An electrometer was developed to measure the voltage on a copper screen used as a passive antenna 1 m above the ground. An array of three antennas, arranged first downwind and then crosswind, provided data concerning the time variation of the atmospheric potential gradient in the range of frequencies between 0.004 and 0.06 Hz. Results indicated that the fluctuations in the field are primarily associated with drifting clouds of space charge 135-425 m in diameter that are near the level of the antenna. The drift velocity is essentially equal to the wind velocity. The rate of variations in the electric field associated with processes effecting changes in the distribution of space charge density within the clouds is about one half the rate associated with the drifting clouds or about one third of the locally observed rate of change in the potential gradient. The time scale for the field fluctuations is about 62 s, and the spectral estimates are proportional to f-•"•', where f is the frequency. Fluctuations in the fair-weather atmospheric potential gradient have been associated with clouds of space charge drifting with the wind. Whitlock and Chalmers [1956] separated two field mills by 500 m on a line parallel to the wind and evaluated a velocity by comparing the time difference between corresponding maximums on the two sets of records. The velocities calculated in this manner were generally larger than the wind velocities obtained from an anemometer at an adjacent observatory. Large[1957] measured the potential gradient with a copper sphere mounted on a pole 7 m high. The observed amplitudes averaged 10 V/m, and the periods, peak to peak, averaged 40 s. Cross-correlation functions were obtained for two records

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The kinematic viscosity, η/ρ, where η is the molecular viscosity and ρ is the density, becomes indefinitely large as the density approaches zero, when η is computed according to conventional concepts. A re‐examination of Maxwell's kinetic theory development of molecular viscosity suggests a change in the limits of integration that leads to a correction term. The resulting equation indicates that the kinematic viscosity reaches a maximum in the F‐region and approaches zero with further increase in height.

A correlation method requiring six values of the correlation coefficient is developed. Expressions for the drift velocity, fading velocity, and characteristic velocity are obtained from a correlation theory extended to include an elliptical contour in the horizontal plane. The physical significance of the derived velocities is considered. Preliminary data indicate that the correlation ellipse exhibits a preferred orientation and that fading associated with random changes is as important as fading associated with a drifting pattern.