Because hot air has more energy, the molecules of air are moving more and pushing each other away more, so its less dense. because of this hot air rises above colder air masses somewhat like a bubble rises in water, only much slower.

as you go higher up in the atmosphere, thanks to gravity the air naturally gets less dense. As the air gets less dense the air is allowed to expand out and cool, a process called Adiabatic Cooling. This same principle is why any liquid or gas sprayed out of a compressed canister, like compressed air or a fire extinguisher is cold.

Lets divide the atmosphere into spheres of equal size. The molecules in warm air bounce off each other more, meaning that each warm sphere will have fewer molecules in it and therefore less mass. Likewise, cold air molecules bounce off each other less, so each cold sphere will have more air molecules in it, and therefore have more mass. Spheres with more mass weigh more than spheres with less mass, so the former descend and the latter rise relative to each other.

As warm spheres rise, two things happen:

1. pressure falls because there are fewer and fewer molecules above a sphere pressing down on the whole atmosphere. As pressure goes down, energetic molecules spread out and bounce into each other less often, which reduces their temperature; and

2. air moves away from its main heat source: Earth's surface. More of its energy simply radiates away into space.

So rising air steadily cools, but continues to rise so long as it remains slightly warmer than adjacent air masses.

Man these answers … this is a classic for being simple yet hard to explain. The first part of the question is easily answered. Hot air is less dense. There’s less “stuff” in a packet of hot air. So it rises up , just like a balloon held under water rises up. All the colder air around the packet of warm air is pushing it up. In fact if you could look really closely you would see that the cold air was pushing on the warm air a littler more on the bottom than on the top, and that slight difference in push means there’s a net upwards force. Now you might think that’s true for a packet of cold air right? But what is the counteracting force ? What’s pulling down on everything? It’s gravity. Gravity pulls down on the cold air and warm air alike right? But remember we said the warm air had less “stuff” in other words it doesn’t weigh as much as cold air. This is the secret sauce of buoyancy. The cold air pushes up with the exact same force as gravity pulling down *on a packet of cold air”. After all that’s where air pressure comes from in this first place, from gravity smashing all this air together at low altitude. But if you replace a chunk of cold air with a chunk of warm air, such as in a hot air balloon, the gravitational force on the balloon is less than the surrounding air. Imagine a more extreme where the balloon was empty. No weight at all except the skin of the balloon. Now all that cold air outside the balloon is pushing at the bottom of the balloon but there’s no downward force from the weight of the balloon because this imaginary balloon is weightless! So up it goes.

For the second half of your question, we have two things going on. But the simplest way to understand this I think is to ask yourself not why it gets colder but why it’s warmer down at sea level. The answer is simple, that’s where the heat is. Where does most of this heat come from? It’s sunlight. Sunlight hitting the earth and warming it up. The ground gets warm and so the air near the ground is also warm. 30,000 feet up , the sunlight is passing through the almost perfectly transparent air without even touching the air molecules. So they stay pretty cold. Go far enough up and you’re almost in space where it seems logical that the air is cold. It’s only down on the ground that it makes sense to feel warmth. There are complications to this , and there are parts of the upper atmosphere that are technically very hot because they are in fact absorbing some high energy light. But there’s so little of that air that you wouldn’t feel the heat. The other issue is pressure. The hot air rising in the first question does something interesting as it rises. It starts to spread apart because the pressure drops as you get higher. As the warm air molecules spread a part from each other , they use up some of their heat energy. So not only is it colder up high; the process of getting up high cools you down from the work of expanding.

Hot air rises because its less dense than cold air. The reason so has to do with the kinetic particle theory. As the temperature increases, the average speed of the molecules increases, and thus they collide with more kinetic energy and spread out more. Thus there are less particles per unit volume, decreasing the density.

Fluids (liquids and gases) form layers based on their density, which we find by dividing their mass (how much stuff there is and how heavy it is) by their volume (how much space they take up).

Gases (and indeed most substances) expand as they get hotter; the amount of stuff, and therefore mass, stays the same but the volume increases - therefore the density falls.

Hotter air is thus less dense than colder air and so it rises, causing the colder, more dense air to fall.

As you get higher in the atmosphere, there's more space to occupy (think concentric circles; they get bigger and bigger as the radius increases). With more space to occupy, again the density is going to decrease as the gases spread out. This also causes them to cool down (Boyle's Law, Charles' Law and the ideal gas law are a bit beyond a 5 year old though).

Also most of the sun's energy that comes to earth hits the surface (gas molecules are TINY so most of the sun's rays will miss them and some of the ones that do hit them bounce off) so the surface warms up far more than the atmosphere does. The surface then loses heat to the atmosphere. Since this heat starts at ground level, the air closest to the ground has more chance to absorb it and therefore the air closer to the ground will be hotter than air higher up.

Lets say you have a chunk of cold air at sea level. The sun heats up the air near the ground by hitting the ground first(regular visible light just passes through the air instead of heating it). The increase in temperature causes it to expand(the weight of the air on top of you isn't changing, so the pressure stays the same for the moment.) Now your chunk of air is less dense than the surrounding gas, so it starts to rise.

The higher up you go the lower the air pressure, so your chunk of air expands even more, pulling heat out of the evaporated water it absorbed near the surface(which condenses into clouds and rain).

High up in the atmosphere it's cooled down a lot from expanding, but it still hasn't actually gotten rid of the energy it absorbed from that sunlight, it does this by radiating infrared light off into space, cooling off even more before it starts to sink back to the ground to start the process over.

Molecular Density + Gravitational Wave + Coriolis Force = ?

Cold air is a lot denser, as the particles that make it up move less. So, hot air gets pushed upwards, as it moves more. Think of it as a person trying to move through a crowd.

Hotness is kinetic energy that's been transferred as heat. If you move a lot, you get hot and sweat, but you stay still, you cool down. There's no active movement in the upper atmosphere, so naturally, it gets cold.