I live in Florida so I know a thing or two about humidity. Good quality technical fabric is the only way to go IMO. I've found that I really like the Under Armour Cool max shirts under a sleeveless jersey. They're really tight and long sleeve which seem counterintuitive to wear when it's 99 degrees with 80% humidity, but somehow the breeze created while riding make my skin underneath feel "cool". I don't get it...I just know it works. In addition, we drink lots of water ( at least a bottle per hour) and use an electrolyte supplement. My skin's surface was so salty after today's ride that the salt was visible all over my arms/chest and I felt itchy all over.
She's going the distance...
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'14 Orbea Orca Dama, Specialized Jett
'10 Giant Avail
'87 Schwinn Cimarron, Brooks B17
Trek mountain bike...don't know what year