An airport's capacity is not measured in terminals or gates but in movements per hour on its runways. That number is set by physics and separation rules rather than by ambition.
Separation is the real limit
Every aircraft leaves a rotating trail of disturbed air behind its wingtips. A following aircraft must wait for that wake to weaken, and the wait depends on the weight of both machines.
A small aircraft following a large one needs the longest gap of all, because the wake it would meet is strong and its own control authority is limited.
Sequencing therefore matters enormously. Grouping similar aircraft together shortens the average gap, so a well-ordered stream moves considerably more traffic than a randomly mixed one.
Arrivals and departures compete for the same surface
On a single runway, every landing consumes a window that a departure could have used. Controllers interleave the two flows, releasing departures into the gaps between arriving aircraft.
Arrivals usually take priority because an aircraft in the air has limited fuel and fewer options. Departures can wait on the ground far more cheaply than an aircraft can hold overhead.
Airports with parallel runways can separate the flows, but only if the runways are far enough apart to be treated as independent. Closely spaced pairs are still partly dependent on each other.
Slots turn capacity into a scheduling currency
Where demand exceeds runway capacity, the hourly total is divided into slots, and airlines schedule against those slots months ahead of the season they cover.
A slot is a permission to use the whole airport system at a stated time, not merely the runway. It carries the associated stand, terminal and handling capacity with it.
Because slots at the busiest airports are effectively finite, they acquire real commercial value, and airlines guard historic holdings closely rather than surrender them to a competitor.
Weather rewrites the plan constantly
Low cloud and poor visibility force wider spacing between arrivals, since aircraft must be established on the approach earlier and the runway cleared more conservatively.
That reduction can be substantial, and it arrives without notice. A schedule built for a clear day suddenly has more flights than the runway can accept.
Flow control then meters aircraft before they depart their origin, holding them at the gate rather than in the air. Delay is cheaper on the ground and easier to unwind.
Recovery takes longer than the disruption
Once a queue forms, it does not clear the moment conditions improve. The backlog has to be worked off at the same hourly rate that created it.
Crew duty limits compound the problem, because a delayed rotation can run a crew out of legal hours and ground an otherwise serviceable aircraft for the night.
This is why a few hours of fog can distort an airport's schedule into the following day, long after the sky has cleared and the runways are dry.