I did 3 of my centuries in the middle of summer in temps between 90-100. I think one of them the temp even reached 109. (sorry I don't know the conversion to celsius) I wear a 100 oz camel back and carry 4 20 oz bottles of water, 2 in the pack and 2 on the bike with eletrolyte replacement in them. Now granted the humidity here in New Mexico stays very low in the summer so I don't have to deal with damp, sticky heat. Every time I stopped I filled up anything that was empty just to make sure I had plenty of water. I also made sure I drank at least 1 bottle per hour, sometimes more but never less.

ps: I love riding in the heat so I may not be the right person to answer this question!