The white smoke that comes out of a rocket’s engine is just a cloud that can produce rain
It seems that scientists at NASA have accidentally found a way of creating man-made rain clouds. In a 13,500 acre-area, surrounded by a 125,000-acre buffer zone in the wetlands of Mississippi, the agency has built John C. Stennis Space Center (SSC), a special place for the testing of their rocket engines. These engines run on liquid oxygen and liquid hydrogen propellants and whenever they are tested, the combustion of these materials creates large clouds of steam that exceed 6,000 °F. The steam rises after the engine test and cools in the air, forming a cloud that floats over the city and can create rain nearby. The experiment's side-effect is delightful, and since the clouds are just water, the plumes aren’t polluting the atmosphere.
An RS-25 engine test at Stennis Space Center. Image: NASA