On very hot days at certain airports, flights depart with seats empty or cargo removed. The cause is air density, and its effect on how an aircraft gets off the ground.
Lift and thrust both depend on density
Wings generate lift by accelerating air, and engines produce thrust by moving mass through them. Both processes weaken when the air contains fewer molecules per unit volume.
Heat expands air and reduces its density, so a hot day behaves aerodynamically like a higher altitude even at the same field elevation.
The combined effect is described as density altitude, and it can make a low airport perform as though it sat thousands of feet higher.
Takeoff distance grows quickly
Reduced thrust means slower acceleration, and reduced lift means a higher speed is needed before the wing will fly. The takeoff run therefore lengthens at both ends of the calculation.
Runway length is fixed, so when the required distance approaches what is available, something has to change. Weight is the variable the operator controls.
Climb performance after takeoff is also reduced, which matters where terrain or obstacles must be cleared on departure.
Weight is removed in a set order
Operators generally reduce fuel first where the route allows, then cargo, then finally passengers, because each reduction has different commercial and operational consequences.
Removing fuel may require a technical stop en route, which is why some flights on hot days take an unexpected intermediate landing to uplift what could not be carried from the departure airport.
Cargo comes off next because it can usually travel on a later service without the complications of rebooking people. Passengers are offloaded last, and normally only when the shortfall is large.
Timing is the cheaper solution
Because temperature peaks in the afternoon, airports prone to this schedule their most performance-limited departures for early morning or late evening.
Travelers on desert and high-elevation routes often notice that long-haul departures cluster at hours that seem inconvenient, and this is usually why.
The schedule is written around the atmosphere rather than around passenger preference.
Aircraft choice absorbs the problem
Operators serving hot or high airports select types with performance margin, accepting higher costs elsewhere in exchange for reliability in these conditions.
An aircraft that is economical on a cool coastal route can be unusable on a hot inland one, which is part of why fleets are assigned to route groups.
The result is that geography and climate quietly determine which aircraft a traveler ends up flying on.