In reply to Ginger McGrath:
I cycled from madrid to france a couple of years ago and live in Madrid
You'll be fine. The hottest month is 15july - 15 june when it can hit 40 quite often. By mid-August though things are more bearable and your lookig at 30 - 35 most days. As others have said, doing most of your riding before lunch is a good idea. The days are long in auguts so you can easily be cycling till 9 in ththe evening. I'd get up at 8 and cycle til about 1, have lunch laze around have a snooze, maybe a swim if you can stop by a river, the get going again around 5ish and ride till 8.
The other thing is, across the centre of spain there's a big mainly flat plateau and bombing across flat ground is no problem at all in the heat so long as you have water. Try to time your big climbs so they're either early in the day or at the end of the day. You dont want to be slogging up an 800m climb at 2pm.
Needless to say take a lot of water. I had 4 bottles of 750ml each and would get through those twice on a hot day (i.e. Each bottle refilled once). drinking so much water, you get sick of the taste of it plus i guess you're losing lots of salts as well which the water isn't replacing. I bought lemons and squeezed lemon juice into one of my water bottles so i could have a swig of lemony water whenever i got sick of plain water. Also keep an eye out of Aquarius. Its s sport drink made by cocacola and sold in spain - i haven't seen it in england. But it is the sh *t for when you are dehydrated.