Simultaneous measurements of radon concentration, wind speed and direction near the surface, and atmospheric mixing height within the first kilometer of the planetary boundary layer were made at a beach site on the U.S. Gulf Coast during November--December 1978. Vertical distribution of temperature, humidity, and wind also were available four times daily at approximately 6-hour intervals from two standard rawinsonde stations about 80 km on either side of a beach station along the coast. The mixing height was determined mainly from an acoustic radar (sounder) recorder, but these measurements were supplemented by rawinsonde data. The mixing height as determined by the sounder was verified at least once a day by the rawinsondes before it was correlated with the radon concentration. It was found that after the passage of cold fronts, the meteorology was dominated not only by cold, dry, offshore wind conditions but also by a well-behaved diurnal cyclic variation of both radon concentration and the mixing height. The radon concentration was found to have an inverse linear dependence on the mixing height, with an unexpectedly high linear correlation coefficient of about -0.96 and conversely to be much less dependent on wind speed than was expected.