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

Also see Cloud ice formationplugin-autotooltip__plain plugin-autotooltip_bigCloud ice formation

The formation of ice crystals have important implications for stratospheric ozone chemistry, cloud dynamics, rock weathering, and hydrate formation etc., however the exact mechanisms by which microscopic particulate matter 'seeds' ice-crystals are unknown. and Lightningplugin-autotooltip__plain plugin-autotooltip_bigLightning

High powered lightning discharges are happening somewhere on Earth 100 times every second.

A possible mechanism for the very substantial electrical charges within the clouds was put forward in 1978. Following lab-based experiments, it was suggested that the charges arise from the static-electric interactions of graupel (slush) and ice crystals moving within the cloud (due to convection currents caused by widely differing air temperatures) .

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