What is a 990-c?

Why are steam burns on the skin so severe?

It is not really more. Steam is the gas form of water, and thus hotter than the liquid form. 1000C (at 1au at sea level) is the boiling point of water. 400C water will not burn you, but 990C will. Steam at 1010C will burn you, but steam at 5000C would burn worse. So your answer is steam is hotter, but how much more of a burn you get also has to do with the amount of steam or water you come in contact with, and the temperature the steam or water is.