size parameter
From Glossary of Meteorology
The ratio of the size of a spherical scattering particle to the wavelength of the radiation being scattered, that is, α = πd/λ, where d is the diameter of the particle, and λ is the wavelength of the incident radiation.
Mie theory describes the general process of extinction, scattering, and absorption of spherical particles as a function of the size parameter and refractive index. For α < ≈0.1, the Rayleigh scattering approximation becomes valid, whereas for α > ≈100, geometrical optics becomes valid.
