Parcels or strata of air having densities sufficiently different from that of their surroundings that they may be discerned by means of refraction anomalies in transmitted light. All of the natural scintillation phenomena in the atmosphere result from the presence of density schlieren developed by turbulent processes. The schlieren method is an experimental technique for optically detecting the presence of slight density, and hence temperature and/or pressure, variations in gases and liquids by virtue of refraction effects.