without an atmosphere, a small star would have an apparent size, an "airy disk", in a telescope image determined by diffraction and would be inversely proportional to the diameter of the telescope. however, when light enters the earth's atmosphere, the different temperature layers and different wind speeds distort the light waves, leading to distortions in the image of a star. the effects of the atmosphere can be modeled as rotating cells of air moving turbulently. at most observatories, the turbulence is only significant on scales larger than "r" (see below—the seeing parameter "r" is 10–20 cm at visible wavelengths under the best conditions) and this limits the resolution of telescopes to be about the same as given by a space-based 10–20 cm telescope.