The cotton works well until it wicks up all your moisture causing the fibers to swell closed giving you a clammy feeling. The newer materials the fibers don't swell closed allowing for better evaporation.
When its extremely hot you want to control the convection heating caused by the wind. The wind dries the sweat so fast it doesn't have time to cool and the heat carried in the wind then begins to cook you.
For instance a 5mph wind in 100 degree heat may cool you while a 30+ mph wind in 100 degree heat will cook you. (Windspeed doing 70 mph on a bike with windshield is what?)
Thermal (Under Armor) gear that wicks the moisture away from your body worn under a light colored shirt that slows the 80 mph wind down to 5-20 mph in 100 degree temps should be more comfortable, especially in humid areas like the Gulf States. (Use a buttoned sleeved shirt where you can control the wind using the buttons to control the long sleeved opening. Same as a vent in the helmet). The newer nylon materials also wash out and air dry many times faster than cotton. Wash it, roll it in a towel, ring it, unroll it, and hang/lay it. There are also garments to help minimize crotch rot (has silver molecules embedded for control of bacteria). Sunburn never feels good so long sleeves helps with that.
Another is liquids for sweating, 85 degrees may require 1 quart an hour while 100 degrees may require 4+ quarts an hour. (This is for long day rides). Freezing drinks makes them more refreshing but you need liquid volume. You can't get volume if it's frozen in the bottle, mix an unfrozen drink into the frozen if need be. Get liquid volume. Drink on a schedule because it's too late once you're feeling thirsty. Drinking large volumes of just plain water that quickly can flush out vital nutrients from your body causing drowning. (Happens at marathon runs.) You need to add supplements like those in Sports Drinks, V-8, and Fruit Drinks. Use additional (not tons) salt when sweating heavily and drinking large volumes as it will flush out quickly. Potassium in bananas helps prevent muscle cramping. Think to use them ahead (BREAKFAST) so they're there when your body wants them. Provide nutrients before your body starts shutting down because it is lacking them. Definitely use supplements if you start feeling symptoms.
Acclimate days in advance. Don't sit in a 74 degree office the weeks before a ride and expect to jump into 90 degree plus temps without problems. Go out an hour at a time and be uncomfortable. Sit in the heat at lunch and then start walking in the heat at lunch. People who sweat early are usually in better shape than slower sweaters. Their bodies know to sweat early. Build up your tolerances. Your body will adjust the insulating fat cells in your blood to the temps outside, it just needs time (days). Certain medications also affect how your body handles heat. (They use certain anesthetizing drugs to create hyperthermia to try to treat certain cancers for instance.) Do a Google search on your medication and heat. You may want to limit caffein due to it's affect on capillary blood vessels until you get acclimated.