What is a nuees ardantes?

Answer 1

A superheated incandescent cloud of gas and ash that swiftly travel down the volcano like an avalanche (also known as "the glowing cloud"). Scientifically, they are called pyroclastic flows or pyroclastic density currents.

Answer 2

When viscous magma, containing much gas, is erupted under reasonably low pressure, a glowing cloud containing ash and pumice may be thrown into the air, this cloud will fall back onto the earth like an avalanche before it can cool off. This avalanche is called a nuée ardente.