I've signed up for this trip in mid-July. Info says to bring "layers for cold weather." Are we just talking base layers, shirts and long pants -- or heavy coat, hats, gloves, etc? I know we're up in the altitude, but it's mid-July and I don't want to take up space in my luggage dragging a bunch of heavy clothes that I won't really need. If anyone has been on this trip in the summer and can give me advice, I'd sure appreciate it.