The Whitefish Summit image: computer animation of the time variation of a superior mirage

Abstract

In this research an attempt is made to produce a computer-animated simulation of the time variation of a superior mirage. Before this is done, however, a well-known parabolic ray-tracing technique is to be used to write a FORTRAN code that can calculate the trajectories of light rays passing through a spherically symmetric atmosphere. The code is used to calculate the paths of light rays as they travel from an observer's eye to a terrestrial object (the rays are traced from the observer's eye to the object, a low-lying hill, rather than vice versa because tracing rays outward from a single point is much simpler than trying to force the rays to converge to the single point of the observer's eye). The heights at which the respective rays strike the hill are stored in a data file, and this file is read by a C graphics program that uses these data to generate images of the object. These images are then compared with photographs taken in the field. Once the procedure for the spherically symmetric case has been perfected, the technique is then modified to enable the tracing of light rays through an atmosphere that is not spherically symmetric, and it is used to calculate the paths of rays through an atmosphere which is disturbed by a gravity (buoyancy) wave propagating towards the observer. The ray-tracing algorithm is used to generate a set of data files, which are in turn read by the graphics program. The images produced by this program are then placed in sequence on videotape, and an animated depiction of the mirage's distortion with time is produced. A total of four films are made: one involving a gravity wave of relatively long wavelength propagating through a four-layered temperature profile, one involving a gravity wave of relatively short wavelength propagating through that same profile, one involving a superposition of long and short waves propagating through that profile, and a wave of short wavelength passing through a six-layered gradient profile. Possible applications of the modified ray-tracing technique are discussed, as are its limitations.