Originally Posted by
Mykle
It has nothing to do with cargo. It's because the main runway is under construction, so they don't have as much room to take off, thus they are limiting the number of passengers.
That would be true if the flight were the reverse, FLL-DCA.
On the performance aspects the other answers are correct - density altitude increases with temperature as well as altitude, so a basically sea level airport would be at quite a bit higher density altitude on a very hot day. In less dense air the wing must make up for the loss of lift with increased speed, which equals more runway required for takeoff. Add in the effect that the higher density altitude has on engine performance and even more runway is required.
There is a very slight chance that weather in S Florida may have required a greater amount of reserve, holding, alternate fuel which would lower the allowable payload for takeoff but with that length of flight it's probably the DCA temps that caused the problem.
Jim