I flew to Wisconsin over the weekend. Leaving Milwaukee on Monday morning, my airplane was a smallish Embraer, three seats across, eighteen rows. That's a total of 54 people, a few less for the one-seat rows near the front.
The temperature was in the low to mid seventies. Milwaukee's altitude is 581.2 feet, pretty close to sea level.
As the temperature goes up, the air thins and it becomes more difficult for a plane to take off. This is called density altitude: warmer air makes the plane perform as though it were at a higher altitude, and it can't carry as much cargo (bodies, luggage).
While we were still in the terminal, the airline personnel were asking two people to volunteer to take a later flight for $400 in vouchers. On the plane, they asked for another volunteer: $500 in vouchers. Plus they were rearranging the luggage in the forward and rear compartments to get a satisfactory weight and balance, the distribution that can cause flight problems. Two of us in the front seats were asked to volunteer to go to the back to help with the weight and balance. I volunteered to get the flight off the ground, and so did a man.
What I'm wondering is what will happen when it gets really warm. If they can't fly with all the seats full in seventy-degree weather, they will have to fly with many fewer when summer comes to Milwaukee. Not to mention Denver and Albuquerque, which are 5000 feet or so altitude.
So why did they cram so many seats in? What is the cost when they have to give vouchers to get people off the plane? Taking out one or two rows of seats might give the others a reasonable amount of room. But then people would be able to contrast that with the much less limited capability to squash people into sardine-like compartments in the larger jets. And we couldn't have that.