Amy,
you need to check with your particular battery manufacturer to be absolutely sure, but I think they base their figures around 25C.
For every 10C above this you halve the life of the batteries.
This is not quite as bad as it sounds as batteries have great thermal inertia and it takes a big battery a long time to get its internal temperature up through high air temperature (quicker if you work it hard).
So after a hot day you often have a cool(er) night which gives it a chance to cool down a bit.
I found this battery primer from Uncle Sam is quite good and has diagrams and graphs.
http://www.hss.energy.gov/nuclearsafety/ns/techstds/standard/hdbk1084/hdbk1084.pdfCheck out the "Avoid High Temperatures" bit on page 34.
