They say, "... when sitting outside on a cold day you will feel "cold" because the heat transferred from the sun to you is less than the radiant heat you are giving off..."
So now what causes you to feel HOT on a cool day. The AIR temperature as reported by weather.com is 80 degrees, but you are getting heat exhaustion. This is because THE SUN IS GETTING HOTTER. So here is the truth: A cool day is a day when heat radiated from the GROUND and the trees and buildings around you as well as the air are all cool. In the past, the sun's radiant heat was not so much of a factor so you would indeed feel cool. Now, even on a cool day, you can feel physically HOT when out in the sun.