I live in Adelaide South Australia. Our summers can peak at 48c+ - and thats in a metro beachside town (not desert).
25-35c weather here is optimal and what we consider perfect riding weather. North of 35c requires good electrolyte and hydration managment.
15-25c is nice weather, also good for riding but not prime.
under 15c and it's getting cold, we will normally start to layer up. I've been riding in the 0-4c mornings lately as we're in the dead of winter, and some of my suishy in the middle riding partners have been bailing out due to the chill.
It's all about what you're acclimatised to.