Abstract
Brilliant displays of the rare noctilucent clouds have recently been seen from central Scotland, and their height, position and speed have been determined from parallactic photographs. Observations of these clouds provide information concerning the physical state of the upper atmosphere at the height of about 80 km at which they invariably occur. Besides yielding direct measurements of wind, they indicate the existence of a temperature minimum at 80 km. Wind speeds of the order of 50 m/sec have been measured; StΓΆrmer has recorded speeds of more than 100 m/sec, and Jesse of about 300 m/sec. On every occasion when the clouds have been observed from Scotland, this movement has been from the eastβnorthβeast. But, since they are made visible by the sunlight scattered by the material of which they are composed, they are observed only on nights when the sun remains close below the horizon β that is, during the summer months in middle latitudes. Presumably they may be present at other times and in other latitudes, but they will not remain sunlit long enough to be observed. Their nature is still in doubt, though their tendency to appear at times of comet and meteor occurrences strongly supports the belief that they consist of meteoric dust. E. G. Bowen has recently pointed out that they appear either on or very near the dates of the daylight meteor streams discovered at Jodrell Bank, but Scottish observations show no such tendency. On one occasion, a portion of the clouds was observed to change colour from vivid blue to white, suggesting growth of the cloud particles by condensation. When opportunity offers, the new spectrograph at St. Andrews will be employed to investigate such changes in the spectrum. Finally, slides were shown illustrating a remarkably sudden change from their characteristically stable structure to quite chaotic shapes that occurred in the early morning of 25 July 1950. Noctilucent clouds have been observed on twentyβtwo nights since 1939; but this was the only occasion when turbulence was seen. It may be significant that it was also the only occasion when aurora occurred simultaneously with the noctilucent clouds.