Scintillation — Scintillation is a generic term for rapid variations in apparent position, brightness, or color of a distant luminous object viewed through the atmosphere.
If the object lies outside the earth’s atmosphere, as in the case of stars and planets, the phenomenon is termed astronomical scintillation; if the luminous source lies within the atmosphere, the phenomenon is termed terrestrial scintillation.
As one of the three principal factors governing astronomical seeing, scintillation is defined as variations in luminance only.
It is clearly established that almost all scintillation effects are caused by anomalous refraction occurring in rather small parcels or strata of air, schlieren, whose temperatures and hence densities differ slightly from those of their surroundings.
Normal wind motions transporting such schlieren across the observer’s line of sight produce the irregular fluctuations characteristic of scintillation.
Scintillation effects are always much more pronounced near the horizon than near the zenith. Parcels of the order of only centimeters to decimeters are believed to produce most of the scintillatory irregularities in the atmosphere.