38 degrees is tough, no doubt about it.
During our 1000 brevet in Israel a couple of years back, climbing up from the Dead Sea at midday, it was 47 degrees and I couldn't even walk up the hill in that. After napping in the shade for a couple of hours at a lookout, it dropped to 37 and we could actually ride uphill, albeit slowly. It was fun watching our average speed vary inversely with temperature through the event. We made the finish with fewer than 10 minutes to spare, so much tighter than we prefer.
35 degrees today in Paris with a 1000km UAF to Nice starting tomorrow. Plenty of sunscreen and water is on the cards.