# Can heat exit the atmosphere?

1. Sep 25, 2006

### mtanti

Here is my theory:

Once heat radiation enters our atmosphere from the sun and gets absorbed by the material inside, it is emmitted back as conduction/convection. In order for this heat to travel it needs a medium such as air. Now isn't it true that once this heat reaches the border between atmosphere and space it stops?

Therefore this would be an explaination for global warming since most of the heat entering the atmosphere gets blocked inside, regardless of carbon dioxide. It would also explain why the further up in altitude you go the colder it becomes, because the density of air goes down the higher you go. Therefore there would be less possibility for heat to travel upwards since the space between 2 neighbouring air molecules increases.

Am I correct?

2. Sep 25, 2006

### Astronuc

Staff Emeritus
Besides conduction and convection, there is radiation as an energy transport mechanism.

3. Sep 25, 2006

### ZapperZ

Staff Emeritus
You are already contradicting yourself. How do you think the heat from the sun reaches the earth?

IR radiation also transfers heat. Since it is an electromagnetic spectrum, it requires no medium. Any body that has any heat content will radiate in the IR region.

Zz.

4. Sep 25, 2006

### mtanti

Yes but once it is absorbed by a body it will release the heat as conduction. No? Would there be more radiation than conduction?

5. Sep 25, 2006

### ZapperZ

Staff Emeritus
It doesn't matter, and it depends on the situation. In any case, any body with heat content WILL radiate, so there is heat loss even in vacuum, which is sufficient to falsify your idea.

Zz.

6. Sep 25, 2006

### Staff: Mentor

That's an improperly worded sentence anyway. "release the heat as conduction" is wrong. Heat is transferred through conduction... and also through radiation.

The atmosphere, through the "greenhouse effect", does indeed trap heat: the sun's radiated heat is centered around the visible to UV part of the spectrum (not sure where exactly) and that radiation mostly passes right through the atmosphere to the ground. When it is re-radiated, it is in the infrared part of the spectrum and the atmosphere is largely opaque to IR radiation. All that said, after eons, the earth reached a reasonably stable equilibrium temperature.

7. Sep 25, 2006

### Elektrotechniker

Think of visible light. Visible light obviously reaches us from the sun. For a long time scientists assumed light must propagate like sound, through a medium. They (usually) called this medium "aether". Relativity was kind of the final blow for aether-theory. Turns out light (a type of electromagnetic radiation) doesn't need a medium. In fact http://physics.uoregon.edu/~jimbrau/BrauImNew/Chap03/FG03_09.jpg" parts of the spectrum). A great deal of this energy becomes heat (how exactly is beyond my understanding - someone else care to fill in?). So earth is a hot object and like all hot objects will in turn radiate EM (mostly in the infrared band).

No heat energy is lost through conduction.

Last edited by a moderator: Apr 22, 2017
8. Sep 25, 2006

### mtanti

So once an object recieves EM heat, it won't transform it into conductive heat? It will return it back as EM heat (of lower frequency perhaps)?

9. Sep 25, 2006

### Bystander

In a word, "No."

See Rohsenow & Hartnett, 15-76; or, in another word, "No."

Earth's atmosphere has been divided into four layers for convenience in analyses of its physical interactions with the solid, or liquid, surface, solar radiation, and the CMB: these are the troposphere, a convective layer extending from the surface to an altitude of 3-10 km; the stratosphere, a vertically non-convective layer extending from the upper boundary of the troposphere (tropopause) to 30-40 km; the mesosphere (ionosphere); and the exosphere. The temperature gradient in the troposphere is from high, surface T, to low, ca. 200 K at the tropopause, and maintained by adiabatic cooling of rising air masses in convection cells driven by surface heat; the temperature gradient in the stratosphere is from low at the tropopause to high at greater altitudes (plus a few odd inflections, wrinkles, and local minima); the mesosphere and exosphere are low enough in density that we're going to ignore them for heat transport purposes.

Conduction of heat through the stratosphere proceeds from high altitudes to the tropopause; conduction and convection of heat proceed in the troposphere from the surface to the tropopause. That is, heat absorbed from solar radiation at high altitude is conducted to the tropopause, and heat absorbed at the surface is conducted and convected to the tropopause, and some fraction is radiated either to the troposphere, or through the troposphere. Heat convected to the tropopause returns to the surface through adiabatic compression in the downward regions of convection cells; that's what makes the temperate climate zones livable.

Everyone "clear" on the fact that the heat transfer problem is NOT as simple as the kindergarten greenhouse model?

10. Sep 27, 2006

### mtanti

Well ok but still if heat inside the earth was only conductive than no matter what the atmosphere does, the heat cannot escape it. So is heat absorbed by matter as radiation transferred back to the air as conduction or as another radiation? Which of the two is most?

11. Sep 27, 2006

### Bystander

Heat is heat. It does not come in flavors. It is transferred by convection, conduction, and radiation. If there is NO convective path, it can only be transferred by conduction and radiation. If there are NO convective or conductive paths, it can only be transferred by radiation. So long as an object, or construct (black body cavity) contains heat, it is above absolute zero temperature, and it radiates heat.

"Which is most?" Either --- it depends upon the physical properties of the system you're analyzing (emissivities, heat capacities, thermal diffusivities, expansivities, viscosities) and of its surroundings.