Fluxes of water vapor, heat, and carbon dioxide associated with a prescribed grass fire were documented quantitatively using a 43-m instrumented flux tower within the burn perimeter and a tethered balloon sounding system immediately downwind of the fire. The measurements revealed significant increases of temperature (up to 20°C), heat flux (greater than 1000 W m^–2), and CO₂ (larger than 2000 parts per million by volume) within the smoke plumes, as well as an intensification of turbulent mixing. Furthermore, the observations revealed an increase in water vapor mixing ratio of more than 2 g kg^–1, or nearly 30% over the ambient air, which is in good agreement with theoretical estimates of the amount of water vapor release expected as a combustion by-product from a grass fire. These observations provide direct evidence that natural fuel-load grass-fire plumes may modify the dynamic environment of the lower atmosphere through not only heat release and intense mixing, but also large addition of water vapor.