We always recommend bringing layers including something waterproof to cover yourself for all eventualities!
With it's south westerly position, Cornwall is lucky to experience regular temperatures higher than the rest of mainland Britain. And as Cornwall is surrounded on three sides by the sea, the weather can often vary from the north to the south coast.
Although Cornwall is famed for its mild climate it is very difficult to predict the weather pattern at any given time. Things can be quite different on the north coast to the south coast in Cornwall - at its narrowest point a stretch of just 6 miles!