[thegers91 (OP)]

Is it to do with radiation of heat into space? im not sure

[daveshorts]

Yes the earth gains heat by sunlight hitting it, and to a much lesser extent radioactive decay, and it looses heat by radiation - it is glowing in the infra red.

The hotter something is the more it radiates. This is why it is so much warmer on a cloudy night. The clouds are warm and radiating heat downwards keeping you warm, on a clear night you just loose heat into cold deep space which hardly radiates back at all.

Adding carbon-dioxide will increase the amount of heat that is reflected back, so the world will heat up until it radiates enough heat to be in equilibrium. Hence global warming.

[Fozzie]

The clouds are warm and radiating heat downwards keeping you warm,

Can you qualify this statement please. I always understood that entropy caused the clouds to equalise to the same temperature as the surrounding air, which is sometimes below freezing, so how can they radiate heat downwards? They need to be a heat source radiator, which they are not. My understanding is that clouds simply prevent heat from below escaping into space.

[Soul Surfer]

The gradual accumulation of small lumps of matter to create a planet (or a star )releases a very great deal of energy turning gravitational potential energy into kinetic heat energy.  This means that the earth once it had first formed was very hot and probably molten.  This is why most of the heavy materials (notably iron) have sunk to form the core of the planet.  Residual nuclear energy  from the small quantities of radioactive materials in the earth is also a significant heating effect so the earth is slowly cooling down but there is a net heat flow out of the solid surface of the planet and it still gets hotter as you go deeper and it is probably very hot at the centre of the earth.  The balance between heat input from the sun and that radiated into space is added to this.  in the absence of the heat from the sun the surface temperature of the earth would be quite low but well above the cosmic microwave background radiation temperature.

[daveshorts]

Can you qualify this statement please. I always understood that entropy caused the clouds to equalise to the same temperature as the surrounding air, which is sometimes below freezing, so how can they radiate heat downwards? They need to be a heat source radiator, which they are not. My understanding is that clouds simply prevent heat from below escaping into space.

At night the clouds are heated by radiation from the ground. This means that the temperature of the cloud will be hotter than the sky. It will then radiate some heat upwards and some back down. This means that the ground will have to be hotter than it would be without the clouds.

[diagram=543_0]

The clouds act as insulation, by reradiating

[litespeed]

SS - I have been absent mindedly pondering this subject and have a few additional thoughts.  Specifically, the earth is under constant bombardment from high energy cosmic rays and such. In addition, a fair number of small meteors are burning up in the atmoshere all the time.

I sort of doubt these are significant contributiors, though cosmic rays carry a lot of energy individually. OH.  Volcanoes release interior heat in a number of ways, such as molten lava. Again, probably not a big net loss but vocanic gasses and dust can reduce received radiation quite a bit.

[Soul Surfer]

To go back to the original question.  The surface  and atmospheric temperatures of the earth are a balance between all the sources of heat input and output by radiation from the planet.  Dark coloured land and sea radiates much more effectively than white cloud cover.  Extra heat tends to create clouds by evaporating water into the atmosphere where it rises cools and condenses into clouds so this is a negative feedback process.  Atmospheric carbon dioxide and methane also has a small but significant part to play in this short term surface temperature balance.  Once you have gone down a shortish distance (a few hundred metres say into the solid earth, rocks or ocean the relatively poor thermal conductivity of the material means that surface temperature changes have no effect on the temperature and it is entirely determined by the heat flow through the material.  Almost all the deep oceans are at an almost constant temperature of 4 degrees centigrade throughout apatr from a few hotb spots.  This is the temperature of the maximum density of water. Once you are into rock the temperature rises steadily as you get deeper although there are some hot spots near volcanoes etc where it is much hotter close to the surface but you only have to go a few miles down in rock to find that the temperature has risen above the boiling point of water (at normal atmospheric pressure.  The temperatue of the core of the earth is estimated to be at 5700 degrees centigrade almost as hot as the surface of the sun!