The atmospheric electric conductivity and aerosol measurements during fog over the Indian Ocean

The atmospheric electric conductivity and aerosol size-distribution measurements made over the Indian Ocean on two occasions of fog occurrence during the XVI Indian Scientific Expedition, 1996–1997, are reported. The locations and the meteorological conditions associated with two fog episodes, one being a warm fog and the other a cold fog, are widely different. Values of both polar conductivities after showing a slight increase start decreasing about 1 h before the onset of fog, and then start increasing about 0.5–2 h prior to the dissipation of fog. The conductivity values then attain their fair-weather values in case of cold fog but continue to increase and give a maximum ∼2 h after the dissipation of fog in case of warm fog. Total number concentration of aerosol particles (between 13- and 1000-nm diameter) increases during the warm fog period but only just after the dissipation of cold fog. Aerosol size distributions show an increase in concentration of all particle sizes in warm fog but mainly in concentration of the nucleation mode particles in cold fog. Possible causes of the changes observed in conductivity and aerosols and their utility in forecasting the onset and dissipation of fog are discussed.