If a hat keeps you warm in cold weather, and keeps you cool in hot weather, is there any weather where it is not a good idea to wear a hat?
I had it pointed out to me and my fellow visiting Americans that you don’t wear a hat into a bar in New Zealand–it instantly pegs you as a foreigner.
I suppose that might translate into other indoor New Zealand environments, but I never went to any of those.
I learned a long time ago, as a young tot, never wear a hat indoors, it just aint proper.
Don’t wear a wool watch cap in the summer, and don’t wear a sun visor in the winter.
Unless your hat has a chinstrap (kinda silly-looking), it is probably not a very good idea to wear a hat in very windy weather. Oh, and there is no weather condition imaginable in which it is acceptable to wear a beanie.
What about the ones with a propeller?
Only moderately acceptable as part of a Halloween candy gathering ritual. 
Unless you’re a Jew or a cowboy, to steal an observation from Kinky Friedman.
What if you’re a Jewish cowboy?
Kinky Friedman and the Texas Jewboys! As he
puts it: Good cowboys and Jewboys always
wear hats inside.
He has a marvelous song called “Ride 'em
Jewboy” about the Holocaust. Unique
remembrance of the event.
So ride, ride 'em Jewboy
Ride them all around the old coral.
I’m, I’m with you boy
If I got to ride six million miles.
Oh, yeah? How about partially frumish with a chance of shul.
Hey, tied two threads together!
Never wear a hat in indoor weather.
FWIW, my uncle worked as a cowboy all his life. He never wore a hat indoors and neither did his kids. I’ve never seen a cowboy wear a hat inside, and neither has my wife.
Of course not! They only wear them on the outside, on top of their heads. (mental picture of cowbows eating their hats)
Maybe he was a reform cowboy instead of orthodox …
Whether it’s a base canard or not, I still like Kinky Friedman’s line about it it.
…And we should trust our manners with Mr. Friedman?
As Kinky once said (to his blushing guitarist, with two slack-jawed young children in front of the stage), “Aw hell, I understand. My momma told me not to say fuck and piss in front of a K-I-D too!”