The authors have studied the diurnal variation of irregular geomagnetic fluctuations in moderately low latitudes using the eight daily K numbers (three-hour-range indices) for the seven years 1940–1946 and for the six observatories Alibag, Watheroo, Honolulu, Tucson, San Juan, and San Fernando. These observatories were chosen because they are fairly uniformly distributed in longitude, being at 73°, 116°, 202°, 249°, 294°, and 354° east, respectively.

A local time variation, to be expected from the work of earlier investigators, is apparent also in these data. The present data indicate a minimum near sunrise and a maximum in the late evening. This diurnal variation appears to have a small seasonal change.

When the local time variation is deducted from the original data for each observatory, the remainders suggest a universal time variation which exhibits a pronounced seasonal change.

A hypothesis is developed to explain the nocturnal prevalence of the irregular fluctuations in terms of atmospheric turbulence in the ionosphere. The turbulent motions are regarded as hindered during the day by electromagnetic damping. It is suggested that features of the large-scale circulation of the atmosphere may contribute to the universal time component.