Effects of Relative Humidity on the Coalescence of Small Precipitation Drops in Free Fall

Abstract

Observations of the effects of relative humidity on coalescence are limited to studies using supported drops or streams of drops, and the results are contradictory. In this paper, findings are presented on the effect of high and low relative humidity on collisions between freely falling drops. Comparisons between the collision outcomes (coalescence, bounce, and temporary coalescence with and without satellite drops) for high-humidity (RH > 95%) and low-humidity (RH ≈ 30%) experiments were made for small precipitation drops at terminal velocity and with minimal electric charge. Coalescence begins after the air-film between colliding drops is drained sufficiently to allow the drops to make contact. For temporary coalescence, the union of the two drops is not permanent because the rotational energy caused by a non-head-on collision is sufficient to pull the coalesing drops apart. One or more satellite drops form during a temporary coalescence when water filament between the separating drops breaks in more than one location. Experiments with higher drop charge were used to examine further the influence of humidity on coalescence. Our results show that relative humidity does not affect the coalescence efficiency for small precipitation drops. The effect of humidity is limited to collisions where permanent coalescence does not occur, and the collision outcome can be temporary coalescence. In cases where bounce is also a possible outcome, it was found that the probability of bounce is enhanced at the expense of temporary coalescence when relative humidity is decreased. For two of the comparisons between high-humidity and low-humidity results, the fraction of temporary coalescence collision outcomes halved at low humidity. Since the colliding drops are at the wet-bulb temperature, this effect is traced to the colder air gap that drains more slowly and retards coalescence at lower humidifies. At high relative humidity the number of satellite drops about doubles with the increased probability of temporary coalescence. Other experiments showed that the increase in satellite drops at higher relative humidities also occurs for cases where collision outcomes are limited to coalescence or temporary coalescence. Since there are more temporary coalescence outcomes at the higher relative humidities in clouds, there are also more satellite drops that can act as embryos for new raindrops. These results apply to rain shafts within and below clouds.