Young men don mitered baseball caps.
Whilst older men put on silly hats.
Now, I have raised this before, but I think this is a relatively serious question: what type of hat should a priest wear with dignity when concelebrating in the hot sun?
I don't know if this picture was taken during Mass itself or during the hour or so wait before Mass. For the balding, being bareheaded can be quite painful.
The traddie answer would be wear a birreta or even a skullcap, the more ancient answer would be to cover the head with an amice, but that would make one look like a druid!
A modern answer might be, oh how I hate it, a baseball cap! Obviously in the proper liturgical colour and issued with instructions for it to be removed and taken to the knee at Holy Name, the Doxology, the name of the Blessed Virgin, and the name of the Supreme Pontiff.