The onset of disintegration and corona in water drops falling at their terminal velocity in a vertical wind tunnel and exposed to horizontal electric fields has been investigated. Contrary to previous observations, the drops elongate in horizontal direction and distort into the shape of a concavo‐convex lens with a convex bottom and a sharp‐edged rim facing upward. Drops of diameter < 6.6 mm which do not break up in absence of electric field in this wind tunnel, break up in presence of the horizontal electric field. The values of horizontal electric field required for instability of the drops are much lower than those either predicted by Taylor's criterion of instability or observed in previous experimental studies. The criterion for instability of water drops freely suspended in presence of horizontal electric fields can be expressed as FH (ro/σ)½ = 0.98 ± 0.03 where FH is the horizontal electric field in esu, ro is the drop radius in centimeters and σ is the surface tension in dynes per centimeter. Most of the drops produce corona just before their breakup. Among various drops that are freely suspended in the wind tunnel, one by one, the number of drops that produce corona and/or breakup increases with increase in the electric field and/or drop size. While all drops of diameter ≥ 7.1 mm produce corona in a horizontal electric field of 500 kV/m, only a small fraction of very large drops of 8.0 mm diameter produce corona when the electric field is equal to 200 kV/m. Comparatively, very low values of instability field observed in our experiment are qualitatively explained because of long exposure of the freely suspended drops to the horizontal electric fields. The drops become unstable and produce corona when the drop's oscillation amplitude overshoots its equilibrium value and the plane of the drop oscillation coincides with the direction of electric field. From the results, it seems likely that horizontal electric fields in the bases of thunderclouds may cause disintegration of large raindrops and the occurrence of corona from their surfaces may trigger a lightning discharge.

Time for which charged or uncharged water drops of different sizes can be suspended over a vertical wind tunnel before their spontaneous breakup and the size distribution of droplets resulting from their breakup have been determined. Probability of spontaneous breakup of a drop has been found to increase with the size and charge of the drop. It has been observed that water drops carrying a charge of 5×10−10 C breakup immediately after their suspension if their diameter > 8 mm. Total number of droplets produced on spontaneous breakup of a drop increases with the size of the drop, and if the drop size is > 6.6 mm, the total number of droplets is more when the drop is uncharged than that when it is charged. However, the number of droplets larger than a critical size is more if the parent drop is charged and the number of droplets smaller than that critical size is more if the parent drop is uncharged. It has been attempted to qualitatively explain the experimental results as the result of enhanced surface charge density around the waist of the drop during its oscillation. Charge on the drop has been suggested to cause an increase in width of the base of suspended drop.

Deformation of the uncharged water drops freely suspended in a vertical wind tunnel and subjected to vertical, horizontal, or no external electric field is investigated using movie photography. Electric field elongates the drop along its direction, and the elongation increases with the increase in electric field. Horizontal electric fields are more efficient than vertical ones in deforming the drop. The larger the drop, the larger is the difference between the distortions in the vertical and horizontal field configurations. As compared to the case of no electric field, horizontal electric field, as low as 1 kV cm−1, changes the ratio of minor‐to‐major axis of the 2.6‐mm diameter drop by ∼3%. An examination of the frequency distribution of the drop’s axis ratio shows that during its oscillations, oblateness decreases more often when subjected to vertical electric field and increases more often when subjected to horizontal electric field. Extreme values of distortion increase and are attained more frequently when drop is oscillating in electric field. It is concluded that the drop size distribution will be wider and thus the rate of drop’s growth faster in those regions of cloud where the electric field direction is vertical rather than horizontal.

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