I try to ride at dawn when it is below 90 degrees or near dusk when the temperature drops below 100, but the last two days, I had to ride a few miles in the middle of the day. Today at 1pm it was 115, yesterday it was 117 degrees.
On a hot day in Pennsylvania (at least what I thought was hot last year--between 95 and 100 degrees) I could ride 17 to 20 mph on a flat road and cool down a little. Uphill I was going to drip sweat and downhill would be very cool. Here there are no hills at all, so the high speed breeze is the best I can hope for. It works in the morning or in the evening when the sun is low in the sky, but the last two days, riding at midday, the air felt like I was riding past an open oven.
A light headwind kicked up, no more than 10 mph, but that felt like I was riding behind a heater blower. The good thing was this evening's ride, when the temp dropped to 100, I was sweating on my 10-mile loop of the base, but the air felt like air, not oven blast.
There is no humidity to speak of here and I suppose the temp would feel worse if it was humid, but 115 degrees is hot--dry heat or not.