The primary destructive effect of a FAE weapon is not the fireball. The first paragraph in the
FAS.org link provided by Veez states,
"The rapidly expanding wave front due to overpressure flattens all objects within close proximity of the epicenter of the aerosol fuel cloud, and produces debilitating damage well beyond the flattened area. The main destructive force of FAE is high overpressure, useful against soft targets such as minefields, armored vehicles, aircraft parked in the open, and bunkers."
It is the tremendous Overpressure front, traveling in excess of 3 km per second for the larger models, which flattens
everything in the immediate blast area and rendering tremendous amounts of damage to anything even remotely close to it. In the open or against civilian targets, the Fireball is almost of an inconsequential effect to the immediate blast area as anything close enough to be incinerated will have already been absolutly battered. Personel in protected positions will suffere tremendous internal damage (any body part that contains air or open spaces taking the brunt of the force) the effect being magnified by being inside hardened facilites, Bunkers, Troop Transports, Body Armor as the rigid nature of the structures acts as a force channeling device.
Thermobaric weapons, FAE's designed and tasked to be used against cave systems and hardened facilities have a two pronged use. The primary blast creates a shockwave that travels down the enclosed cave systems crushing and battering everything inside. Much like a pop-gun will shoot a cork out one end the shockwave will smash into anything not nailed down but unlike the pop-gun the cork (personell) have only more cave system to crash into and not open air. The secondary effect the FAE device has is the consumption of all the oxygen in the area of the blast effect. Combine that with the resultant vacuum from such a tremendous explosion, any remaining survivors inside the caves ( if not pulled right back out of the earthworks) will be asphyxiated when the atmosphere retreats to fill the hole left by the rapidly expanding explosion at the entrace to the cave.
-PFC joe