Atmospheric Turbulence Measurements at Mount Wilson Observatory

Simultaneous measurements of atmospheric temperature fluctuations at various altitudes and locations at Mount Wilson Observatory provide a quantitative description of local turbulence characteristics. The average rms value of the temperature fluctuations at an altitude of 150 feet is found to be 26% of the rms value at 9 feet, showing a substantial decrease with altitude for temperature fluctuations close to the ground. These rms values are found to be more heavily dependent on wind speed than on time of night, in contrast to prior belief. For measurements made close to the ground, the power spectra of the temperature fluctuations closely fit the Kolmogorov-Taylor prediction of -1.67 at frequencies up to 8 Hz, although at higher elevations some discrepancy is observed: the mean slope at 70 feet is approximately -1.50. The average size of major turbulent eddies is found to depend on wind speed, varying approximately linearly from about 1 to 25 m in both the horizontal and vertical directions for wind speeds between 0 and 8 m s-1. Analysis of the correlation between horizontally separated sensors indicates that Taylor's approximation remains useful on timescales of 10-15 s, after which time the turbulence changes. This suggests that local measurements of temperature fluctuations could provide some appreciable correction to variations in path length in interferometric observations of stars caused by atmospheric turbulence.