A theory is developed to describe light scattering by clouds of statistically irregular particles using the Rayleigh-Gans-Debye approximation. The theory reproduces the expected behaviour due to irregularity, that is insensitivity to shape in forward scattering, smoothing of the polar diagram and an increase in backscattering. It also suggests that the particles can be represented by spheres with a size distribution equal to the probability distribution of the radius, provided that the scale height of the irregularity is not too large.