The iPad's lithium polymer battery is very sensitive to temperature swings—it doesn't like environments colder than 32 degrees Fahrenheit or hotter than 95 degrees Fahrenheit.
As a result, if you're out in the sun for too long, the system runs the risk of overheating and automatically shutting down. You should allow the device to cool down and restart the system.
On warmer days, use your iPad in the shade and never put it in a hot place, like the back shelf of a car. If you've just come in from the cold, you may want to let your iPad warm up a bit before restarting it.