The traditional hat, known as a camauro, was commonly worn by popes in
the medieval period to keep their heads warm on cold days and it
featured on many paintings at the time, but it has rarely been worn in
modern times. Various reports have taken note of how this pope is using once again various items of papal clothing not used for many decades. Read more about this here.
Link: Scotsman.com News – International – Pope caps audience with his ‘Santa hat’.