A few months back we took a road trip to ooty or ootacamund, a hill station in tamil nadu, India. This is the conversation between my son and I.
Son : "Ma, aren't we climbing up a mountain?" .
I : "Yes".
He : "Aren't we moving closer to the sky and the sun ? Why are we feeling cold?".
I : "Hmmm.. well.."
And flashback time it is! You know in the olden day movies, to depict a flashback, they show us a spiralling thing. Anyways, all I tried telling was, it got me back to my school days.
I'm sure all of you would've learnt about water cycle in your good old school days. I think I learnt it somewhere in my 2nd standard. My son is learning about it in the lower kindergarten. Things are moving fast now a days. Anyways, besides the point. From the water cycle, we all know one thing. Water from the seas, oceans and the rivers, evaporates and we distinctly remember seeing red lines or rather red curves representing the water vapors, rising up. With the explanation, hot air rises. Well, if that be the case, shouldn't the hills and the mountains all be warm or maybe even hot ? (hot from all the hot air that rises)
One more perspective to look at this whole thing, like how my 3 year old pointed out, we know our main source of heat is the sun. And the sun is high up in the sky. No denying that. One might argue that closer to the heat source, the hotter you feel. That is, the hills being closer to the sun than the ground, the hills should feel warmer than the plain land.
Two different perspectives, with good amount of logic, all telling us one thing. Hills should be hot or atleast warm. But from our experience, we know that the hills or hill stations, are cold. How could that be? What went wrong in my logic? Was my water cycle theory, the one that is clearly etched in my mind through the years wrong, or ..is the sun not high up in the sky?? Let's think this one through!
The water cycle is not wrong. Just think one step ahead. Why does hot air rise? When air gets heated, what happens actually? The molecules in the air, gain some energy. (heat is a form of energy) and the molecules, with their newly acquired energy move faster and bombard into each other and they tend to push the surrounding cold (not hot) molecules aside. And in the process, they've expanded and spent some energy too. Once they expand, they become less dense. And from our previous post, we know that the less dense substance when immersed in a denser substance, rises up due to buoyancy force. That's precisely why the hot air rises.
If you would've carefully read the above paragraph, you would've already said "voila, now I know why the hills are cold". The hot air spends some energy in expanding. Which means it becomes cold. The air that rises, eventually becomes cold. ( Now, a very good follow up question would be, why didn't it sink back then. Now that it is is cold, shouldn't it reverse all the effects that the heat caused and sink back down? There air on top is comparatively less denser than that of the lower layers. So, this cold air needn't sink down. Same density as the surroundings.)
That's one explanation. I've done some reading on the internet and I've found some different perspectives on this. Let me share them too, for completeness. Oh hold on, I still didn't explain what was wrong with the it-should-be-hotter-in-the-hills-because-its-closer-to-the-sun theory. Do you know how far up in the sky is the sun? It is a whopping 150 million kms. Compare that with the height of the heighest mountain you've ever heard of.. 9kms maybe? So whether you are on the top of the highest mountain or on the plain, it doesn't make a difference. We are still the same distance away from the sun, which is, very very very far away.
One more way of looking at why the hills are cold will be, the lower layers of the atmosphere are under pressure from the upper layers. So are usually compressed and under pressure. The upper layers experience lesser pressure. When the gas is under more pressure, it tends to be warmer. Because when a lot of gas molecules are trapped under a small space, the molecules bombard against each other and are in constant motion and hence get heated up. More the pressure, more the heat. And lesser the pressure, lesser the heat.
Another explanation, the sun's heat reaches us as light rays. not as heat waves. Through radiation that is. Once the surface of the earth absorbs it, it releases it as heat waves. One way to see it is that , the air near the earth is dense, so it absorbs more of this heat energy. The air near the mountains is less dense, and it absorbs less of the heat energy. And the layers of air closer to the earth are more warm as compared to the ones near the hills. Also, as the pointed mountain has lot more surface area than the plain, it tends to dissipate more of the absorbed heat, making it a cooler place.
Another one, imagine the layers of air above the earth as blankets you use to cover yourself. If you have one blanket over you, you feel warm. If you have two, you feel warmer. If you have three, you feel hot probably.. You get it? The more number of atmospheric layers on the plains, the more of the heat they hold within them. So you can say, as you go higher up, you start losing the blankets on top of you.
I didn't try explaining atmospheric pressure and density and buoyancy to my 3 year old. I just used the blanket analogy and he seemed happy. For a whole week, he went around asking this question to the adults he came across, and whether they gave him an answer or not, he said, "No. Let me tell you. It is because the hills have only two layers of air, whereas we have 10." And immediately thereafter, he made it a point to find me to give me a hi-fi! Oh, the joys of being a mom.
So essentially, it all boils down to these. The air in the plains are under a lot of pressure. So they are dense. They absorb heat faster, and they also act like a blanket and keep the heat there for us. The air that gets warm, tends to have lot of energy and tends to push the surrounding air particles, thereby expanding and hence rises due to buoyancy, and loses heat in the process, thereby becoming cold. The mountains being pointy and having more surface area, tend to dissipate their heat quicker. And that is why the air in the mountains feel cold. Aaha!
Son : "Ma, aren't we climbing up a mountain?" .
I : "Yes".
He : "Aren't we moving closer to the sky and the sun ? Why are we feeling cold?".
I : "Hmmm.. well.."
And flashback time it is! You know in the olden day movies, to depict a flashback, they show us a spiralling thing. Anyways, all I tried telling was, it got me back to my school days.
I'm sure all of you would've learnt about water cycle in your good old school days. I think I learnt it somewhere in my 2nd standard. My son is learning about it in the lower kindergarten. Things are moving fast now a days. Anyways, besides the point. From the water cycle, we all know one thing. Water from the seas, oceans and the rivers, evaporates and we distinctly remember seeing red lines or rather red curves representing the water vapors, rising up. With the explanation, hot air rises. Well, if that be the case, shouldn't the hills and the mountains all be warm or maybe even hot ? (hot from all the hot air that rises)
One more perspective to look at this whole thing, like how my 3 year old pointed out, we know our main source of heat is the sun. And the sun is high up in the sky. No denying that. One might argue that closer to the heat source, the hotter you feel. That is, the hills being closer to the sun than the ground, the hills should feel warmer than the plain land.
Two different perspectives, with good amount of logic, all telling us one thing. Hills should be hot or atleast warm. But from our experience, we know that the hills or hill stations, are cold. How could that be? What went wrong in my logic? Was my water cycle theory, the one that is clearly etched in my mind through the years wrong, or ..is the sun not high up in the sky?? Let's think this one through!
The water cycle is not wrong. Just think one step ahead. Why does hot air rise? When air gets heated, what happens actually? The molecules in the air, gain some energy. (heat is a form of energy) and the molecules, with their newly acquired energy move faster and bombard into each other and they tend to push the surrounding cold (not hot) molecules aside. And in the process, they've expanded and spent some energy too. Once they expand, they become less dense. And from our previous post, we know that the less dense substance when immersed in a denser substance, rises up due to buoyancy force. That's precisely why the hot air rises.
If you would've carefully read the above paragraph, you would've already said "voila, now I know why the hills are cold". The hot air spends some energy in expanding. Which means it becomes cold. The air that rises, eventually becomes cold. ( Now, a very good follow up question would be, why didn't it sink back then. Now that it is is cold, shouldn't it reverse all the effects that the heat caused and sink back down? There air on top is comparatively less denser than that of the lower layers. So, this cold air needn't sink down. Same density as the surroundings.)
That's one explanation. I've done some reading on the internet and I've found some different perspectives on this. Let me share them too, for completeness. Oh hold on, I still didn't explain what was wrong with the it-should-be-hotter-in-the-hills-because-its-closer-to-the-sun theory. Do you know how far up in the sky is the sun? It is a whopping 150 million kms. Compare that with the height of the heighest mountain you've ever heard of.. 9kms maybe? So whether you are on the top of the highest mountain or on the plain, it doesn't make a difference. We are still the same distance away from the sun, which is, very very very far away.
One more way of looking at why the hills are cold will be, the lower layers of the atmosphere are under pressure from the upper layers. So are usually compressed and under pressure. The upper layers experience lesser pressure. When the gas is under more pressure, it tends to be warmer. Because when a lot of gas molecules are trapped under a small space, the molecules bombard against each other and are in constant motion and hence get heated up. More the pressure, more the heat. And lesser the pressure, lesser the heat.
Another explanation, the sun's heat reaches us as light rays. not as heat waves. Through radiation that is. Once the surface of the earth absorbs it, it releases it as heat waves. One way to see it is that , the air near the earth is dense, so it absorbs more of this heat energy. The air near the mountains is less dense, and it absorbs less of the heat energy. And the layers of air closer to the earth are more warm as compared to the ones near the hills. Also, as the pointed mountain has lot more surface area than the plain, it tends to dissipate more of the absorbed heat, making it a cooler place.
Another one, imagine the layers of air above the earth as blankets you use to cover yourself. If you have one blanket over you, you feel warm. If you have two, you feel warmer. If you have three, you feel hot probably.. You get it? The more number of atmospheric layers on the plains, the more of the heat they hold within them. So you can say, as you go higher up, you start losing the blankets on top of you.
I didn't try explaining atmospheric pressure and density and buoyancy to my 3 year old. I just used the blanket analogy and he seemed happy. For a whole week, he went around asking this question to the adults he came across, and whether they gave him an answer or not, he said, "No. Let me tell you. It is because the hills have only two layers of air, whereas we have 10." And immediately thereafter, he made it a point to find me to give me a hi-fi! Oh, the joys of being a mom.
So essentially, it all boils down to these. The air in the plains are under a lot of pressure. So they are dense. They absorb heat faster, and they also act like a blanket and keep the heat there for us. The air that gets warm, tends to have lot of energy and tends to push the surrounding air particles, thereby expanding and hence rises due to buoyancy, and loses heat in the process, thereby becoming cold. The mountains being pointy and having more surface area, tend to dissipate their heat quicker. And that is why the air in the mountains feel cold. Aaha!
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