"It"s a symptom of something extremely dangerous happening on the ground," says Fromm. "Because to get a cloud like this, you need some crazy intense fire behaviour. A pyroCb is at the very limit of how high, cold and opaque a cloud can get, and means that the fire which is causing it is at the extreme limit of intensity too. And that means the fire itself is more intense, more unpredictable, and more hazardous than fires in a more lazy state."