the groups of equations (1) and (2) allow determination of the radon concensration from the alpha activity of the filter by means of a computer. Ve EXPERIMENTAL RESULTS We present a certain number of radon profiles obtained during several sets of measurements. A meteorological study made with the help of daily measuremerts of the Weather Bureau plus measurements on board the aircraft (humidity, wind and temperature gradient) allow interpretation of the results. A. Horizontal Flight On May 9, 1966 we made a horizontal flight at 500 meters altitude between Mont-de-Marsan and Toulouse (150 km over a region cut with valleys and of the same geologic origin). The south of France is normally in a flow from the north=- west, a cold circulation at high altitude maintains a strong unstable activity above France. The region surveyed was situated at the edge of the disturbance. There was partial cloud cover of low cumulus. Curves 1 and 2 of Figure 3 show the variations in filter activity during the trip out and the return. Curve 3 shows the alpha activity of the filter for the case where the radon and daughter concentrations remair constant during sampiing. The three curves are identical indicating a constant radon concentration during a flight. There were no significant variations due to uneven emanation from the soil. On June 6, 1966 this same region was in a flow from west, south-west. Figure 4 shows the concentration profile at the beginning of afternoon at around 70 km from the coast. At 1500 meters the rapid decrease of radon concen- tration indicated a non-uniformity of emanation from the source plane. emanation above the ocean is negligible. The The air takes up radon passing across Fs TREE GS SEPeTSO eter petey Influence of an Emanation Discontinuity + Be