. Compendium of meteorology. Meteorology. 68 METEOROLOGICAL OPTICS the ground such a parcel must have an extremely small diameter to cause scintillation. The mechanism of intensity fluctuations was ex- plained by K. Exner [c. 42] as follows: In Fig. 8, density schlieren around S are embedded at certain intervals in an otherwise more or less homogeneous field of density and cause concavities (or convexities) in an originally plane wave-front of light and, thus, divergence of the rays at A and A' and convergence at B and B'. If the system of schlieren moves horizontally, an observer— say at B—wi
. Compendium of meteorology. Meteorology. 68 METEOROLOGICAL OPTICS the ground such a parcel must have an extremely small diameter to cause scintillation. The mechanism of intensity fluctuations was ex- plained by K. Exner [c. 42] as follows: In Fig. 8, density schlieren around S are embedded at certain intervals in an otherwise more or less homogeneous field of density and cause concavities (or convexities) in an originally plane wave-front of light and, thus, divergence of the rays at A and A' and convergence at B and B'. If the system of schlieren moves horizontally, an observer— say at B—will perceive alternate increases in flux dens- ity where the rays converge, and decreases where they diverge. He wifl also see the light come from slightly varying directions, that is, apparent vibratory motions of the star. It is easily envisioned that the same varia- tions occur if the schlieren system moves vertically. K. Exner measured the radius of curvature r of the wave-front deformation to be roughly between 2 and 20 km. In addition to discrete schlieren, we may also consider the wavy structure of surfaces of temperature or wind discontinuity as a cause of variations in flux density. LIGHT RAYS. Fig. 8.—Scintillation resulting from density schlieren {after K. Exner). The short-term fluctuations of images received from terrestrial light sources or objects, or terrestrial scin- tillation, is of great practical importance; in particular, strong scintillation may interfere with blinker signaling. Scintillation also limits the precision of telescope point- ing and the useful magnification of telescopic devices [46]. Siedentopf and Wisshak'' recently investigated the case of collimated light sources over a range 1 km long and 1 m above the ground, employing an objective receiver. With strong scintillation, the frequency of apparent intensity fluctuations was most often observed between 5 and 9 sec~^ with lesser scintillation between 1 and 3 sec^^ The whole range covered
Size: 2123px × 1177px
Photo credit: © The Book Worm / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectmeteorology
