Flights departing early in the morning are cancelled and delayed less often than those departing later. The reason is how disruption accumulates over the course of a day.
Aircraft are already in position overnight
An early departure usually flies an aircraft that arrived the previous evening and stayed at the airport overnight.
Later flights depend on an aircraft arriving from somewhere else that morning, so any delay to that inbound flight transfers directly to the departure.
The first flight of the day is the only one with no inbound dependency, which removes the single largest source of delay.
Crews start their duty rather than continue it
Flight and cabin crew operate under duty time limits designed to manage fatigue, and those limits are absolute.
A crew starting the day has its full duty period available, while a crew late in a disrupted day may run out of legal hours before completing a rotation.
Cancellations caused by crew hours cluster in the evening for exactly this reason, and they cannot be resolved by waiting.
Delay propagates through the day
Airline schedules are built with limited slack, since idle aircraft earn nothing and utilisation drives the economics.
A morning delay is therefore absorbed only partially and passes into the next flight, growing as it moves through a rotation of four or five sectors.
By evening the accumulated delay across a network is at its highest, which is visible in on-time statistics broken down by departure hour.
Weather patterns are not neutral
Convective storms build through the afternoon in many climates as the surface heats, so thunderstorm disruption is concentrated later in the day.
Early morning fog is the counterexample and can be severe, but it affects specific airports and seasons rather than the whole network.
On balance the afternoon exposure is larger and more widespread, particularly in summer and in continental interiors.
The trade-offs are real
Early flights require a start that often costs a night's sleep and sometimes an extra night's accommodation near the airport.
They also leave fewer options on the day if something goes wrong, since a rebooking onto a later flight is available while the reverse is not.
For connections, tight schedules and anything that cannot be missed, the reliability advantage generally outweighs the inconvenience, which is why experienced travellers book them.