A fumarole, also called a steam vent, occurs when a hydrothermal feature has so little water in its system that the water boils away before reaching the surface. Steam and other gases emerge from the feature's vent, sometimes hissing or whistling. Steam vents are often superheated, with temperatures as high as 238°F (114°C).
Black Growler Steam Vent: Located in Norris Geyser Basin, Black Growler is one of Yellowstone's most famous steam vents and has been measured at nearly 300°F (149°C). This feature has a history of shifting its location several times. It has been active since 1878 at least, often roaring in a noisy stream of hot vapor.