Electric potential gradient and atmospheric opacity at Kew Observatory

It has long been known at Kew Observatory that thick fog is generally accompanied by a high value of the electrical potential gradient. During fog the oscillations of potential, which are common even in the electrically quietest of days, are sometimes very large; but whether they are exceptionally large, when expressed as a percentage of the mean gradient for the hour, has not been investigated. There is this peculiarity, however, that sometimes during fog the gradient oscillates between a positive and a negative value, a phenomenon which is quite exceptional except during rain or a thunderstorm. It occurred to me to examine the records for a series of years to see whether any information could be derived as to the existence or formation of fog. The appearance of the electrogram during heavy fog is usually so characteristic that from it alone one could say with a close approach to certainty that fog existed. Thus electrograms may be of considerable value as confirmatory evidence of fog when direct evidence is lacking. But the influence of light fog or mist does not seem sufficiently regular and dominant to encourage the hope that potential gradient curves by themselves can serve satisfactorily as a prognostic of fog.