The warmest I've commuted home from work was 105 F (40.5 C). I've been riding a few deg close to that many times. Open all the vents in the protective riding gear.
NEVER ride in those temps with any skin exposed. Research has shown that the air layer on your skin is about 93 F (33.9 C). When the ambient air that is blowing over exposed skin is lower than that body heat is drawn away from you. Your body is designed to work best and is most comfortable with a little heat is being drawn away... like what happens when temps are around 77 F while riding (25 C) give or take a few degrees. When the ambient air is warmer than 93 F the warmer air ADDS heat to your body and your core temp increases. This happens faster on exposed skin in those warmer temps while riding. The faster the air blows over your skin the faster the temp transfer happens, NOT faster when that skin is covered to shield it from the air blowing by while riding. So shield all skin from the sun and from too much air blowing on it. Better to drink lots of water and have a sweaty or wet shirt under the riding gear. Did that many times and felt like I had an air conditioner turned on under my gear. Was even chilled a little at extreme warm temps when my shirt was wet and vents open. Yep, even sweat feels good when that happens.
