Norway rats do not tolerate heat well: they die fairly easily of heat-stroke, and even if they don't die of it they can suffer permanent kidney-damage. Temperatures over 80°F may be dangerous, especially to large bucks (or to tailless rats, since rats use their tails to offload excess heat). Close, clingy, humid heat is particularly bad.
Turn a fan on, and/or place a closed bottle (glass, not plastic which would be eaten) of iced water in the cage, and/or provide a bowl of water with ice-cubes in it to drink and paddle in, and/or give frozen peas as ratty iced lollies. An ordinary house-brick, left in the freezer for a while and then placed in their cage, provides a cool place to sit.
An NFRS member called Nikki Janman recommends making a virtue of necessity and providing a bowl of water containing not only ice-cubes but assorted frozen vegetables, so they can play a ratty version of "bobbing for apples".
When transporting rats to another location it may be difficult to provide drinking water while in motion, so give them plenty of very moist food such as grapes and cucumber.
If transporting rats by car in hot weather, even for a short journey, make sure their carrying-box is very well ventilated, and take along a water-spray with which to dampen their fur if they start to overheat: this will reduce their temperature, and they will also lick it off and drink it. Moistening the tail with a wet sponge also provides relief.