Height, structure and displacements of noctilucent clouds

Abstract

Observations of noctilucent clouds have been carried out during the summer of 1958 at Torsta (63.3° N; 14.6° E) in Central Sweden as part of the IGY working program of the Institute of Meteorology, University of Stockholm. Simultaneous pairs of cloud photographs have been taken with accurate phototheodolite cameras from the end-points of a geodetically determined base-line of length 51.5 km. The picture pairs were subsequently analyzed in stereo instruments (autographs) by which Cartesian space coordinates were obtained for various points in the cloud system. These coordinates, duly corrected for atmospheric refraction, were used for determination of the height of the individual features. Through the stereoscopic effect, measurements could be made on diffuse parts of the cloud system as well as on marked details. Additional information about movements of the cloud system was obtained from a time-lapse film in Kodachrome. The results were plotted and analyzed by conventional methods and maps of the cloud topography at consecutive time intervals could be prepared. In addition to these maps, vertical cross-sections through the cloud system were made as well as detailed studies of particularly interesting cloud features. This paper gives a presentation and interpretation of the results obtained so far and a brief description of the photogrammetric technique applied. The results presented below were obtained during a very bright cloud display with good visibility conditions on August 10–11th, 1958. Thirty pairs of pictures were taken of various parts of the cloud system, which covered the entire northern horizon. Eight of these have been analyzed so far. The results can be summarized as follows. The cloud system moved in a direction north-east to south-west with velocities of the order of 50 to 100 m/s. It consisted of a continuous diffuse layer interchanging with regions of sharply defined features such as systems of parallel billows and bands, blobs and other smaller-scale irregularities of various shapes. The measured heights varied between 81.5 and 85.5 km. The long parallel bands were identified as a system of waves with wavelengths of the order of 50 km and amplitudes up to 4 km which propagated in a direction nearly opposite to that of the cloud system with absolute velocities of the order of 10 to 20 m/s. The wave crests were oriented nearly perpendicular to the main air flow and were continuous over distances of hundreds of kilometers and exhibited local refraction effects. The smaller billows had wavelengths of the order of 5–10 km and amplitudes about 0.5–1.0 km; they moved with the cloud sysem. The billows showed no preferred orientation and were observed to pass through the crests of the longer waves. It is indicated by the analysis the regular changes in the brightness of these clouds are due to changes of the optical thickness of the cloud layer, particularly at the rear slope of the long waves. The measured details had well-defined lower boundarie. DOI: 10.1111/j.2153-3490.1962.tb00115.x