When I used to go caving (not hard-core, but crawling more than the average tourist), I was somewhat bemused by people going into a cave wearing shorts, tank tops and flip-flops and looking at me coming out wearing long pants, boots, at least two shirts - one long sleeve, and usually flannel (and usually covered in mud/dirt from crawling). The expression is priceless.

Most people don't remember their grade school science or history. Most caves stay at a relatively constant temperature year-round, with maybe 5-10 degrees (F) fluctuation. Things underground usually stay cooler in the spring/summer than those aboveground and vice versa in the winter (northern hemisphere). That's why folks would have root cellars under the house/nearby to keep food stuffs.

As far as wearing a sweater with the kilt, I haven't yet. I don't often get an opportunity to wear my kilt casually. (I prefer to wear my 16 oz. - the only kilt I have - on special occasions).