Balloons ascending simultaneously at various points around thunderstorms are followed by radar or radio-direction finding (rawins and rawinsondes) to determine horizontal inflow or outflow of the air. It is found that in the early stages of development of a thunderstorm cell, inflow or horizontal convergence is present at the ground as well as all heights reached by the cloud. In a mature thunderstorm cell, the observations show outflow or horizontal divergence under the cloud base and convergence at all heights between about 4,000 and 23,000 feet, with divergence again in the uppermost levels. Relationships to rainfall and to cloud entrainment of environmental air are shown.