回复：有个无法控制的因素，那就是SFO夏天常起雾。尽量要走道靠前的座位

Thank you! I have just called the airline customer service, she said there is no need to go through the security or picking up baggage at PHL for the outgoing trip. For the returning trip, however, I need to pick up my baggage and go through the customs. But for the return trip I have 3 hours at Charlotte airport.