You don't know what to wear.
Maybe you're going from one climate to another.
Things to know:
*Airports are typically kept at medium temperatures: they'll feel pretty much the same in the winter and in the summer. I've been in a few low-budget terminals in southeast Asia, and those don't have temp control. However, I think that phenomenon is not normal.
*You'll be going through security and may be required to take off jackets, sweaters, scarves, and shoes.
- wear well-fitting shirts and pants
- wear shoes that are easy to remove/put back on
- wear a sweater or sweatshirt -- regardless of the weather outside
- have/wear socks
- wear super loose-fitting clothing-- you're asking to be pat-down!
- try to avoid sandals-- unless its a quick flight (planes can be cold and I've been stepped on more in airports than I have anywhere else- ever)
- wear a ton of jewelry. I usually limit it to a bracelet I never take off and earrings.
- be a carrying excess baggage. Keep carry-ons to a minimum.
- wear too many layers. Yes, it might be cool on the plane, but 10 layers plus a winter coat is excessive.
- wear clothes you don't want to risk getting sweaty. Odds are, you'll end up running the gamut from boiling to freezing and back again.
I think my biggest attire tip is:
...always pack your winter coat. Don't bring it on the plane with you- stow it in your checked baggage. The airport temp is controlled and the plane is controlled. Except for that nano-second or two when you're between the gate and the door of the plane, you're not really even going to get a hint of the outside temp. You'll have access to the coat as you're walking into the airport before you check, and then as you're walking out of the airport after the baggage claim. Huge cumbersome coats are definitely a DON'T for on-board!