I wear this scullcap under my helmet. It is thin, it doesn't bulk up and make the helmet fit tight. Although it is designed for warm weather, I find it helps cut a bit of chill on cool days too. It does not go over the ears. For me, it's a good piece to wear from high 50's through low 70's. Since it does retain a small amount of heat, I tend not to wear the cap when it's really hot, although, it is a trade off from a sun burned scalp (as I'm bald).
I've tried many skullcaps to keep the sweat off my brow and so far this is the best one out there. It fits tight and keep the sweat from dripping on my face as I ride long miles in the sun all summer long.
This skullcap fits well. I like the design with no tying knot at the back. It makes it more comfortable under the helmet, and looks more streamlined. Works well to wick the sweat from your head and prevent it from dripping in your eyes.
This skull cap does an excellent job of wicking away sweat and keeping it out of your eyes, even on high output rides in humid 95 degree conditions. Airflow through the top is also excellent so no over heating from trapped heat. Its low volume adds virtually no bulk under your helmet and the trim-free design does not leave you with a fossil forehead after your ride. I plan to purchase another one.
I use them every ride starting when temps hit 50 degrees, I live in Wisconsin and early season rides start out in the 30's, keeps my hair from ripping out, comfortable one size fits all, no sweat running into my sunglasses, protects bald spot on my head from direct sun. The only thing I don't like is I don't have a couple more of them.