I went the second week of December in 2008 and we layered things. It was chilly in the morning and later evenings, but for the afternoons t-shirts and shorts/jeans were all we needed. (December 2009 was colder from what I hear.) I made short sleeve shirts and we had sweatshirts and light jackets for in the morning and evenings, but I don't think we would have been hot in long sleeve t-shirts either.