The diurnal variation of atmospheric condensation‐nuclei

Records of the diurnal variation of the density of atmospheric condensation‐nuclei have been obtained over a period of four months. The apparatus employed consists of a Wilson cloud‐chamber with an expansion ratio of 1.15. At regular intervals the air in the chamber is replaced by a fresh sample, saturated with water‐vapor, and subjected to an expansion. A beam of light passing through the chamber is diminished in intensity by the resulting fog, and the degree of extinction may be related to the nuclei‐density. The diurnal variation in nuclei‐density shows a definite maximum in the forenoon and in the evening with a pronounced minimum prior to sunrise and a midday minimum of lesser extent. Such a variation might be expected in the normal daily convective cycle. The average hourly values of nuclei‐density have been compared with the hourly averages of the local component of the diurnal variation of the Earth's electric field for this Station. The results indicate that in a large measure, if not entirely, the variations in the local component of the Earth's potential‐gradient arise out of variations in the nuclei‐density.