As everyone knows (and take care to remember) Roy is also known as the Flame Alchemist, code name given to him by Fuhrer King Bradley himself when he received his Certified National Alchemist license. As Roy was sent to the Ishval front in 1908, when he was 23 years old, we can safely state that he became a certified National Alchemist before that age.

Chapter 58 revealed that the person responsible for Roy's alchemy learning was none other than Hawkeye's father. Most facts are still unknown, but he was the man that taught all the basics to Roy and considered to rely the secret of his alchemy to him. Roy joined the military before that happened, in hopes of aiding his country as a soldier (and as an alchemist in the near future) and tried to convince his teacher to do the same, as a way to use his power for the good of everyone, through the military. Roy proved to be naive beyond hope at this point, to say the least. Life would teach him the hard way.

Roy's teacher, in fact, passed away before he could actually rely his most powerful alchemy to him. However, he somehow left all his knowledge within his daughter, Riza Hawkeye, and left it up to her to trust Roy with it or not. She eventually decided to confide him with her father's research, as Roy's dream for the future inspired her with newly found hope.

Going back to his alchemy. Even if his code name is "Flame", his transmutations have nothing to do with fire itself. Actually, his alchemy revolves around oxygen (but I guess Oxygen Alchemist wasn't fancy enough). He manipulates it to create a path in order to direct fire wherever he wants. Basically, what he does is splitting water molecules in the air for hydrogen and oxygen.

     

His gloves, then, have a double purpose. One is they have the alchemy array he designed sewn on them. That way, there's no need for Roy to draw it whenever he needs to use it, and as the transmutation is always the same, the alchemy array needed is always the same one too. The other purpose being to produce the fire. The gloves are made with spark cloth, Roy just needs to flick against it hard enough and it'll produce a spark.

So just flick, adjust the oxygen via alchemic transmutation and "boom". It would seem a very easy technique for alchemists to learn and apply, but the truth is Roy is the only one we know that uses it. And it seems his teacher was very careful choosing a successor for his "power". Why, then? Well, gases are extremely volatile, so a simple mess-up could end up blowing you up in little pieces.

There needs to be accurate control over the oxygen, something which probably is not affordable to every alchemist. Besides, if we consider the alchemy performed in FMA's world to be based on willpower (because, honestly, I don't know what else it could be) then Roy's strength of will is formidable, as he makes such a difficult task seem almost easy.

It's sadly become an understandment that Roy is "useless" in the rain. Let me clarify that the only thing that can get in Roy's alchemy's way during rain is his spark gloves getting wet. However, the spark Roy needs can come for literally anywhere, be it a lighter or a campfire. He just needs to carry around a lighter and the problem is solved.

Ironically, you could say that Roy is most deadlier with high concentration of water in the air or with water itself (like chapter 38 proved), since he can cause a chain reaction -- although perhaps it'd be harder to control the fire. Either way, I have no doubt that at this moment (clapping-hands alchemy or not) Roy is the most powerful alchemist there is in the FMA universe. With the possible exception of Izumi Curtis (Ed and Al's teacher).

More over, as you probably noticed, Roy's alchemy array includes a salamander. Well, the mythical salamander makes its home in fires, the hotter the better. Early travelers to China were shown garments which were said to had been woven of wool from the salamander, and so the cloth was completely unharmed by fire (it was actually woven from asbestos -- an incombustible mineral). Later, Paracelsus suggested that the salamander was the elemental of fire.

Aside from his alchemy specialty and the basics he was taught at the beginning, Roy has stated a couple of times how biological alchemy isn't something he's skillful at, but that he can do it regardless of it. He even manages to make a false corpse when trying to save Maria Ross.