[chris (OP)]

What determines the size of a raindrop or a drip from a tap?

Presumably, the droplet from a cloud will correspond to how big the hydrometeor was that formed in the cloud and then fell groundwards? But, as it falls through the air does the drop disintegrate into threshold-sized blobs dictated by the surface tension of the water?

Also, with a dripping tap, are those drops regular and all the same size?

[chiralSPO]

Raindrops are definitely not all the same size.

From experience I can say rain (at least liquid water precipitation) can vary between mist-like droplets that barely fall to large drops that "splash out" into ~1 inch circles on landing on pavement.

I would imagine that there is an upper limit on the size of a raindrop, but that limit may be quite sensitive to environmental conditions and where the drop forms. (Unlike drips from a faucet, where the material and shape of the faucet probably play the dominant role in the size of the drop before it falls.) I would postulate that the temperature, atmospheric pressure and wind conditions are key parameters. The surface tension and density of the water will depend on the temperature (and also, possibly on what else is in the rain--dust, salt, dissolved gases, etc.). The drag on the raindrops will depend on the atmospheric conditions. There may also be a significant role played by how the droplets form initially.

Perhaps a meteorologist or hydrodynamicist could shed more light on this?

[alancalverd]

In the absence of a gravitational field there is no upper limit to the size of a drop of liquid. Surface tension will eventually form a sphere if the drop is undisturbed.

However any perturbation adds energy to the system and may result in it separating into more energetic droplets: the free surface energy of a droplet radius r depends on 1/r. If these are separated by, e.g. air turbulence, they may not recombine and if small enough will evaporate completely.

[chris (OP)]

However any perturbation adds energy to the system and may result in it separating into more energetic droplets: the free surface energy of a droplet radius r depends on 1/r. If these are separated by, e.g. air turbulence, they may not recombine and if small enough will evaporate completely.

This is what I was wondering; could the density of the atmosphere affect the potential sizes of raindrops?

[RD]

The supernumerary-bands on rainbows are due to raindrops being surprisingly uniform in size ... http://www.atoptics.co.uk/rainbows/supers.htm  [ something is constraining their size-distribution ].

Include electrical-charge to the factors determining the distribution of droplet-size : tiny charged drops will repel one another, rather than coalesce into bigger drops ...

https://youtu.be/ZObsVP13xpE?t=1m43s