4) The green house effect

4) The green house effect

Even though the temperature at the surface of the Earth is +15 [°C] (59°F), when observed from space the temperature of the planet is -19 [°C] (- 2.2°F).


The fact that the surface temperature of the Earth remains +15 [°C] (59°F) is of course due to the solar rays that are not immediately reflected towards space or immediately absorbed by the atmosphere.

A large part of these rays therefore go through the atmosphere and reach the ground. In return, the heated ground sends infrared rays towards the atmosphere.
These infrared rays coming from the ground are partially captured by the atmosphere, which allows the planet to maintain its temperature around +15 [°C] (59°F) close to the ground, while its temperature around 5,000 [m] altitude is -19 [°C] (- 2.2°F).

We are thus “protected from the -19 [°C] (- 2.2°F)” by a sort of “atmospheric cocoon” which occurs through a process called Greenhouse Effect.
In the end, everything happens a bit as if the atmosphere was a layer of transparent isolation (such as a layer of air), and whose thermal resistance generates the difference in temperature of 15 – (–19) = 34 [°C] (34 K).

We can certainly use the insulated glazing unit (IGU) analogy here.

For a flux of 235 [W/m²] leaving the surface of the Earth and a temperature difference of 15 – (–19) = 34 [°C] (34 K) “at the extremities”, we can say therefore that the thermal resistance of the atmosphere is:
R = 34 / 235= 0.15 [m².K/W]

This value will come as no surprise to thermal technicians in the construction industry…

The thermal resistance of layers of air does not increase with the thickness because of the increase in convective movements of air, but it can increase depending on the nature of the gases that constitute the layer…

If the thermal resistance of the atmosphere towards a heat flow increases, the Earth’s temperature at the ground surface has to increase so that the Earth can maintain its evacuation of energy towards space.