Archimedes number
The Archimedes number serves as a correlation between two hydrodynamic or thermal phenomena when determining factors include buoyant and viscous forces. (consider rain droplet falling in air)

(1) 
where
 A_{r} = Archimedes number [dimensionless quantity]
 D_{p} = Diameter of particle [m]
 g = gravity [m/s^{2}]
 ρ_{air} = density of air (or other dominant fluid) [kg/m^{3}]
 ρ_{p} = density of particle [kg/m^{3}]
 μ = viscosity of air [kg/m*s]
The characteristic velocity of the particle in a particular medium may be obtained from the square root of kinetic energy and mass forces of the particle such that

(2) 
The Reynolds number can then be acquired from the correlation:

(3) 
Once the Reynolds number is known, it may be used to estimate the terminal velocity of a particle from:

(4) 
The Archimedes number has also been characterized as a ratio of lift forces to internal friction forces.
These correlations are not mine, in search of source to be posted**