Noctilucent Clouds

An atmospheric phenomenon is considered as 'explained' when we have succeeded in deducing it on the basis of accepted principles of physics. (H. Ertel, Methods and problems of dynamical meteorolo­ gy,p. l) Until recently, noctilucent clouds were regarded merely as a curious atmos­ pheric phenomenon, the occurrence of which aroused only limited scientific attention. However, in the last two decades the interest they have been given has markedly increased. The clouds, usually pale blue in colour, may be seen on a clear night at high latitudes. Typical examples are illustrated in the book. Clouds looking like these in daytime would be classified as cirrostratus. What sets noctilucent clouds apart is their occurrence in the middle of the night, their very obvious pale blue colour, and their disappearance into the dawn close to the onset of civil twilight when the Sun is 6° below the horizon. as being set apart from ordinary Noctilucent clouds were first recognized clouds in 1884/1885 and in a series of sightings that followed their return in the summer of 1885. That year marked the beginning of observations and the interpretation of twilight phenomena. The impetus came from the extraordi­ nary Krakatoa eruption, which was not only one of the most spectacular volcanic eruptions in recorded history, but which also turned out to be a startling event in atmospheric science with repercussions even in our time.

Inhalt

1 Noctilucent Clouds.- 1.1 Introduction.- 1.2 How, When and Where Noctilucent Clouds Are Seen.- 1.3 Amateur Observations.- 1.4 CloudTypes.- 1.5 Structure of the Upper Atmosphere.- 2 History.- 2.1 Introduction.- 2.2 The Discovery of the "Shining Night-Clouds".- 2.3 Measurements of Noctilucent Clouds.- 2.4 The Middle Period of Noctilucent Cloud Research.- 3 Observations from Ground Level.- 3.1 Introduction.- 3.2 The Geometry of Twilight Scattering.- 3.3 Latitude of Observation.- 3.4 Absorption of Light in the Atmosphere.- 3.5 Height of Noctilucent Clouds.- 3.6 Drift Motions.- 3.7 Wave Structure.- 4 Spectrophotometry.- 4.1 Introduction.- 4.2 Spectroscopic Observations.- 4.3 Spectrophotometry from Ground Level.- 4.4 Rocket-Borne Photometers.- 4.5 Spectrophotometry from Satellites.- 4.6 Conclusions About Cloud Particle Sizes.- 5 Polarimetry.- 5.1 Introduction.- 5.2 Polarization by Scattering.- 5.3 Measurement of Polarized Light.- 5.4 Polarization Measured from Ground Level.- 5.5 Measurements of Polarization from Rockets.- 5.6 Conclusions About Cloud Particle Sizes.- 6 Rocket-Borne Sampling.- 6.1 Introduction.- 6.2 Flights over Sweden in 1962 and 1967.- 6.3 Rights over Sweden in 1970 and 1971.- 6.4 Flights over Canada in 1968 and 1970.- 6.5 Collectors Flown by Max-Planck-Institut Researchers, 1968 to 1971.- 6.6 Conclusions About Cloud Particle Sizes.- 7 Variation of Occurrence.- 7.1 Introduction.- 7.2 Sunspot Cycle.- 7.3 Seasonal Frequency of Noctilucent Clouds.- 7.4 Climatology of the Mesosphere.- 8 Other Observations.- 8.1 Introduction.- 8.2 Association with Hydroxyl Airglow Emission.- 8.3 Association with Aurora and Planetary Magnetic Activity.- 8.4 Lunar Effects.- 8.5 Lidar Observations.- 8.6 Artificial Noctilucent Clouds.- 8.7 Abnormal Observations.- 9 Environment of Noctilucent Clouds.- 9.1 Introduction.- 9.2 Atmospheric in Temperature.- 9.3 D-Region.- 9.4 Dust.- 9.5 Water Vapour in the Mesosphere.- 9.6 Radiation.- 9.7 Rates of Growth.- 9.8 Nucleation of Ice.- 9.9 Settling of Particles.- 9.10 Modelling Noctilucent Clouds by Numerical Simulation.- 10 The Nature of Noctilucent Clouds.- 10.1 Introduction.- 10.2 Formation in Noctilucent Clouds.- 10.3 Growth of Noctilucent Cloud Particles.- 10.4 Evaporation of Noctilucent Cloud Particles.- 10.5 The Relationship Between Polar Mesospheric Clouds and Noctilucent Clouds.- 10.6 Summary.- 11 Bibliography.- A) Before 1900.- B) 1900-1950.- C) Bibliography since 1950.- Appendix 1: Atmospheric Refraction.- Appendix 2: Atmospheric Transmission Along Grazing Pays.- Name Index.