[2] The weather can cause large variations in capacity; strong winds may limit the number of runways available, and poor visibility may necessitate increases in separation between aircraft.
However, aircraft flying in circles is an inefficient and costly way of delaying aircraft, so it is preferable to keep them on the ground at their place of departure, called a ground delay program.
[1] This way, the delay can be waited out on the ground with engines off, saving considerable amounts of fuel.
All IFR flight plans are tracked by the Central Flow Management Unit (CFMU).
Every airport and air traffic control sector has a published maximum capacity.