The Earth is surrounded by space, known to be almost empty and very cold. Cruising on a plane at an altitude of 10,000 [m], the pilot announces temperatures in the -40 [°C] (-40°F) range. The temperature in space has been calculated to be -270 [°C] (- 454°F) (3 degrees Kelvin).
Surface temperature in land and ocean is on average +15 [°C] (59°F), but when observed from space the temperature of the Earth is -19 [°C] (- 2.2°F). The latter temperature allows the planet to discharge as radiation the heat it receives from the sun and from the center of the Earth. Across the planet, this temperature is approximately equal to the temperature of the atmosphere at 5,000 [m] altitude, according to measurements obtained with weather balloon (See Global warming - Part 2 ).
The difference between surface temperature (+15[°C], 59°F) and “exterior surface” temperature (-19 [°C], - 2.2°F) is due to the “greenhouse effect” (We could hardly do without it, because the average surface temperature would be -19 [°C], - 2.2°F)
The earth is surrounded by space, which we know is essentially a vacuum of extremely cold temperature (-270 [°C], (-454°F)).
Space being almost empty of matter, the earth cannot discharge its heat by conduction or by convection. The remaining possibility is a discharge by radiation.
Observed from space at -270 [°C] (-454°F), the earth is "hot", given its -19 [°C] (-2.2°F).
The earth therefore sends a strong radiation towards space.
The power of this radiation is strictly equal to the power the planet receives (from both inside and outside), at least as long as the temperature of the planet does not fluctuate.
Similar to all thermal exchanges, the magnitude of the transmission is a function of the difference in temperature that triggered the exchange.
In the case of an exchange by radiation between two systems, the power transmitted is proportionate to the difference in the two temperatures raised to the fourth power (the temperatures being expressed in degree Kelvin):
Flux = k x (T14- T24)
Flux: Power exchanged by radiation, in [W]
T1 : Temperature of the warmest element, in [ K degree]
T2 : Temperature of the coldest element, in [K degree]
k : Coefficient of proportionality, in [W/K 4]
The planet discharges its energy towards space through radiation. As part of an exchange of heat through radiation between two systems, the transmitted power is proportional to:
- The magnitude of the difference between their temperature raised to the 4th power (the temperature values in degree Kelvin)
- A coefficient of emissivity
The coefficient of emissivity of the planet is a function of the nature of the surface of the Earth, of the cloud cover and of the nature of the gases in the atmosphere. If this coefficient was decreasing, the planet’s temperature would have to increase so that the planet could maintain its discharge of energy towards space.