Seeing units, arcsec VS cm I am very used to working with seeing in arcseconds, but sometimes I come across the seeing in cm, e.g. here.

I have looked online for a way to convert from one to the other, but I can't find any. How would this be done?
Seeing is typically measured as the FWHM of the seeing disk, but can also be expressed though the Fried parameter $r_0$, which measures the size, or strength, of the parcels of gas that cause the turbulence in the atmosphere. The relation between the two is (e.g. Vernin &amp; Munoz-Tunon 1995)
$$
\text{seeing} = 0.98\frac{\lambda}{r_0},
$$
where $\lambda$ is the wavelength of the light.

$r_0$ can by calculated by integrating the "turbulence strength" along the line of sight$^\dagger\!\!$, and  scales as $\lambda^{6/5}\!$. Because of this dependence, the "usual" seeing has only a weak ($\lambda^{-1/5}$) dependence on wavelength.

From the Wikipedia article on seeing:


  At visible wavelengths, $r_0$ varies from 20 cm at the best locations to 5 cm at typical sea-level sites




$\dagger$Actually, $r_0$ refers to a distance corresponding to the line of sight towars zenith; observing at an angle $\zeta$ from zenith introduces a factor $\cos^{3/5}\!\zeta$.