Despite what you might think about someone who’s stayed married to me for 32 years and counting, my wife, Pat, is a smart woman. But every year about this time, she makes the same complaint. Usually, it’s in the evening after sunset.
“The house is 68℉. Why am I cold?”
For at least the first two decades of our marriage, I didn’t have a good answer—and by good answer, I mean one that satisfies my own curiosity. Pat doesn’t necessarily want an answer. She just wants to be warm.
Sixty-eight degrees on a spring day is glorious. Yet there is no question that she is cold—I’ve held her hand on the couch watching TV and felt her bare feet in bed. My Yankee response is to say, “Put on a sweater,” but that doesn’t answer the fundamental question, nor does it make me popular. In fact, 68℉ inside a New England house in winter does feel colder than the same temperature on a May day, when the lawn is yellow with dandelions.
Radiant heat transfer
What Pat is experiencing is radiative cooling, which is independent of the sensible air temperature measured by thermometers. A lot of people understand heat transfer better when thinking about warming. The basis of all heat transfer is that heat moves from warm to cold. Radiation is one of three ways that heat moves. Radiation is what you feel from the sun, or when sitting around a campfire. The other two ways that heat moves are conduction, which you experience when you burn your hand on the ladle that’s been sitting in the soup pot, and convection, which you experience when warming your cold hands above that same pot of boiling soup.
Warming is what happens when heat is transferred…