Theoretical considerations and empirical evidence indicate that atmospheric turbidity, a function of aerosol loading, is an important factor in the heat balance of the earth-atmosphere system. Turbidity increase over the past few decades may be primarily responsible for the decrease in worldwide air temperatures since the 1940's.

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Observations of a considerable number of vertical-velocity spectra are combined to show that under near-neutral and unstable conditions, the shape of the spectrum is essentially invariant up to heights of order 100 m, and the ' scale ' of turbulence is proportional to height within this regime.A semi-empirical working hypothesis is suggested which relates the spectrum to wind speed, lapse rate, height and roughness.

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SUMhWRYThe properties of spectra of vertical and horizontal turbulence and of their cross-spectra, at elevations near 100 m, are summarized ; a few spectra from lower levels are considered. The following tentative conclusions are drawn :At frequencies of the order of 100 c/hr and above, the spectral intensity is proportional to the square of the mean wind speed and essentially independent of the radiation intensity. At lower frequencies, the ratio of spectral intensity to the square of the wind speed increases with increasing incoming radiation.2. The intensity-ratio of the co-spectrum of vertical and horizontal velocity to the spectrum of vertical velocity increases with increasing wind shear. It also decreases with increasing frequency, n, approximately as n-'. 3. At low frequencies, the spectral intensity of the horizontal velocity components is generally much larger than that of the vertical velocity. At high frequencies, the spectral intensities of all velocity components are of the same order of magnitude. At low frequencies, the ratio of vertical to horizontal spectral density increases with increasing radiation.The quadrature spectra at a given frequency tend to be negative with large radiation, positive with little radiation. This indicates that eddies are low and wide with little radiation, and tall with much radiation.The frequency at the maximum of the vertical velocity spectrum decreases with increasing height and increasing radiation intensity.The ratio of turbulent to mean energy is somewhat larger for air with over-land trajectories than for air with oceanic trajectories. 1. 4. 5.6.