"Why can heat get in through the atmosphere, but not out" - Aey
Global Warming, the boogeyman of science class, is all about the balance of Earth's heating and cooling properties. Our lucky little Goldilock's planet has blessed us with being close enough to the sun to keep us warm[1], but not so close as to burn us up in a great ball of fire, akin to Mercury. Think of it like the perfectly toasted marshmallow over a campfire; too far away and it'll never get that delicious golden brown, but too close and whoosh, welcome to hell.
In comes our atmosphere, that delicately thin sweater that keeps the toasty warm thermal energy in amidst the icy cold vastness of space[2]. Our atmosphere traps heat in, preventing it from going out, but still lets new energy in. How does it do that? Well, it's not as complicated as the iPhone you're probably reading this on, but it's still science.
Our sun produces energy via nuclear fusion between two hydrogen atoms[3], Fusion is when an atom splits apart, while fission is putting atoms together. The nucleus of an atom has a lot of energy [4] and when that nucleus splits open, it releases all that energy in a form of radiant energy. Radiant energy is special because it is just that, energy. There is no mass associated with it. It is just pure energy, unlike those energy drinks we enjoy so much that come with a buttload of calories. Radiant energy is the only form of energy that can travel through space. It comes in various different flavors that we use every day: X-rays, radio waves, Visible light, infrared radiation and even ultraviolet. Some of these waves are moving fast enough that they can go through even solid matter [5]. Others get bounced away even from gaseous matter in our atmosphere. However, most of the radiant energy from our sun can go through our atmosphere and penetrate into the crust of our Earth.
Once the radiant energy enters the crust, the atoms absorb the energy and get hot. Just like your food absorbs energy in the microwave (another form of radiant energy) and gets hot. Once the radiant energy gets absorbed by atoms, it transforms into thermal energy. Thermal energy travels in ways different than radiant energy. It need atoms to move from place to place. There are two main ways that thermal energy can move: conduction and convection.[6] Let's look at each of these separately.
Conduction
When atoms are given energy, they get excited and move around very quickly. In their clumsiness, they bump into other atoms around them and give off a little of their energy to the atom they bumped into. This chain of events continues down a long line until all of the atoms in the mass have the same amount of energy (equilibrium).
Convection
One of the first things we learn in fire safety is to get to the floor. That is because heat rises and cold sinks. The hot smoke rises, leaving the cold air down at the floor with us. Heat rises because of density. Less dense things (like pool noodles) float on top of more dense things (water). This is why we boil water with a coil from the bottom and not with a flamethrower from the top.
Through conduction and convection, the thermal energy that got put into our ground transfers to the air, which moves about the Earth keeping our weather stable[7]. The reason why all that thermal energy doesn't escape into space is because there are no atoms to give the thermal energy to in space, so we keep it all in. Space tries to take it away by taking atoms away from the atmosphere, but it can't do that because of a property revolutionized by one of our favorite scientists: Isaac Newton. Newton's Universal Law of Gravitation proves that all the atoms in our atmosphere are constantly being pulled into the Earth, leaving very few atoms with enough energy to escape Earth's gravity.
In short:
Energy enters the atmosphere as radiant energy which does not need atoms to move from point A (the sun) to point B (Earth). When it hits the Earth, it changes to thermal energy which moves through atoms, but can't escape because gravity prevents those thermal energy-carrying atoms from escaping Earth's gravitational pull. The atmosphere can let energy in (radiant energy), but traps the heat in (thermal energy) because it uses atoms to store all that energy.
