Originally Posted by
MSPeconomist
Do they take account of the climate and weather at the origin and destination of the flight or (more sophisticated, but would be a better predictor of passenger's winter clothing) of each passenger's itinerary including connections?
No, all major airlines just use a standard assumed weight for passengers and carry-ons together. In the winter they add something like 5 or 10 pounds to the average based on the idea that passengers have more "weight" on their person and in their carry-ons due to heavier clothes in the winter.
The overall average figure has been revised upward over time, but it may be due for another given the continued expansion of waistlines as well as heavier carry-ons due to checked bag fees. I think the last revision was around 2006.