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It’s a clear sky over San Francisco, with no clouds in sight, and there’s an audible groan as your captain announces that your aircraft’s approach is on hold due to weather conditions. It’s not the pilot, the aircraft, or the airline – there’s a lot more that is going on under the hood of your cross-country trip than what meets the eye. Orbiting somewhere over the Bay Area, you and the117-foot wide aircraft supporting you are waiting for clearance from the ruler of the airspace  you’re inhabiting – the air traffic control tower.