based on Feb 2005, when we went, it's all of the above. USUALLY it's cool in the morning (light jacket, or sweatshirt), and then warm during the days (in the sun), so shorts and tshirts are the norm. HOWEVER, it can be cold, especially if it's overcast, so you will want long pants and a jacket or sweatshirt. We didn't need anything heavier than that.
Nights can be kind of cold, depending on how hard the wind is blowing. Again, jacket and or sweatshirt.