In Star Trek, a thermobaric cloud is a type of energy formation that defined the perimeter of the Delphic Expanse. The clouds were several million kilometers thick, and took at least six hours to cross at sublight speeds. (ENT: "The Expanse") The thermobaric clouds permitted entry into the Expanse, but prevented ships from leaving. The clouds were almost certainly a product of the spheres found in the Expanse. (ENT: "Anomaly")

The Vulcan cruiser Seleya attempted to chart the thermobaric perimeter, but it was caught in a subspace eddy and pushed into the Expanse. They were never heard from again, until Enterprise (NX-01) found them adrift inside an asteroid field. (ENT: "Impulse"). When the sphere network was destroyed in early 2154, the cloud barrier dissipated. (ENT: "Zero Hour")

Thermobaric clouds allude to the many different forms of clouds that are found in the real universe. Among the largest of these discovered in April 2007 - a cloud of intergalactic plasma that stretches more than 8 million light years across - or about 80 times the width of our galaxy in the general neighbourhood of a dense collection of galaxies called the Coma Cluster, which lies 300 million light years from Earth. The cloud contains several galaxy-sized black holes.

Then there is the molecular clouds - sometimes called a stellar nursery if star formation is occurring within. This is a type of interstellar cloud whose density and size permits the formation of molecules, most commonly molecular hydrogen (H2). Giant molecular clouds (GMCs) are huge complexes of interstellar gas and dust, composed mostly of molecular hydrogen but also containing many other types of interstellar molecule. GMCs are the coolest (10 to 20 K) and densest (106 to 1010 particles/cm3) portions of the interstellar medium. Stretching typically over 150 light-years and containing several hundred thousand solar masses of material, they are the largest gravitationally bound objects in the Galaxy and, in fact, the largest known objects in the universe made of molecular material. Molecular clouds are the only places where star formation (and planet formation) is known to occur.

There are also the Magellanic clouds, which are actually irregular dwarf galaxies, which are members of our Local Group of galaxies. The Large Magellanic Cloud and its neighbor and relative, the Small Magellanic Cloud, are visbible from the Southern Hemisphere as misty patches in the night sky, like separated pieces of the Milky Way. Together with at least two other satellite galaxies of the Milky Way, the Draco Dwarf and the Ursa Minor Dwarf, they move within an enormous river of hydrogen gas known as the Magellanic Stream. The Large Magellanic Cloud is located approximately 160,000 light-years from Earth. It's about one-twentieth as large as our galaxy in diameter and holds about one-tenth as many stars. The Small Magellanic Cloud is located around 200,000 light-years from Earth. It's about ten times smaller than its companion and a hundred times smaller than the Milky Way.

Then there is the Solar System's Oort cloud - a hypothetical spherical cloud of comets. The outer extent of the Oort cloud defines the gravitational boundary of our Solar System.

The term thermobaric is derived from the Greek words for "heat" and "pressure". The term is used today to refer to weapons and explosives that use the gaseous products (H2, H2O, CO and CO2) of an initial explosion for an afterburning of reactive solids. Because their reaction with atmospheric oxygen only produces solid oxides the blast wave is primarily generated by heat of combustion ("thermobaric") instead of expanding explosion gases. This makes thermobaric explosives more effective in oxygen deficient environments such as tunnels, caves or underground bunker.