There are three ways in which objects can have a thermal interaction with other objects. The commonest method is thru heat conduction. This happens when two objects of various temperatures are involved, and thermal energy is transferred from the hotter object to the colder object—like whenever you hold a can of cold soda in your hand. The can warms up and your hand cools down.
The next heat transfer method is convection, and it only works with gases and fluids. Let’s use air for example. Suppose you’ve got a heat source like a stovetop. The air near the stove burner will increase in temperature through a heat conduction interaction. This hotter air now could have a lower density than the colder air above it. It will rise and colder air will take its place. Then the new air can have one other heat conduction interaction with the stuff above it, like perhaps the ceiling. The indirect transfer of warmth from the stove to the ceiling is convection.
The third variety of thermal interaction is radiation—and that is the one we actually need. When a hot object emits infrared radiation, that radiation will be absorbed by other objects. This is precisely how your oven works. You put stuff that you wish to cook inside, and the heating elements get highly regarded, producing thermal radiation. (Yes, that is the same as infrared.) The food absorbs this and increases in temperature.
Now imagine that you just preheat your oven, then turn it off and stick a potato inside. The hot oven emits thermal radiation and the potato absorbs most of it. The result: The potato gets hotter and the oven gets cooler. This isn’t really a traditional option to bake a potato, but the purpose is that when objects produce thermal radiation, they cool off.
But if the whole lot around us is emitting electromagnetic radiation within the infrared, then shouldn’t the whole lot be getting cooler? Not really. If you’re taking an apple and place it on a table, it emits thermal radiation. But it also absorbs radiation from the whole lot else: the table, the air, the partitions. So when all of the objects in the identical vicinity are already the identical temperature, they don’t seem to be going to chill off by radiation.
Reflectivity vs. Emissivity
There’s one other very necessary property to contemplate to totally understand how radiative cooling works: the difference between reflectivity and emissivity. Imagine you’ve got an ideal mirror. All the sunshine that hits it reflects off of it. That mirror would have a reflectivity of 1, which implies that one hundred pc of the sunshine that hits it bounces off.
A sheet of aluminum foil also reflects quite a little bit of light—but not all the sunshine. It might need a reflectivity of around 0.88, meaning that 88 percent reflects. The other 12 percent of sunshine that falls on the foil is absorbed, increasing the temperature of the foil.
Now imagine an object that does not reflect light in any respect. Of course it still emits light, but only due to its temperature and never because light is reflecting from it. This object would have an emissivity of 1 and we might call it a “perfect black body,” meaning that it absorbs all electromagnetic radiation. So emissivity is basically the alternative of reflectivity.
