While a rocket is waiting on the platform during the final countdown, it often seems to have clouds of steam or something similar escaping from it.
Is this normal, and what are these clouds?
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This occurs when cryogenic boosters are used (Liquid Oxygen / Liquid Hydrogen) - as these warm up on the launch pad, some of the liquids boil and to release pressure, bleeder valves allow this gas to escape. If it remained within the boosters, there is a risk of the pressure blowing seals or damaging other components.
Most of what you can see is water vapour as it condenses around the O2 and H2, which are still extremely cold despite being in gaseous form.
Typical spacecraft fuels/oxidizers are super-cooled. The vapour visible is formed due to the extreme temperature difference.
The Apollo 11 flight journal has this to say
Filling these tanks with such cold contents requires a little finesse. Initially, the LOX is fed at a slow rate which furiously boils as it contacts the relatively warm tank structure. The vapourisation of the LOX takes heat away until a pool of liquid begins to form. When enough liquid has collected, filling steps up to a fast rate until the tanks are nearly full and the slow rate is reestablished to top them off. From then on, until three minutes before launch, the level is replenished as the volatile LOX continues to boil off from heat leaking into the tank.]