At the Dome, the reunions have been ended by an apocalyptic party-crasher. Those on the inside have something more important than relatives to occupy them now: the giant mushroom cloud that’s growing to the northwest of their position, rising on a muscle of fire already almost a mile high. The first feather of wind—the wind that has sent Carter and Big Jim fleeing for the fallout shelter—strikes them, and they cringe against the Dome, mostly ignoring the people behind them. In any case, the people behind them are retreating. They’re lucky; they can.
Question: What’s the meaning of “mostly” in this case?