On the extreme, when I walk outside on a cold cloudy day, my head quickly informs my brain that a warm cap is in order lest all my body heat escape directly through my shiny dome like hot air rising up a chimney. That cold air hits my warm scalp and convective heat exchange is immediate. An infrared photo would light up my head like a beacon and show waves of heat traveling skyward. When the day is cool and sunny, that same patch of skin sends a different signal but with a similar message: "put on a cap." This time the sun is beating down on my sensitive bare spot and threatening to cook it to a sunset red if not acted upon in short order.
Operative temperature is that temperature which combines both the air temperature and the radiant temperature effect. Unfortunately, conventional heating (and cooling) systems measure only the air temperature. If my biological sensor was like a typical wall thermostat and only measured air temperature, on a cool sunny day my scalp would cook from the radiant energy and I would be no more the wiser until I looked in the mirror the next morning.