# Fire and sun: transmission of heat

As we know there are three well known ways in which heat is transmitted:

1. conduction
2. convection

I suppose fire and sun make us warm in the same way: radiation. Of course, near the fire there should be convection and conduction, but about $$1$$ meter away, heat should be tranferred to our body mainly by IR radiation.

Questions

Is the previous reasoning correct?

Then, if heat trasmites mainly through radiation -as in a fire-, but why there is such a difference out of atmosphere if heat comes from radiation?

• Re, "out of the atmosphere...nearer the Sun"; If you're talking about a spacecraft orbiting three or four hundred miles up, then don't forget that the Sun is ninety three million miles away. – Solomon Slow Feb 26 '18 at 18:09
• @jameslarge right, but it is not only that space isn't hotter, it is colder.. – santimirandarp Feb 26 '18 at 18:14
• Space isn't hot or cold. Things in space get hot when they absorb solar radiation, and they get cold when they are shielded from the Sun and all of their heat radiates away. You've probably seen pictures of spacecraft wrapped in shiny metal foil--often gold colored? The purpose of that foil is to reflect away as much solar energy as possible to help keep the spacecraft cool. – Solomon Slow Feb 26 '18 at 18:19
• @SolomonSlow didn't pay attention to the comment. That's exactly what I'm in doubt with...you should post an answer XD, so they are kinda isolated of space "heat"? – santimirandarp Sep 30 '18 at 14:49

If heat trasmites mainly though radiation, why is colder out of the atmosphere, when we are nearer the sun?

Heat tranmission is different than heat retention. The atmosphere retains the heat to the level that life can exist. It is the so called "green house effect".

The moon which has no atmosphere gets fried in the sun and near zero in the shadow.

Why is the temperature in the green house hotter than outside the green house?

answer:there is no direct convection to remove heat from the greenhouse, the bulk can just cool by radiation, and inner convection which will transfer heat by conduction to the outside glass for outside convection. This raises the temperature in the greenhouse with respect to the outside conditions. Same with a closed car where children die when left there in the sun.

Then, if heat trasmites mainly through radiation

within the atmosphere convection plays a huge role in heat transmission.

-as in a fire-,

Within the atmosphere of the room convection plays a large role in homogenizing temperature. Radiation has to be absorbed by something, transfer kinetic energy to the molecules , to be counted in the average kinetic energy as temperature.

but why there is such a difference out of atmosphere if heat comes from radiation?

Change in heat is connected with a change in temperature, and temperature is defined by the kinetic energy of the molecules.

So objects get hotter when the kinetic energy of the molecules that composes them gets larger. Out of the atmosphere there are very few molecules to absorb the radiation and increase their kinetic energy .Heat is energy and in the vacuum of space there are very few molecules to carry any energy.

• Good..And the heat we feel is mainly transmited by radiation or by what? – santimirandarp Feb 26 '18 at 18:17
• The heat we feel is the temperature the atmosphere has at sea level mostly, unless we live in the mountains. It is radiation on our skin, to start with, tempered by convection , which removes heat or transfers from hotter regions – anna v Feb 26 '18 at 18:21
• But why a fire emits so much heat if it is just radiation? Isn't the same with the sun? – santimirandarp Sep 29 '18 at 20:36
• I have edited the answer – anna v Sep 30 '18 at 3:07

Earth of course has a Greenhouse Effect--where absorbed solar radiation is remitted in the infrared from the surface. That is absorbed by the adjacent layer of atmosphere, which reradiates isotropically, leading to heating of the layer above that, and so on until you get to 1 optical depth from the top, and radiation can escape to space. (It is this top layer that has a temperature that matches the power absorbed from the sun). The surface temp (in a weather-free model) just depends on the total IR optical depth--so adding Greenhouse gases increases it (c.f. Venus).

An aside on fire: if you watch a fire spread through the neighborhoods of LA, direct IR is a huge factor. When a 100ft wall of fire is a "safe" distance away with respect to convetion and conduction, you will see houses' paint bubble and smoke and then the whole thing bursts into flames. Nevertheless, I have seen houses without a line-of-sight view of flames but bathed in hot wind burst into flames too.

That is the greenhouse effect. The Earth as a whole is radiating as much energy as it receives from the Sun. Seen from space, Earth has a temperature of about 255 kelvin. That radiation comes from IR-active molecules in the atmosphere, mainly carbon dioxide, ozone and water.

Now, these molecules radiate both upward and downward. So the surface is heated both by direct sunlight and by IR radiation from the upper atmosphere. These two contribution are roughly equally important.

The surface must also radiate as much as it absorbs. According the the Stephan-Boltzmann law, radiation is proportional to $T^4$. To radiate twice as much as the upper atmosphere, the surface should be warmer with a factor $\sqrt[4]{2} \approx 1.19$. This rough estimate would result in a surface temperature of about 303 K.

• But the top layer of atmosphere is hotter, why it is so? – santimirandarp Feb 26 '18 at 23:37
• @santimirandarp Those outermost molecules are so few. They are not in thermodynamic equilibrium with the atmosphere. And their temperature is not what the IR satellites see. – Pieter Feb 28 '18 at 14:00