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So much for looking at the vertical temperature of the atmosphere. Now let's do something about it.
Suppose you're a bit of air in the atmosphere. You're floating along in the atmosphere, not rising or sinking, because you're neutrally buoyant. In other words, you're just as dense (or light) as the air that surrounds you, so there's no tendency for you to rise or fall.
Now suppose that some force comes along and forces you upward a bit. You will become cooler. Remember how a dry air parcel, if lifted, will cool at the rate of 10 C/km? Well, that's you now. So now you're colder (and denser) than your surroundings. So you'll sink back down. If you sink too far, you'll warm up too much, become positively buoyant (as meteorologists say), and rise. But the bottom line is, you'll always be accelerating back toward your initial level of equilibrium.
Simple concept, right? Unfortunately, the atmosphere is not quite so simple. The preceding paragraph implicitly assumed that the temperature of your surroundings didn't change with height. But we just looked at a few sounding diagrams in which temperatures get colder aloft. So it's not necessarily true that, if displaced upward, you will accelerate back down.
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