@Some1Somewhere : You need to keep in mind that being in the sun is not a good place for a phone, or anything else that has a maximum temperature.
The human body can sweat to keep the temperature down in high temperatures. Your phone cannot. Its sitting there with the temperature going up higher the longer it sits out there, and if its on, its also generating heat.
One thing it does when its in the sun, is it cranks the display up HIGHER than maximum brightness. They have a boost mode to make it readable in the sunlight that exceeds the "normal" maximum brightness. More bright means more power, which drains the battery faster, which being a CHEMICAL reaction, will drive the temperature up pretty fast. Add to that if you're doing anything intense with the CPU, and its no wonder you'll get to the shutdown temperature eventually.