I am fair skinned (have been all my life!) and bald (have been for most of my life!) and have learnt through experience what happens when I go out in the sun without sunscreen, a hat and, if walking, a raft of other precautions as well (like water and rehydration salts). To me it just makes sense - I'd rather be comfortable and safe!
I know some are happy to wander in temperatures like we have been experiencing without a hat or sunscreen - that is up to them. But each individual is different.
Also, pretty much everyone is in agreement that too much exposure to the sun's rays and heat is dangerous: it damages the skin, causes it premature aging, brings on sunstroke or heatstroke, and can lead to skin cancer. And it may take years or decades to manifest the problems.
Everyone has their own limit, but I don't want to find out where mine was the hard way!