less for 
larger droplets. The mean values of the ratios were 0.44,0.58, 0.71 for thin, moderate 
and thick layers respectively. The curves for different layers are nearly parallel (Fig. 1). 
(ii) Carbon soot 
Results for the mean flow rate for the moderate layer are given in Table 11. The ratio 
values are plotted in Fig. 1. The mean .ialue of the ratios was 0.55.  The ratio 
C,\I.Il~RI\TIO\' FOR CLOUI' DROPLEr SIZE OISTRIHL'IION 
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CALliillATION FOR CLOUI) DROPLET SIZE DISTRlWlJTlON 
i b 3 L  I 1-l 
0 4 B I2 16 20 ZT 18 32 36 40 14 48 
CLOUD DROPLET D I A M E T E R  l a m >  
FIG. 1. Crater diameter seriur droplet to crater size ratio in oxide and soot layers (mean impact "do- 
city : I m se~-~) .  
values in different cla5res and the mean valne are nearly identical to those for the 
model-ate oxide layer. 
3. Field investigations 
A programme for studying rnicrostn~cture af ground-bascd clouds was undertaken for 
two days, i.e., on September 13 and 11, 1975, a t  Lmavala, a hill station, on the windward 
side of Wcstern Ghats. Slides of 1 cm width, half coated with magnesium oxide and 
hnlf coated with carbon soot layers o r  maderate thickltess were used for collecting drop- 
lets. The total number of clouds sampled i~ 21 for the oxide layer and 18 for  the 
carbon soot. 
The droplet size distribution was determined by measuring the crater sires on the 
exposed slides under an 'optical microscope. From craters, droplet diameters were 
evaluated using calibrations presented in Fig. 1. The overage wind velocity (Im sec l )  
110 S .  K ,  PAUL et 01 
durins sampling periods was about the same as theimpactvelocity for the calibraeo, 
curves. True droplet concentrations were obtained by app1)ing corrections for collec. 
tiou efiiciencies of the slide for droplets of various sizes (Ranz and Wongll). 7he mean 
droplet size distributions for the two days taken together, in the oxide and soot lagers 
are shown in Fig. 3. The droplet concentrations have been plotted a t  intervals of 
3,um. From the mean droplet size distributions, cloud liquid water content and median 
volume diameter of the drops were computed. The values of total droplet concentra. 
tion, liquid water content and median volume diameter are respectively, 133 cm-3, 0.07: 
g mn-J, I l . 8 p m  for the oxide layer and 93 cnl-', 0.030 g I T 3 ,  11.3pm for the soot layer. 
---e--- Maderote layer Soot 
----rt-- Thrn toyer Mgo 
-r- Moderate layer Mgo 
C R A T E R  ' D I A M E T E R  l p m i  
FIG. 2. Mean droplet size distributions of gtound-based clouds. 
It is seen that the pattern of variation of cloud droplet size distribution is similar in 
both oxide and carbon soot. The maximum size encountered in each case is limited to 
about 5 0 ~ .  Both the distributions are unimodal with mode a t  about 8 ~ m .  Siaiia- 
rity of distributions and comp~rable values of various microphysical parameters SU@ 
that the presen; calibrations can be applied for such studies. 
CALIBRATION FOR CLOUD DROPLET SIZE DISTRIBIJTIOX 141 
4. Conclusion 
Calibrations o f  spresding factors of  water droplets o f  various sizes, impinging a t  a low 
on oxide a n d  soot  layers af measured thickness, have been obtained in the labo- 
ratory and their applicstions t o  field studies a re  dacumented. 
5. Acknowledgements 
The authors are  greatly indebted t o  Dr.  Bh. V. Ramana Murty for  his valuable guidance 
during the course of this work. They are also thankful to  Shri P. M. Bhanage for 
:!.ping the manuscript o f  this paper. 
References 
The measurement of airborne droplets by the magnesium oxide 
method, J. Scie~zt. Ind~wn., 1950, 27, 128-130. 
The size of cloud particles in stratocumulus cloud, Qriwt. J. Ro): 
Met. Soc, 1951, 77, 441-444. 
SQU~RES, P. AND A cloud droplet sampler for use on aircraft, Qriart. J. Roy. Met. 
GILLESPIE, C. A. Sot., 1952, 78, 387-393. 
SRIVASTAVA, R. C. AND Dropsize distribution and liquid water in a winter fog at Deihi, 
K A ~ R ,  R. K. Indian 3. Meteor. Geoph)~~., 1960, 11, 157-162. 
GATHMAN, S. G. Airbonie droplet sampler for use in fogs and low-level stratus, 
J. AppL Meteor., 1975, 14, 1293-1296. 
Kawon, R K., PAUL, Measurements of cloud droplet size distributions in seeded warm 
S. K., RAMACHANDRA cumulus clouds, J. Pim mid Appl. Geophys., 1976, 114, 379-392. 
MURW A. S., KRISHNA, K., 
Smnan, S. K. AND 
RAMANA MURTY, Bh. V. 
The microstructure and colloidal stability of warm clouds. Pt. I. 
The relation between structure and stability, Tellrr, 1958, 10, 256- 
261. 
The microstructure of cumulus cloud, Part I. General features 
of the droplet spectrum, J.  AAros. Sci., 1969, 26, 1049-1059. 
The microstructure of cumulus cloud, Part V. Changes in drop- 
let size distribution with cloud age, J. Afmos. Sci., 1973, 30, 1724 
1726. 
GCODMAN, J. The microstructure of California coastal fog and stratus, J. Appl. 
Meteor., 1977, 16 (lo), 10556-1067. 
h ~ z ,  W. E. AND Impaction of dust and smoke particles on surface and body collec- 
WONG, J. B. tors, Zndusir. Engg. Chem. (Indusfr.), 1953, 44, 1371. 


Calibration for measurements of droplet size distributions of ground based clouds - a laboratory investigation

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Calibration for measurements of droplet size distributions of ground based clouds - a laboratory investigation

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