Science Questions

Why does it get cooler as we climb higher if we are getting closer to the sun?

Mon, 5th Sep 2016

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Question

Nozinhle Bee asked:

I want to ask, that they say the higher you go the cooler it becomes, the thing but when we are on planet earth its hot , like climbing a mountain you find that its freezing as you go up. what happens to the sun when climbing its supposed to be hot. just asking for a clarification

Answer

We put this question to physicist Caroline Steel...Mountain

Caroline - Firstly, the sun is 150 million kilometres away. So actually, climbing up a mountain or sort of moving a little bit further away from sea level is really quite negligible in comparison - there's 150 million kilometres. So that in itself shouldnít really have an effect on temperature. But it does get colder as we increase in altitude. There's lots of ways to look at this but one way to think about it is, as air rises, it feels less pressure the higher up it rises. So it expands. And to expand, the gas must do work and to do work, it must lose energy, and therefore, the individual gas particles have slightly less energy. And the definition of temperature is sort of the average energy of these molecules. So, if the molecules have less energy at a higher altitude, there's a lower temperature.

Kat - So basically, temperature, something thatís hot, itís all the molecules that are squished together and then moving a lot and thatís really hot. And then if you're going up into space, the air just kind of goes wooh!

Caroline - Yeah, exactly. It spreads out and by spreading out, you have less molecules with less energy and therefore, lower temperature.

Kat - David?

David - Yeah. What Caroline is describing is for lower atmospheres. This is probably what the questioner is asking about. But you get above the troposphere into the stratosphere, it starts to get warmer again because thatís where ozone molecules are absorbing solar ultraviolet energy so it does get warmer. But there's another layer beyond that called the thermosphere where the temperature is officially thousands of degrees, but you wouldnít feel hot if you were there because itís almost a vacuum. Itís just the energy of the individual molecules. Itís absorption of solar photons that shake at the molecules.

Kat - Ah, so they're really going as weíre up there.

David - They're going for it but they are so few and far between. There's high temperature but almost no heat.

Kat - And then when you get out of the atmosphere then you're really in trouble.

Caroline - So what would you sort of feel if there's a thermometer there? Would it have a high temperature or a low one, or does it sort of depend where it is Ė is it near the molecules or not?

David - What a thermometer is filled with mercury or something? I donít know. It depends how you define temperature. The molecular temperature is hot, but you would feel cold. You would need to be wrapped really, really warm apart from being almost in a vacuum.

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The intensity of solar radiation in the absense of obstructions varies inversely as the square of the distanct to the sun.  The sun is about 93,000,000 miles away. If you climb a 7 mile high mountain, that is an insignificant change. However, there will be a more significant increase in radiation because the air at that altitude is thinner, but even that is not enough to offset the change in pressure of the air between the bottom and top of the mountain, which is the main reason for the colder air. Atomic-S, Tue, 26th Jul 2016

If you imagine a breeze hitting the side of a mountain, and the air is deflected upwards.

As the air rises, the air pressure reduces, and the average distance between air molecules increase.

The air molecules are slightly attracted to each other (by Van Der Waals forces), and it takes energy to pull them farther apart. This energy comes from the kinetic energy of the air molecules; at higher altitudes they don't move quite as fast, and we measure this as a reduction in air temperature as we go up the mountain. evan_au, Tue, 26th Jul 2016

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