From AMS Glossary
The Arago distance is sensitive to the presence of foreign scattering particles in the atmosphere since these increase the contribution of the negative (horizontal) component of skylight polarization and hence shift the location of the point where the negative component is just equalled by the positive component. Thus the Arago distance is a useful measure of atmospheric turbidity. Its value is generally close to 20°, and is a function of solar elevation angle and of the wavelength of the light with which polarization is studied.
Sekera, Z. 1951. Compendium of Meteorology. 79–90.