# Why / How do we feel heat from the sun

MikeeMiracle
TL;DR Summary
Why / How do we feel heat from the sun
Probably a silly question and a simple one for yourselves which I can't figure out myself. Hopefully I can explain it properly.

Lets say I am standing on the equator and the sun is directly overhead. I am probably in the middle of a desert and it's around 50 degress centigrade. Assuming I could fly, as I fly upwards through the atmosphere towards the sun I would get colder and assuming I am 200 miles up, I assume I would measure the heat of space as a few degress K as most of space is?

If 200 miles up I feel little heat, then how is it the surface of the planet is as hot as it is. I am assuming it's something to do with heating air in the atmosphere, but if outside the atmosphere is so cold, how did the heat get to the surface? It surely cannot be a gradual trapping of solar radiation otherwise there would not be such big temeprature variations between night and day.

It must be an interaction between some kind of solar radiation and it's interaction with the atmosphere, but this must be some kind of "indirect" heat as opposed to feeling the sun heat "directly" otherwise I would feel hot when i fly out of the atmosphere.

Homework Helper
It's the sunlight. It can power solar arrays and it can heat up a dark surface very quickly.

I would also note that satellites orbiting the Earth are also warmed by the sun - and the difference in temperature between the dark and sunlit side of a satellite is an engineering issue - as is a low-orbiting satellites rapid transition to night and day and night and day...

MikeeMiracle
So the sunlight heats the surface, and the surface radiates that heat into the atmosphere? Or it heats both at the same time?

I guess I am just seriously underestimating the amount of energy provided by the sunlight.

So by that token, 200 miles up I do not feel any heat as there is no medium for the energy in the sunlight to be transferred to then I assume.

Homework Helper
The sun heats the atmosphere and the Earth's surface. The Earth's surface and its atmosphere exchange heat - through conductance and radiance. When you bundle up against a frigid wind, you are more concerned with heat conduction directly from the surface of your skin to the air than you are with radiance.

Speaking of "flying out of the atmosphere", I suppose I did that once - at least as a simulation. It was a hypobaric chamber exercise. The air was rapidly vented from the chamber I was into simulate the rapid decompression that can occur with high-flying jet aircraft. It immediately became foggy and the temperature dropped to about -40F. I was in a tee shirt - but it was not uncomfortable. The air was too thin to have a strong affect through conduction and the walls of the chamber were still at room temperature. So I was still getting thermal infrared radiance from those walls.

Motore
You do feel heat from sun's radiation, so in Earth's orbit in direct sunlight you would actually fell around 100°C or more. Go into some shade however and you would freeze. Space is a really good insulator and heat can only escape through radiation and not conduction as here on Earth (where there's an atmosphere). That's why you need a pressurized space suit to not be cooked or turned into an ice cubicle.

Here on Earth the story is of course different.
Check this for further insight:
https://en.wikipedia.org/wiki/Earth's_energy_budget

anorlunda
lomidrevo
Our bodies are not directly sensitive to the temperature of the environment. What we feel as hot or cold, are changes in the in/out thermal energy budget, with a constraint to maintain a constant body temperature, roughly speeking. As mentioned in above posts, you can exchange the heat with environment either by conduction or by radiation, so both of these mechanisms have to be accounted.

At the Earth's distance from the sun the solar radiance is about 1.36 kw/m^2. An average human body has a surface area of 1.6 to 1.9 m^2; so if you full face the sun (exposing 0.8 m^2) your body would be receiving nearly 1 kw -- and that's a lot of heat.