The big truck on the highway is convection. If you want serious heat transfer, that's the way to go. It's orders of magnitude more efficient than the others (factors of 10). If you have a gaming PC, you know that conduction and radiation isn't going to cool your processor. You need moving fluid, and that's convection.
Natural convection is the air rising from the hot land. Gliders and big hawks use the thermals generated by convection. It quickly cools the desert at night. It is one of the most powerful things in Nature. Ignore it at your peril!
The biggest mover of heat by convection is the Deep Earth, or the Mantle. There is a huge amount of heat generated by radiation deep in the Earth. There might even some left-over formation heat. The Outer Core is also fluid and has severe convection. Our magnetic field is created by convection, and if we didn't have it, the warmies would have something serious to worry about. The latest scare story is about this convection changing course and flipping the magnetic poles. That's because severe convection is chaotic, and anything can happen. I wouldn't worry about it.
You can see convection in your morning oatmeal, but nobody really defines it well. In fact, all of this stuff is poorly explained. The explanations assume you know the answer in the first place. This doesn't work with English Majors or FB readers.
Convection is just billiard balls, but the billiard balls are moving. This puts a lot of oomph into it. You can move the billiard balls by pumping fluid, but there is a good natural trick that allows us all to live (physical law). If we didn't have the physics laws that we do, we wouldn't be here to talk about them. Think of them as God's will, if you want. Do not get in their way!
The billiard balls in fluid have a neat trick. As they vibrate, they expand the space around them. This makes them less dense. Although I said that density is an inviolate property of matter, it changes with temperature. Less dense fluid rises. Don't ask me why, that's another lesson, just go with it. You can look at a hot air balloon, and say "Ah, the hot air inside is less dense and it rises." English majors will be so impressed!
- to be continued