Optical effects of thermal structure in the lower atmosphere

Ascending convective plumes of inhomogeneous warm air interspersed with regions of air that are remarkably free from temperature fluctuations are sometimes observed in the lower layers of the atmosphere. A close correlation is demonstrated between intervals of good optical seeing along an upwardslanting path 20 m long and such periods of below average, air-temperature fluctuation. This correlation is sensitive to the azimuthal angle between wind direction and the vertical plane containing the optical path. The occurrence of temperature-quiescent periods at a given height is also shown to require the horizontal wind speed to be less than a critical value, and there is evidence that the thermal structure tends to be elongated in the direction of the wind. The range of amplitudes of image dancing observed in these experiments is comparable with that encountered in solar observations. The atmospheric temperature structure coefficients calculated from these optical experiments compare favorably with independent direct measurements.