[Robert DeLong]

From what I am reading they involve 2 explosions. The first explosion releases gas clouds, and then a second explosion occurs which ignites the gases, as seen in this video (advance video until the the 7:52 mark):

Testing Fuel-Air Explosives / Thermobaric Bomb |

So, if I am correct it would have to explode pretty close to the ground to disperse the gases over a considerable amount of area to do significant damage. But I know not how they truly work, just for doing a quick research.

[Boogieman]

The burning gases don’t do the majority of the damage. The shockwave created by the rapid expansion of the gases as they ignite causes the real damage.

[Vermont Lt]

It’s not the “explosives” that cause the damage. The pressure under the explosion exerts very high pressure downward. That causes physical damage to buildings and what not. Then, as the pressure recedes it causes a vacuum to form—and that sucks the atmosphere up into the fireball. It could suck your lungs dry.

Look up MOAB Or Daisy Cutters.

These things are BIG.

[Liaison]

You forgot that after the flash over explosion of the the gas that displaced the surrounding air, a vacuum is left and the air rushes back with a vengeance.