For security and convenience reasons, Air Force One can be refueled in the sky by other planes. If there's a reason why it isn't safe to land on the ground, the US can't afford to take the chance of the President's plane running out of fuel and being forced into an emergency landing.
The tank capacity of this impressive plane is fifty-three thousand gallons. Once it's been filled up, it can stay in the sky and wait for the danger to pass, or turn around and go home.