The first bank of departures each morning is more reliable than anything later in the day. The advantage comes from the state of the network at dawn rather than from the weather being kinder.
Aircraft begin the day where they finished
Overnight, most aircraft sit at the airport where their final service landed. The morning departure therefore starts with the aeroplane already parked at the stand it needs.
A midday flight, by contrast, depends on an aircraft arriving from somewhere else. That inbound leg is a dependency, and every dependency is a chance for the schedule to slip.
The same applies to crews, who are rested and positioned at base rather than working their way through a sequence of legs across the country.
Delay accumulates through a rotation
An aircraft typically flies several sectors in a day, and the turnaround between them is short by design. There is little padding for anything to go wrong.
A modest delay on the first sector does not disappear. It is carried into the next departure and the one after, growing as each turnaround fails to absorb it.
By late afternoon a fleet can be running well behind, which is why cancellations cluster in the evening as airlines cut flights to reset the following morning.
Weather has a daily rhythm
In warm seasons convective storms build through the afternoon as the surface heats, which is why thunderstorm disruption is concentrated in the second half of the day.
Morning conditions are more often stable, and the main early hazard is fog, which tends to be local and short-lived rather than a spreading disruption.
Airports also run at lower traffic density early on, so when something does go wrong there is more room in the sequence to recover from it.
Crew duty limits bite late
Flight and duty time are legally capped, and the limit applies to the whole working day rather than to any single sector.
A crew that has absorbed several hours of delay can reach that ceiling before the final flight operates, at which point the service cannot legally depart.
Reserve crews exist for exactly this situation, but they are finite. Once the pool is used, a cancellation is the only remaining option.
What the early slot does not protect against
Nothing about a dawn departure helps if the aircraft has a technical fault or if the destination is closed. Reliability is improved, not guaranteed.
An early flight also leaves fewer options behind it in the day if something does go wrong, since the traveller is at the start of the schedule rather than near the end.
The practical value is on connections: a first-wave departure gives the whole day for recovery, which is worth more than the modest inconvenience of the alarm clock.