Off put from the engine testing. NASA just happens to use Liquid Hydrogen as the fuel for some of their rockets with Liquid Oxygen as an oxidizer to make it burn. Hydrogen is a fantastic rocket fuel. This fuel combination of course means that its exhaust is H2O. There's also some rockets that use kerosene, and rockets that use solid fuel, but the hydrogen based fuel is really efficient compared to other rocket fuels. It really is just a chemical coincidence that the best rocket fuel also makes rain.

Watch this amazing video of artificial rain created by NASA. : videos Friday, December 13, 2013 @ 11:28pm