Thermobaric clouds are a hazardous type of gaseous formation which can be extremely dangerous to starship navigation.
The region of space known as "the Delphic Expanse" was surrounded by a thick layer of thermobaric clouds which made it extremely difficult for ships to enter and exit, and took at least six hours to traverse at sublight velocities. In 2153, the Vulcan starship Seleya attempted to chart the thermobaric perimeter of the Expanse. After several days, they encountered a subspace eddy and were pulled inside. Later that year, the Earth starship Enterprise NX-01 penetrated the perimeter in their mission to enter the Expanse and locate the Xindi, a species which had launched a devastating attack on Earth. While traversing the clouds, Enterprise came under attack by three Klingon Bird-of-Prey, under the command of Duras, son of Toral, who was attempting to apprehend Captain Archer. In an effort to elude the Klingons, Archer ordered Enterprise's speed increased to full impulse, despite the fact that the ship's intake manifolds were experiencing difficulty in processing the gases. In addition, the thermobaric clouds prevented the Klingons from acquiring a precise weapons lock on Enterprise, necessitating a reduction in the distance between the ships. Eventually, two of the Birds-of-Prey retreated, their crews fearful of entering the Expanse, and Archer and his crew were able to destroy the last attacking vessel, immediately before gaining access to the Expanse. (ENT: "The Expanse", "Impulse")
The thermobaric clouds surrounding the Delphic Expanse were dissipated in 2154, upon the destruction of the Sphere network which was responsible for the unusual properties of that area of space. (ENT: "Zero Hour")
In the final draft script of "The Expanse", the thermobaric clouds were commonly referred to as "murky perimeter" and were described as "a VAST WALL of dense, murky clouds. Their grey-brown surface is rippling with turbulence." The clouds were also referred to as "eerie", "roiling", and "turbulent".