It depends. Wet heat is the worst, can't take that much above 22C. Dry Heat I don't start melting until 30C. I'd take -10 over either. Put on my gloves, hat, woolly jumper, coat and drink delicious warm beverages and I stay at a toasty temperature anyway. Also, I am skinny enough to be able to wear two pairs of jeans without anyone noticing, which is very effective.
Edit, also whilst not red haired, I share their pain in the face of bright sunlight, and so getting progressively nakeder in hot weather isn't really an option.