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Make sure your coolant is full and in good condition. Make sure there is no plant debris blocking the radiator airflow. Make sure your tires are in good condition and drop the pressure 3-5 psi , especially if you are running near max pressure. And as always it is a good idea to carry a little emergency water for you, just in case. Otherwise your Prius is well equipped to deal with extreme temps and is programmed to deal with them accordingly.
everything I read said not run the tires under inflated. i guess it also makes sense to not run them too full as heat expands.
no concerns about batteries?
Absolutely! You do not want to run underinflated because they can't slough off heat, but many people run tires on hybrids real high near maximum and that isn't good if they get real hot. Your batteries have a cooling system and it is just important to make sure the coolant isn't low or nasty, but the computer will keep them regulated properly.
You may also lower mileage because the batteries may be shut off some to keep them happy, but it is all automatic, nothing for you to do.