Radon (222Rn) measurements provided a simple, reliable, in situ indicator of the continental component of the resident air masses during marine fog events off Nova Scotia in August 1975. Frequent and substantial changes in the continential component of the air, including an increase from 5 pCi m-3 (mostly maritime air) to 80 pCi m-3 (continental air) within 6 hours on August 5 at a location 100 km off shore illustrate the variations in the nature of over-ocean air and the importance of continuous sampling of the surroundings during shipboard atmospheric experiments. The recent 1- or 2-day histories of the air for several fog events were investigated by relating the air mass movement as suggested by weather maps to the measured radon concentration. The reliability of determination of atmospheric radon concentrations by measurement of the beta radioactivity of daughter products collected on filter paper was verified as being applicable in fog.