During transition from the cooler weather to flat out hot I will tend to to not wear full gear that I would normally wear during the winter months.
For about 2 weeks I rode home wearing just a short sleeve shirt. This was under very hot conditions with the sun blasting me every mile I rode.
My arms are the only exposed skin parts minus a small section on my face that is not covered by the helmet.
You can see the intense redness after another hot 100+ temperature ride. My arm looks like it is swollen and very red in color.
My wifey and I purchased long sleeve shirts.