Have ridden in this about 10-12 times now in temps in the mid 40's to 50's and am comfortable with just a base layer. Think you need a long sleeve base layer with this as this has no insulating material at all, therefore when wearing just the jersey, the chill will come right through. Even though it's windproof, you will feel like it isn't. With a base layer, no problem. This one hugs you. I'm 5'11", 152 lbs and 31" waist and the medium is fine but snug. Can't layer much under it, so if you want to do that, order up a size.