In case this question is about keeping your pet safe, I'll go a step further.
To directly answer your question, cats act as all animals do during a heat wave. They seek shade, breezes and water. They avoid activity during the worst hours of the afternoon.
If a cat (or dog) is kept in a home without air conditioning, they can perish because of the heat. If you are a pet owner in an unairconditioned house --- be sure to open screened windows, use fans, close lightweight curtains to reduce direct sunlight while allowing air to flow, provide plenty of fresh water, and have someone check on kitty if you are gone for an extended length of time.
Allow access to the bathroom. Bathrooms are usually tiled. The tub and tiled floor are usually cooler than the carpeted areas.
Ice is great! You can freeze bottles of water overnight, then surround the kitty bed with the contained ice. You can wet 3-4 washcloths, lie them flat in a large Ziplock bag, and freeze them. Place these under the kitty bed mattress. You can provide a second water bowl filled with ice cubes to create cool water upon melting.
Since cats don't sweat as we do, they don't get the cooling effects of evaporation on their own. Pet kitty with a cool wet washcloth to dampen the fur.
Never leave your pet in a vehicle. The greenhouse effect soars temperatures quickly to lethal levels.
If you see signs of heat stroke like panting, bright red gums and staggering while walking - go to a vet immediately.
