Toyota 2008 Prius Quick Reference Manual page 242

3-2. Using the air conditioning system and defogger
CAUTION
To prevent the windshield from fogging up
n
Do not use
The difference between the temperature of the outside air and that of the
windshield can cause the outer surface of the windshield to fog up, blocking
your vision.
NOTICE
n To prevent 12 volt battery discharge
Do not leave the automatic air conditioning system on longer than necessary
when the hybrid system is not running.
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during cool air operation in extremely humid weather.