I learned it the hard way. Went with my Spanish class to Puerto Rico in June, so I figured, hey, it'll be warm. Wore only a thin pair of jeans and a t-shirt. -_- Froze the whole way down and then again on the way back up, as I didn't have any long-sleeved shirts or sweaters.

Next time I fly, I don't care if it's to the Sahara desert, I'm wearing long johns and a sweater!