This is the time of year to think a bit about summerizing your soles. Ken Bob Saxton, author of Barefoot Running Step by Step over at The Running Barefoot. says this about Summerizing Soles:
I’ve heard some interesting arguments against barefoot running, for example “you couldn’t run a full marathon barefoot in Phoenix, Arizona, in July at noon – therefore barefoot running isn’t worth trying”…
Well, reasonable people wouldn’t run marathons in shoes in Phoenix, in July, at noon either. That’s why Marathons in Phoenix are rarely, if ever, run in July at noon!
Remember, the soles can be early warning systems for all sorts of things. And that can apply to overheating, too:
Soles are extremely sensitive to warn us when we’re running badly, or at a bad time of day, or on the wrong surfaces for that time of day, for too far, and/or too often, too soon. These warnings are not simply to protect the feet, they protect the rest of our body from impact, excess or improper torque, and let’s include overheating too.
He also says
That warning stated, each summer, there will be days in which the pavement becomes very hot, compared to other days in spring, and some of those day, the weather might not be nearly as hot as the pavement. On those days, limit how far you run, but don’t avoid the pavement all-together, as there is some conditioning (re-conditioning) needed each spring season. (what some of us have called “summerizing” our soles). As with any new activity that we haven’t done for months (over the winter), conditioning our soles for hot surfaces will take time, and several outings, and we should be careful to limit these outings to avoid blistering, as I don’t believe that healing blisters is any faster than a more conservative toughening without blistering approach.
Go read the whole thing.
I play quite a bit of tennis, barefoot, over the summer, usually about 5 hours a week. That includes hot and sunny days. I can report that one’s feet really do get conditioned, but it can take a bit of time. One problem with barefoot tennis compared to barefoot running is that, while waiting between points, you don’t get to keep your feet in the air like you do when running.
When you run, each foot gets to spend about half its time in the air, cooling back off. Not so between points. For that, I’ve found standing on the white lines provides a cooler area if you just have to stand there.
I also agree with Ken Bob on blisters. It is better to condition your soles gradually than to let them blister and heal. When they are getting on the edge of blistering, your soles will have a slightly “loose” feel about them. It’s hard to describe, but you’ll know if when you feel it. That is a warning that it is time to stop. At that point, it is time to use shoes as the tools that they are — to extend the natural capabilities of the human body (but not supplant them).
I’ve had to put shoes on to play tennis because of a hot, sunny court only once. It was somewhat later in the day, and the sun had been beating down on the court all day. I might have been able to tough it out, but why bother? Why risk putting myself out of commission for a while? After all, this is supposed to be fun; it’s not a contest.
Within half an hour, the sun had sunk lower, the court had radiated off enough heat, and I went back to playing shoeless. And quite enjoyably, I might add.
Ahcuah 
That “loose” feeling on the soles is when the burning goes from sharply burning (where the heat begins to feel prickly on the soles), to actual tingling on the soles. If the tingling does not subside within a couple of seconds, it is likely that blisters will occur very soon. This is for hot surfaces.
For longer distances even when the surface is not as hot, the warning sign that blisters will form soon is the formation of white hotspots and an eventual sharp jabbing sensation with each step as the hotspot starts to become a blister.
The soles actually go momentarily numb in the location that actual blisters have started to form. Feeling a fluid under the soles sensation in some part of the sole ultimately confirms the formation of blisters.
A way to extend your range and gage how your feet are doing (if you haven’t toughened them up) is to stop in the shade every so often. You’ll feel the difference and it will help your feet.