When [an] ascending parcel of moist air reaches the condensation level, the initial mist of small, micron-size water droplets is formed, which are suspended in the air. In the super-saturated environment water droplets grow due to condensation of water vapor from the surrounding atmosphere. However, to form the raindrops, which can fall down triggering rain, they must grow up to about 50μm size droplets, which would take a very long time. Observations indicate that the average time for rainfall initiation is approximately 15 − 20 minutes, while existing theories predict that the duration of a time interval, required for droplets to grow up to 50μm in radius, is of the order of hours. Indeed, though the actual time of large droplets formation depends on the initial droplet size spectrum and cloud water content, the predicted growth time differs considerably from the observations.“

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The unexplained physical mechanism of such fast growth is crucial for the understanding and modelling of rain, and is known as the “Size Gap, or the Condensation-Coalescence Bottleneck” in warm rain formation.

It's regarded as an important unresolved problem in cloud physics.

The quote above comes from Acceleration of raindrops formation due to tangling-clustering instability in turbulent stratified atmosphere arXiv, 2015, which offers a possible explanation.

Further reading Growth of Cloud Dropletsin a Turbulent Environment Annu. Rev. Fluid Mech. 45:293–324

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