We went on the first trip in May which would probably be comparable since May and September are the "bookends" of the Alaska cruise season. It was very cold. Everyone, including the crew, bundled up in warm jackets and many also wrapped up in blankets. Even in port, there was a constant cold wind, which was especially bad on the public decks. Our balcony helped block some of the wind, so we were much more comfortable there. The highest temp we had was mid-forties, but it felt much colder because of the wind. It was especially cold at the glaciers. Be sure to take something with a hood. Having a hood to raise helped much more than simply wearing a hat. (I tried it both ways.) You'll love Alaska! Have a great cruise!