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An examination of Zacharov's results on the wavelength variation of attenuation of solar radiation following the Perseid meteor shower shows that the effects could be produced either by absorbing particles of diameter about 10-5 cm or 'less, or by transparent particles of diameter; in the range 5 X 10-5 to 10-< cm. The rapid disappearance of attenuation, however, can be explained only if the particles evaporate on falling and it is concluded that they are ice crystals formed on nuclei of meteoric origin at a height of about 80 km where there is a temperature minimum. From estimates of the light scattered by these ice crystals it is deduced that they would be visible as noctilucent clouds.

The Attenuation of Light by Meteoric Dust in the Upper Atmosphere