A long series of ground-based salt seeding experiments in north India during the southwest monsoonseasons (June-September) of 1957-66 showed significant increases of about 20 percent in rainfall on seeded days (Ramana Murty and Biswas, 1968). In September of 1974, salt seeding from aircraft on isolated maritime warm cumulus clouds within 50 km off the coast at Bombay(18 15’N, 72v 49’D, II m ASL) was followed by increases in radar echo area coverage in the vertical and in echo intensity (Chatterjee et al., 1978). seek similar results in a different area, a randomized salt-seeding experiment, using a single ground-based generator, was conducted in 1973 and 1975-77 just west of Madras, on the Bengal coast 1,000 km southeast of Bombay. There in the state of Tamil Nadu, South India, the main rainy season is during the northeast monsoon(Oct-Dec), whereas in most parts of India, about 75 percent of the annual rainfall is received during the southwest monsoon(Ananthakrishnan, 1977

Kapoor, RK

Murty, ASR

Pillai, AG

Ramanamurty, BhV

Reddy, RS

Selvam, AM

Vijayakumar, R

Ministry of Earth Sciences, Government of India

"REVIEWED"
GROUND-BASED SALT SEEDING IN TAMIL NADU STATE, SOUTH INDIA, 1973-1977
A.G. Pillai, R.S. Reddy, R. Vijayakumar, R.K. Kapoor,
A.S. Ramachandra Murty, A. Mary Selvam and Bh.V. Ramana Murty
Indian Institute of Tropical Meteorology, Poona-411005, India
1. INTRODUCTION
A long series of ground-based salt seeding
experiments in north India during the southwest
monsoon seasons (June-September) of 1957-66 showed
significant increases of about 20 percent in rain-
fall on seeded days (Ramana Murty and Biswas,
1968). In September of 1974, salt seeding from
aircraft on isolated maritime warm cumul~s clouds
within 50 km off the coast at Bombay (18 15’N,
72v 49’D, II m ASL) was followed by increases in
radar echo area coverage in the vertical and in
echo intensity (Chatterjee et al., 1978). 
seek similar results in a different area, a ran-
domized salt-seeding experiment, using a single
ground-based generator, was conducted in 1973 and
1975-77 just west of Madras, on the Bengal coast
1,000 km southeast of Bombay. There in the state
of Tamil Nadu, South India, the main rainy season
is during the northeast monsoon (Oct-Dec), whereas
in most parts of India, about 75 percent of the
annual rainfall is received during the southwest
monsoon (Ananthakrishnan, 1977).
Precipitable water in the Tamil Nadu region
is maximum in July and August, but rainfall is
greatest in October and November. Mean daily
rainfalls are 9.4 mm and 20.0 mm, respectively,
during the southwest and northeast monsoon months.
During the northeast monsoon season some of the
cyclonic disturbances in the south Bay of Bengal
move towards the Tamil Nadu coast and give heavy
rainfall over the coastal belt. Since rainfall in
Tamil Nadu during the southwest monsoon is low
despite the high precipitable water vapour present,
the feasibility of increasing rainfall through
the modification of cloud microphysical conditions
was explored. Aircraft observations made in north
India had indicated fewer than one giant condensa-
tion nucleus per litre at cloud base under certain
meteorological conditions during the southwest
monsoon season (Biswas et al., 1968). Correcting
this deficiency of giant condensation nuclei may
help accelerate rain-formation through collision-
coalescence.
A salt seeding experiment using ground based
generator was undertaken during the years 1973
and 1975-1977, for exploring the feasibility of
increasing rainfall through cloud seeding in the
region. For the type of research described above,
aircraft seeding would be ideal since it ensures
precise targeting of the salt particles into the
clouds and also facilitates to make in-cloud
microphysical observations. However, the prohibi-
tive cost of the aircraft operations has become
the main factor for adopting the ground seeding.
During the above experiment surface measurements
of the concentration of giant (r~ipm) conden-
sation nuclei (GCN) were also made.
The experimental area with it~ network of
about 1 raingauge per 30 to 160 km (Fig. 1) was
about 40 k~ inland (Nest) from the coast 
Madras (13u O0’N, 80v 11’E, 16 m ASL). ~arget
and control areas were 10000 to 12000 km .
During the southwest monsoon season (July-
Sept) at Madras, moderate southwesterly to
westerly winds prevail in the lower troposphere
(Table 1). When the wind is southwesterly 
westerly, airmasses travel over land before
passing over the experimental area and may have
fewer giant condensation nuclei. The average
cloud base height is about 5000 ft. ASL.
During the northeast monsoon season (Oct-
Dec) winds in the lower troposphere are mostly
northeasterly to easterly, and are most gusty -
during December. Skies are overcast on 8 to 12
days per month (Table i). When it is not raining,
low clouds are between 1000-3000 feet ASL in
most cases. Maximum rainfall is received during
November, when maritime airmasses bring giant
condensation nuclei traveling over Bay of Bengal
before crossing the east coast.
2. EXPERIMENTAL DESIGN AND RESULTS
The fixed target-control design was adopted
in the experiment with day randomization. Target
and control areas were delineated by the location
of the seeding generator and the predominant wind
direction in that area. The angula~ spread of
the target area covers the 70 to 90 quadrant
opposite to the mean wind direction, as indicated
by the analysis. The control and target sectors
fixed accordingly for the southwest and northwest
monsoon seasons were:
Southwest (June-Sept)
Northeast (Oct-Dec)
Target Control
50o- 120o 230o - 300°
180o- 270° 0° _ 90o
Correlations between average daily rainfalls
of the target and control areas, for the 5-year
period 1968-72, were 0.69 for the southwest monsoon
and 0.62 for the northeast monsoon seasons. The
criteria for seedable day were based on (i) fore-
cast wind at 1.0 and 1.5 km levels, (ii) actual
or forecast low cloud amount, and (iii) probability
of rain occurrence in the region during the 12
hour period preceding the time of seeding. Days
on which intermittent to continuous rain occurred,
preceding and during the scheduled period of seed-
ing, were not considered as seedable.
On the afternoons (1400-1700 IST) of seed-
able days, randomly selected for seeding, a pul-
verised mixture of salt and soapstone, in the ratio
177
10:1 with particle mode diameter of about 10 pm,
was injected into the atmosphere from a single
ground-based generator at T~ruvallur (Madras).
Seeding at about 300 kg hr- continued for about
3 hours commencing from 1400 IST. ~e seeding
rates adopted should yield about 10 giant size
hygroscopic particles per second. Cloud bases
are about 5000 feet during southwest and 2000
feet during the northeast monsoon seasons. At
25 km downwind particle concentration will be of
the order of 20000 m- (Biswas et al., 1967),
Hence only a fraction of the salt particles
released at the ground should reach the cloud base
levels under favorable meteorological conditions
and influence the microphysical state of the cloud.
After the initial year, 1973, with 20 days
seeded and 16 not, no experiment could be under-
taken during 1974. Thereafter the experiment could
be carried out only sporadically (Table 2) which
ultimately led to its termination in 1977 before
statistically significant results could be
obtained.
During the SOUTHWEST MONSOON (Jun-Sept), rain-
fall on 40 seeded days was compared to that on 34
non-seeded days (Table 2), The result was posi-
tive in 2 seasons and negative in 2 seasons, with
increases of 7 to 355 percent and decreases of
5 to 8 percent (Table 3). The four seasons
showed an increase in rainfall on seeded days of
32 percent, significant at 15 percent level accord-
ing to Mann-Whitney test (Table 4).
During the southwest monsoon, the air has
enough water vapour for cloud formation, but the
windflow in the lower troposphere southesterly
to westerly causes the air masses to travel over
land. Such air masses may contain few of the
hygroscopic nuclei required for rain-formation,
and seeding them with giant condensation nuclei
may help accelerate rain-formation through colli-
sion coalescence process. The surface observa-
tions showed fewer giant condensation nuclei
during the southest monsoon season. Numbers per
litre of giant nuclei were:
2)
3)
A 17 percent rainfall decrease, significant
at 33 percent level, during the northwest
monsoon;
Concentrations of giant condensation nuclei
at the surface higher up to a factor of two
during the northeast monsoon season compared
to the southwest monsoon season.
ACKNOWLEDGEMENTS
The authors express sincere gratitude to the
India Meteorological Department for cooperation
received in the installation and maintenance of
the raingauge network in the experimental area.
REFERENCES
Ananthakrishnan, R., 1977: Some aspects of the
monsoon circulation and monsoon rainfall.
Pure and Applied Geophysics, 115, !209-L249.
Biswas, K. R., R. K. Kapoor, K. K. Kanuga and
Bh. V. Ramana Nutty, 1967: Cloud seeding
experiment using common salt. J. Appl.
Meteor., 6, 914-923.
Biswas, K. R., S. K. Paul and Bh. V. Ramana Murty,
1968: Giant s~ze aerosols in lower tropo-
sphere at De~hi. Proc. International Con-
ference on Cloud Physics, Toronto, Canada,
August 26-30, 1968.
Chatterjee, R. N., A. S. R. Murty, K. Kris~na and
Bh. V. Ramana M~rty, 1978: Radar evaluation
of the effect of salt seeding on warm mari-
time cumulus clouds. J. Weather Modification,
I0, 54-61.
Ramana Murty, Bh. V. and K, R. Biswas, 1968:
Weather Modification in India. J. Met Soc.,
Japan, 46, 160-165.
Year: 1973 1975 1976 1977 1978
Southwest monsoon: 10.2 2.6 4.1 5.9 4.4
Northeast monsoon: 23.8 3.2 7.0 7.0 5.5
During the Northeast Monsoon (Oct-Dec) of 1976
and 1977 rainfall on 16 seeded days (Table 2), com-
pared to that on 20 non-seeded days, decreased by
17 percent, significant at 33 percent level (Table
4). The GCN concentrations were higher up to 
factor of two during the northeast monsoon.
3. SUMMARY
The ground-based salt seeding experiment, with
a fixed control target design and day randomiza-
tion, on 56 days with 54 days as control during
the monsoon seasons of 1973-1977 in Tamil Nadu
State, South India showed:
1) Rainfall incrase of 32 percent, significant
at the 15 percent level, during the south-
west monsoon;
178
NB
A
Y
0
F
8
E
G
A
L
FIG. 1. Location of the experimental area (inside the
circle) and the raingauge network.
179
Table i. Meteorological Parameters, 1931-60, at Madras
(13°O0’N, 80°ll’E 16 m ASL)
JUN JUL AU6 SEP OCT NOV DEC
Relative humidity (%) 5g 63 67 71 79 79
Days with low cloud~ 3 ok 14 15 14 I0 8 i2
No. of rainy days 4.0 G.G 8.4 7.3 I0.0 9.8
Rainfall (mm) 52.6 83.5 i24.3 118.0 2~?.0 308.7
Mean wind speed (Km/h) 16.4 14.6 13.6 li.1 9.2 11.7
Percentage number ~ 5W 8 13 9 8 3 0
of days with wind
at surface from
| NE i 2 2 2 22 45
£ 8 7 ll 17 24 Ii
77
12
5.~
139 .l
12.6
0
53
13
Table 2. Seeding details.
Year Period Number of days
Seeded Not-seeded
Seeding material
(kilograms)
1973 JUN-SEP 20 16
1974 - - -
1975 JUN-JUL 3 7
1976 JUL-SEP 5 4
" OCT-DEC lO lO
1977 JUN-SEP 12 7
. OCT-NOV 6 i0
TOTAL
16,050
1,825
3,450
6,175
10,500
4,850
56 54 42,850
180
,,REVIEWED"
Table 3. Total rainfall (mm) per station in Target (T) 
Control (C) sectors on seeded and not-seeded days.
Year Month
Seeded Not-seeded
T C’ Result
T C T/C T’ C’ T’/C’ C T’
1973 dUN 6.24 10.05 0.641 3.03 6.83 0.444 1.444 +
JUL 5.59 4.07 1.373 1.55 5.68 0.273 5.029 +
AUG 31.02 1.35 22.977 1.32 0.00 - - 0
5£P 33.51 27.28 1.228 2.88 9.76 0.295 4.163 +
1974 .... No experiments - -
1975 JUN 0.99 0.34 2.912 25.06 9.54 2.627 1.108 +
JUL 4.08 1.77 2.305 0.00 O.O0 - - O
1976 JUL 13.63 9.22 1.478 46.46 48.24 0.963 1.535 +
SEP 0.00 4.02 O,OOD 0.00 0.00 - - 0
1977 dUN 1.27 0.26 0.488 1.65 0.00 1.650 0.488 -
JUL 2.41 10.55 0.228 0.64 0.00 0.640 0.228 -
AUG 26.57 27.97 0.950 10.16 15.62 0.650 1.462 +
5EP 0.00 1.82 0.000 0.00 0.23 - - 0
1976 OCT 5.41 0.82 6.598 1.27 0.13 9.769 0.675
NOV 3.54 4.05 0.874 4.28 i0.ii 0.423 2.066 +
DEC 3.11 1.04 2.990 0.32 0.00 - - -
1977 OCT 9.54 32.35 0.295 67°25 103.07 0.652 0.452 -
NOV 3.79 6.62 0.573 15.92 17.05 0.934 0.613 -
Table 4. Average ratio values of total rainfall in
Target (T) and Control (C) sectors on seeded
and not-seeded days.
Seeded Not-seeded T C’
~Year
T/C T’/C’ C T’
1973 1.791
1974 -
1975 2.403
1976 1.029
1977 0.745
Total 1.272
Southwest Monsoon (JUN-SEP)
0.394 4.55 + 355
No experiment -
2.627 0.92 - 8
0.963 1.07 + 7
0.785 0.95 - 5
0.967 1.32 + 32*
Northeast Monsoon IOCT-DEC)
1976 2.040 0.573 3.56 + 256
1977 0.295 0.652 0.45 - 55
Total 0.566 0.633 0.83 - 17"*
Significance levels by one-tailed Mann-Whitney test : * = 15%
¯ * = 33%
181


Ground-based salt seeding in Tamil nadu state, south India, 1973-1977

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Ground-based salt seeding in Tamil nadu state, south India, 1973-1977

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