Is it bad to use or leave a MacBook in the sun?

Working outside in the sun is fine. Apple rates Mac laptops for ambient temperatures of 50-95°F (10-35°C), and a shaded balcony or cafe table sits well inside that. The risk is baking the machine. A closed laptop in a parked car can hit 140°F/60°C inside. Direct sun on a dark surface for an hour is what burns batteries and triggers thermal shutdown. The fix is shade: work in open shade, keep the vents clear, and bring it in before the case feels hot. And MacBrightness adds no heat. It uses brightness Apple already sustains, with no noticeable CPU overhead.