From Glossary of Meteorology
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In cloud physics, the merging of two water drops into a single larger drop after collision.

Coalescence between colliding drops is affected by the impact energy, which tends to increase with the higher fall velocities of larger drops. Colliding drops having negligible impact energy compared to their surface energy behave as water spheres that collide with a collision efficiency (the fraction of small drops that collide with a large drop within the geometric collision cross section) predicted by the theory for falling spheres. The result of increasing impact energy is to flatten the colliding drops at the point of impact, impeding the drainage of the air and delaying contact between them. As the distortion relaxes, the drops rebound, reducing the coalescence efficiency for cloud drops and drizzle drops colliding with smaller drops. At larger impact energy, separation will occur if the rotational energy (fixed by conservation of angular momentum) is higher than the surface energy of the coalescing drops. This phenomenon, termed temporary coalescence, can result in satellite droplets considerably smaller than either of the parent drops. This phenomenon is also called partial coalescence because the large drop may gain mass as a result of the higher internal pressure in the small drop. At still larger impact energy, drop breakup occurs for the smaller drop. About 20% of the high-energy collisions between large raindrops (d > 3 mm) and drizzle drops (d > 0.2 mm) result in the disintegration of both drops. Other factors that affect coalescence are electric charge and electric field, both of which promote coalescence, leading to earlier onset of coalescence during an interaction so that coalescence efficiencies are increased by suppression of rebound and temporary coalescence. All of these processes are important in formation of precipitation in all liquid clouds both above and below 0°C.
See collision– coalescence process.