# Heat (Energy) distribution during day-night cycle

1. May 2, 2013

### Trafiq

Hello everyone,

This is my first post on PhysicsForums (although not my first encounter) which I would like to start with asking a question. How does heat from the sun get distributed on earth so that we have temperature variations between day and night?

In other words, if an area/country/continent gets nice and warm to say, 25°C during the day, where does that heat go during the night when the temp drops to say, 18°C?

My approach so far has been to think of the earth as a whole particle, with one half being heated by the sun and the other half in darkness. Obviously the heat doesn't just dissipate in vacuum by rising to a "cooler" ~3K vacuum (which is an insulator by itself anyway), so what could it be?

This is not a homework question, this is just a question I've asked myself and have yet to answer properly.

If someone could enlighten me, then that'd be great.

Regards,
Trafiq

Last edited: May 2, 2013
2. May 2, 2013

### Bandersnatch

Hello, welcome to PF!

In general, heat can be transferred in three ways: by conduction, by convection, and by radiation.
Space is a near-perfect heat insulator, so conduction is out. Convection can hardly apply, as there is nothing in space to form convection bubbles either.
This leaves radiation, which indeed does account for all the energy losses.

On planets with atmospheres, there are more factors to keep in mind when considering heat distribution, e.g., the greenhouse effect or air mass movement(wind).
Still, in general terms, after reaching thermal equilibrium, the Earth reradiates all the extra heat into space.

So, in actuality, the "obvious" bit in your question is incorrect.

Look up black body radiation and Stefan's Law for more insights on the subject.

3. May 2, 2013

### Trafiq

Right, so it doesn't dissipate via conduction or convection apart from wind, but it does dissipate via radiation. I was actually thinking the means could be radiation via infrared or perhaps microwave photons as I was typing the OP, but wasn't sure. Ok so apart from energy transport via air masses, the heat just simply radiates away. Cool.

Thank you for the clarification, Bandersnatch.

4. May 2, 2013

### sophiecentaur

Absolutely the ground cools very quickly on a clear night because a lot of the infra red radiation goes straight out into space. When there are clouds up there, they absorb the radiation and, because of their temperature ( not far below that of the ground) they radiate loads of heat back down again and the ground stays warmer.

This is scratching at the surface of a topic that always gets people going on PF - the thermal situation in the atmosphere is very very complex and there is not just one mechanism at work.

5. May 3, 2013