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WritingTheRongs t1_j67m8bi wrote

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.

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