It's 30+ degrees outside. The humidity level is far beyond tolerable, situated somewhere in the oppressive, hellhole realm of summer days. You’re sweaty, wish you could rip off every shred of clothing, and would pay (nearly) any sum of money for access to a pool. Your makeup has melted off of your face and onto your previously white shirt. At this point, you’re probably thinking about how you’re surviving in the heat, not your phone.
Alas, you should be thinking about the latter, too. That's because the "acceptable operating temperature" for iOS devices falls between 0 and 35 degrees. (The temperature sweet spot is 16 to 22 degrees.) Go lower or higher and you might notice your iPhone's performance begin to change, and not for the better.
When using your phone in cold temperatures your battery life can shorten, though it will be fine when you return to warmer temperatures. (In the past years, some iPhone X users found that their screens were freezing up in chilly weather.) Hot temperatures — above 35 degrees — on the other hand, can cause lasting damage. Apple warns that using your iPhone on a hot day like that can "permanently shorten your battery life."
It's normal for your phone to feel warmer when it's doing something power-intensive, such as operating an augmented reality app. But a phone that feels warm because it's been left in a hot car, or because you're using that AR app in direct sunlight, is basically sending out an SOS. At that point, you might notice that your phone signal is weaker, your battery drains, or your phone slows down. You might also see the "temperature" warning screen shown above.
If this starts happening, what you shouldn't do is try to charge your phone — that could result in more damage. Instead, turn your phone off and remove your phone case so it can have a chance to cool down. If you absolutely can't turn your phone off, avoid using power-intensive tools such as location services (Settings > Privacy > Location Services). You probably want to save your Instagram Story watching for later, too. Most importantly, go somewhere cooler, where the heat isn't pounding on your head and you iPhone's screen. Doing so will benefit you both.