# eli5 Where does the heat go?

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How does the Earth and its various spheres lose heat?

This has stumped me since high school physics. I have an elementary understanding of how my coffee cup for radiates heat. However, I’ve always imagined that the vacuum of space would insulate the planet, or anything, like an invisible Yeti cup.

So, where does the heat go, and how does it get there?

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Radiation doesn’t need matter, just like the Sun heat the earth despite the vaccum between them.

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The Earth, like all objects, emits light based on its temperature. This radiation, called [thermal radiation](https://en.wikipedia.org/wiki/Thermal_radiation), is what makes the Sun glow white, hot metal glow red, or your body glow in the infrared. At Earth’s typical temperatures, the glow is mostly in the infrared.

Since the light emitted contains energy, emitting it tends to cool an object.

You’re correct that the vacuum of space acts as an excellent insulator at Earth-like temperatures. At Earth’s temperature, conduction (warm and cold objects in contact with one another exchanging heat) and convection (movement of fluids based on temperature differences) are much more efficient ways to transfer heat than radiation is. But radiation still matters, and it’s the means by which the Earth releases heat into space.

At higher temperatures, radiation – which scales up faster than the other forms of heat transfer – is more efficient. For example, a fire in a fireplace heats the room mostly by radiation, which is why it warms the room even though the air it’s heating up escapes up the chimney (i.e., it cannot transmit its heat by convection or, mostly, by conduction). It’s also why the Sun can heat Earth efficiently: the Sun is much hotter than Earth and therefore radiates much more efficiently.

Heat is just energy. It gets radiated off as infrared light. Everything with temperature emits light, the wavelengths of which are determined by its temperature. This is how the sun emits light, the fusion reaction in the core releases a ton of energy, that energy turns into heat, and that heat turns into light. The reason the sun is yellow is because it emits colors that add up to look yellow, almost white. See [black body radiation](http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html) for more about how objects with temperature emit light. Also see the ultraviolet catastrophe for how fitting this into our models was problematic, and the birth of quantum physics.

Heat can be transferred through conduction, convection (which is just conduction with extra steps) or radiation.

Conduction is when the atoms are wobbling (because being warm means they have more energy) and bump into other atoms, making them wobble too. That’s why when you touch things it warms you up. Despite the name, radiators are mostly doing this.

Radiation is when the atoms release that energy by throwing out photons (light), (specifically but not only in the infra red variety). Instead of the atoms bumping into you and making you move, they yeet tiny tennis balls at you, a d that makes you move.

Conduction is WAY more efficient than radiation. That warmth you feel what you step into sunlight is because the sun puts out A LOT of radiation. And the reason you cool down so quick when a cloud goes over is because the colder air is touching you and it sucks all the heat out of you much more efficiently.

In space it’s a vacuum so a planet can only lose heat by radiation.

You are right though that atmospheres do insulate. Air isn’t all that good at being warmed up by *sunlight*, but some of it (say, greenhouse gases) are great at abosrbing (and thus blocking) infra red.

What happens there is the sunlight (which is partly infra red, but also lots of other visible light) radiates from the sun towards the earth. Some of that gets caught by the atmosphere but most hits the planet itself, which warms up. But as we just said, things that are warm then start throwing infra red tennis balls right back out again to shed that excess energy. Or in science terms the ground absorbs sunlight and re radiates infrared light. THAT heat is what gets blocked by a thick atmosphere.

So an airless rock like the moon will heat up in sunlight, and then cool down a lot because even though it’s only radiating heat, there’s nothing to absorb that radiation and keep it around. So objects in space tend to be cold +unless in direct sunlight) but space itself doesn’t cool you down.