David L. Fried
David L. Fried is a scientist, best known for his classic article, "Optical resolution through a randomly inhomogeneous medium for very long and very short exposures.". In it, Fried describes what has come to be known as Fried's seeing diameter, or r0 (usually pronounced r-naught). The seeing diameter is effectively a limiting aperture due to atmospheric turbulence, and is found either empirically or statistically. The seeing diameter limits optical resolution.
Typical values for Fried's seeing diameter in the visible spectrum may range from less than 1 cm (you can detect turbulence with your eye) to 20 cm on Mt. Haleakala.
References
- "Optical resolution through a randomly inhomogeneous medium for very long and very short exposures.", J. Opt. Soc. Amer. 56:1372-9, 1966