What is a nuee ardent?
This is a geological term. It is nuee ardente, a French phrase meaning "burning cloud" (there is an acute accent over the first "e" in nuee). It is used to describe a particular type of pyroclastic flow. A pyroclastic flow occurs when a volcano erupts and sends streams of hot gasses and rock particles flowing down the sides of the volcano at extremely high velocities. Pyroclast is the technical term for solid rock material blown into the air from the volcanic blast. These flows are quite dense and behave as a liquid as they visibly flow down the terrain. The temperatures of these flows can exceed 1,000 degrees C and their velocities can reach 450 mph.
A pyroclastic flow that is termed a nuee ardente means that it appears as a burning red cloud as it flows down the volcano at night. The term was first used to describe flows from the eruption of Mount Pelee in 1902 in the French colonial territory of Martinique in the Caribbean Sea.
Nuée ardentes contain dense lava fragments that forms after a lava dome- a cold, viscous lump of low-silica high-iron lava- collapses. The French geologist Alfred Lacroix discovered it, which is why the term nuée ardente, meaning "glowing cloud," is French. Because of the collapsed lava dome of Mount Pelee, nuée ardente eruptions are also referred to as "Peléen eruptions."
"A more precise term would be glowing avalanche. The bulk of these hot block-and-ash flows hug the ground surface, but are disguised by an overlying cloud of fine ash particles..."
Here we have another name for it, then. Nuée ardentes are sometimes called block-and-ash flows because their deposits are made of blocks (lava fragments >64 mm) in a matrix of ash.