I seem to recall an article somewhere that proposed that the old highlanders would wear their kilts at varying lengths based on the weather (winter - long kilt, summer - short kilt) I have no way of knowing if it is accurate or true, but the logic makes sense. Because we aren't wearing the belted plaid as much, this is less possible today, but even lowering the kilt an inch or two would greatly reduce the exposure at the knee and retain more body heat.