The effect of wind upon the Earth's electric field at the surface

The effect of wind upon the potential-gradient has long been a subject of study and speculation. Probably the most significant fact so far observed is that stated by C. W. Allen,1 namely, that on days of continuous wind the diurnal variation of the gradient at Mount Stromlo, Australia, approaches the unitary, or world, variation observed over the oceans, while on days with calm the departure from the variation over the ocean is much greater. In a recent paper by the author2 a method of obtaining the local diurnal-variation by eliminating the unitary-variation from mean curves was adopted and a theory of the local variation based upon the daily cycle of turbulence, convection, and subsidence in the atmosphere was proposed. Allen's observation is of considerable interest in connection with this theory, for if the wind blows continuously throughout the 24 hours the constant turbulence due to the wind should greatly reduce the effect of the turbulence and convection due to the heating of the surface and of the subsidence at night. On the other hand, days with calm nights and no general wind should allow these processes to take place with the greatest effect upon the gradient. To examine this result more fully, the mean local-variations of the days of continuous wind and the days with calm at Mount Stromlo have been computed using the method referred to above. These are shown in graph A of Figure 1 along with the local variation computed from all electrically-quiet days. It is evident from these curves that wind is an important factor in determining the amplitude and form of the local variation; it is believed that the differences here found are of the sort to be expected from a consideration of the proposed theory of the local variation.