On the diurnal variation of the potential gradient of atmospheric electricity

New observations obtained aboard the Carnegie during the year ending November, 1921, increased by 50 per cent the observational data regarding the diurnal variation of the potential gradient over the oceans. A separate analysis of the new data has confirmed the results announced by the author in 1921 regarding the predominance, over the oceans, of a 24‐hour wave progressing approximately according to universal rather than local time. It is found, however, that at least in the Pacific, there is also a well‐defined secondary wave during the months of northern summer. The present investigation has been extended to include also the diurnal variation of the potential gradient over the several continental areas. From harmonic analyses of the diurnal variation from many widely separated stations, varying in latitude from 78° north to 77° south, it is found that, despite the greater complexity of the variation over land, a large percentage of the stations considered show sufficiently good agreement with the ocean results to furnish strong evidence for the assumption of a world‐wide effect of 24‐hour period progressing approximately according to universal time. The average amplitude of this wave is about 20 per cent of the mean‐of‐day value and the average time of its greatest phase apparently varies somewhat with time of year, ranging from about 16h to 19h G. M. T. While there are land stations in the tropical and temperate regions for which the analytical results are not in accord with the foregoing, there is considerable evidence indicating that this may be due to local disturbances of large amplitude rather than to an absence of the general phenomenon. The fact that for ϕ2, the phase angle of the 12‐hour wave at local midnight, marked departures from the all‐stations mean are largely confined to places where the amplitude of the secondary wave is relatively small lends further support to the view that the limitations of harmonic analyses in the form here used may be partly responsible for the lack of more complete agreement between the land and ocean results.