Layers, layers, layers! We went in August a few years ago and had warmer than usual weather for the most part. I would take a heavy sweater, a polartec type jacket to go over it, and a raincoat. Also, gloves (it can be pretty chilly cruising through the glaciers and if you are outside trying to get pictures, those fingers get cold), also some type of hat. It can be breezy! Some type of thin long underwear like Under Armour Cold Gear would be handy. Have fun!