Overview of the weather modification research in India

A Rain and Cloud Physics Research (RCPR) Centre was set up in 1955 at the National Physical Laboratory, New Delhi for undertaking scientific studies on Cloud Physics and Rainmaking. The RCPR Centre conducted a long series of ground based salt seeding experiments in north India during the southwest monsoon seasons (June-Sept.) of 1957-1966. The results of these experiments showed on the average, an increase of about 20 percent in rainfall on seeded days

Chemical composition of rain water and rain characteristics at Delhi

Rain water samples collected with the progress of rain shower on a number of rain occasions in different seasons at Delhi have been analysed for chloride, sulphate, sodium, potassium and calcium. The trend of variations in the concentration of chloride ion in rain water with the progress of a rain shower would seem to suggest that the growth of rain drops in convective clouds at Delhi would involve a process which might be analogous to what was envisaged by Woodcock for Hawaiian rains. Further, it was noticed that the characteristics of precipitation at Delhi are dependent on the amounts of air-borne soluble particles present in the cloud forming air. Smaller cation concentration ( 1.8) distinguish what may be called “freezing” rain (rain from cold layer clouds) from that of the other type in which both freezing and non-freezing mechanisms operate (rain from cold convective clouds). Smaller ratio values of sulphate to chloride ( 1.8) characterize heavier rainfall. Probable explanations have been suggested for these observed features

Conditions governing drop freezing at warm temperatures

In an attempt to investigate conditions under which ice crystals can form at comparatively warm temperatures in supercooled regions of a cloud, the scope of experiments on drop freezing, previously undertaken by the authors, has been enlarged. The results of the present extensive series of experiments have confirmed that supercooling drops, when subjected to evaporation, freeze more readily due to what has been called the “dynamic effect” of evaporation. When sodium sulphate was added to the drops in arbitrary concentration, their freezing probability also showed a significant inctease. These findings indicate that, in supercooled regions of a cloud where prevailing conditions are similar to those of the reported experiments, the ice crystal concentration should be two to three orders of magnitude higher than the existing ice nucleus concentration

Features of lower troposphere on occasions of contrasting rainfall at a tropical coastal station

The radiosonde data and the synoptic situations relating to pairs of consecutive days of contrasting rainfall recorded at the tropical coastal station, Bombay, were analysed for two contrasting summer monsoons. The synoptic situations relating to the pairs of days were classified into two categories, namely, not similar and nearly similar. The results pointed out that the majority of the higher rainfall days in the pairs, irrespective of the similarity in the synoptic situation, were associated with one or more of the following: (i) higher static stability, (ii) lower convective instability, (iii) higher precipitable water content and (iv) lower lifting condensation level. The associations noted with (i) and (iii) were not statistically significant on both the categories of synoptic situations. The association with (ii) was significant at the 10% level when the synoptic situations were nearly similar and was not significant when the situations were not similar. The association with (iv) was significant at the 5% level on both categories of days. The features noticed were discussed in the light of the available knowledge for tropical oceanic and land regions

High level warmings over a tropical station

Examination of the available rocketsonde data for the tropical station, Thumba, India, has shown that warmings, less pronounced than in the case of middle and high-latitude stations but of noticeable magnitude, and coolings of similar magnitude occurred in the mesopheric and upper stratospheric levels during the period, December 1970–March 1971. This was the only winter period when observations were made at the station during all of the 4 mo. No prominent change in wind has been observed in association with the warnings, however. The maximum warming observed over a period of 1 week in the upper stratosphere was 26°C at 45 km. The upper mesosphere had been subjected to a continuous process of warming for over 3 weeks in December–January 1971 during which period the temperature rose by 48°C at 70 km. There is no definitive indication that these warmings were of the propagating type. However, they appear to have moved in the vertical at a rate of 3–5 km/day. The temperature behavior of the lower stratosphere (50 mb) and the upper troposphere (300 mb) in the winter of 1970–71 was different than that which was observed in the preceding 2 winters

Freezing characteristics of rain water drops with different solutes and their implications on anomalous ice crystal concentration in clouds

In an attempt to examine the probable cause for the reported anomaly between the ice crystal-and ice nucleus concentrations in clouds, the authors have carried out laboratory experiments on the freezing of rain water drops, under various conditions of temperature, with various solutes. Sulphates, when present in adequate concentration, have been found to accelerate remarkably the process of ice-formation in the drops. The contrast noticed has been greater at warmer temperatures. The ice-forming ability of silver iodide has also improved remarkably in the presence of sulphates. Chlorides, on the contrary, not only decelerated natural ice-formation in the drops but also deactivated silver iodide. Based on certain consideration

Experimental investigation of the influence of electric field on the collision - coalescence of water drops

Laboratory experiments were conducted on the collision-coalescence of pairs of water drops of equal size, in two oil media (kerosene and mustard), with and without external vertical electric field (F). The radii of the water drops used were in the range 1.6 to 1.7 mm and the external electric field varied from 0 to 375 V cm-'. Collision frequencies were determined for various combinations of mean lateral (X)a nd mean vertical (Z) separations of the drop pairs as fixed combinations of X and Z could not be reproduced in any given set of experiments due to the limitations of the mechanical set up of the apparatu

Numercial simulation of cloud seeding experiments in Maharashtra state, India

Two numerical cloud seeding simulation experiments of 5-, 8- and 10-years duration, with a double area cross-over design and area randomization, were performed using historic rainfall data for the Deccan plateau region in Maharashtra state. The first numerical experiment (EXP-TR) used the simulation technique of Twomey and Roberstson (1973), second (EXP-MMM) used different simulation technique proposed in the present study. The results of the two numerical experiments have agreed closely. The EXP-MMM technique not only reduces computational time by an order of magnitude but also defines the exact lower limit for the double ration value which can be detected at 5 per cent level of significance. The results of the numerical experiments suggest that 15 and 20 percent increases in rainfall due to seeding in Maharashtra could be detected, with 80 percent or more probability, in 5 years. In a 10-year experiment the probabilities of detecting 5 and 10 percent increases in rainfall due to seeding are 27 and 65 percent, respectivel

Dynamic responses of warm monsoon clouds to salt seeding

High resolution temperature measurements during single-level air- craft penetrations through warm monsoon clouds before and after salt seeding had a significant wave-length of about 2 km. The slope of the spectra relating to not-seeded traverses followed a -5/3 power law. The slope of the spectra relating to seeded traverses increased when liquid water content increased and rain formed. The temperature spectra of the seeded traverses showed a net energy gain in the larger wave-lengths ( >540 m) and a net energy loss in the shorter wavelengths. The net-energy gain could be due to condensation of water vapor on the salt parti- cles, the net energy loss to the decrease in the small scale turbulence resulting from the invigoration of the updraft. These features could be manifestations of the alteration of the dynamics of the cloud through salt seeding

Diurnal and seasonal variations of space charge, electric field and cloud condensation nuclei in the lowest layer of the atmosphere

The association between the onset of the monsoon and the subsequent rainfall distribution over three west coast subdivision of of India has been examined for the 50 years period