It sounds counterintuitive, but your iPhone actually dims its screen in direct, hot sunlight. This is a safety feature designed to prevent the internal components and battery from overheating and sustaining permanent damage.
Why This Happens and How to Fix It
You cannot turn this specific thermal throttling feature off in Settings.
- The screen dims because the phone’s internal temperature has exceeded its safe operating limit (usually from a combination of direct sun, screen brightness, and CPU usage like navigation).
- To fix it, immediately remove the phone from direct sunlight.
- Take it out of its case to allow heat to dissipate faster.
- Turn off the screen for 2-3 minutes. The brightness will return to normal once the device cools down.