Roy Mustang

Roy Mustang (ロイ・マスタング, Roi Masutangu), the Flame Alchemist, is a State Alchemist and officer in the Amestrian State Military. A hero of the Ishval Civil War and Edward Elric's superior officer, Colonel Mustang is a remarkably capable commander who plans to become the next Führer of Amestris.
Even among the elite State Alchemists, Colonel Mustang is a particularly skilled alchemist adequately versed in standard physical transmutation, experimental theory and biological alchemy. However, the brilliant skill that has earned him his state license, national notoriety and the status of "hero" during the Ishval Civil War is his remarkably powerful fire-based combat style. Mustang's unique style of combat alchemy is made possible by his custom made gloves. Made of "pyrotex/ignition cloth" and embroidered with unique flame alchemy Transmutation Circles, these gloves create a spark when Roy rubs his fingers together and allow him to manipulate the concentration of oxygen in the surrounding air, raising its density to a level at which it becomes combustible and creating pathways of oxygen through which he can direct the ensuing flame birthed by the spark. The 'snap' sound is caused by the instantaneous speed at which the gases and sparks react together and pop (not his fingers actually 'snapping', to common belief.)
While Mustang's Flame Alchemy cannot be used on rainy days because the humidity makes it impossible for his gloves to produce sparks, it becomes clear that water itself does not make the Colonel entirely as "useless" as his subordinates assume. Using the transmutation circles on his gloves, Roy is able to separate oxygen atoms from the hydrogen atoms in water molecules, isolating the two combustible gases. Though this method can only be used when a large quantity of water is present in a relatively closed-off space (and not in the rain where the air is open and separating hydrogen and oxygen for the purpose of combustion would pose far too much danger to anyone in the vicinity), with an outside source of ignition such as a cigarette lighter or a match, Roy can produce extremely powerful, localized explosions.