What is the atmospheric temperature in space

Energy balance of the earth

The earth's energy or radiation balance is divided into a short-wave and a long-wave part. The energy input of the sun into the earth's climate system is on average 342 W / m².

Almost all of the energy available on earth comes in the form of electromagnetic radiation from the sun. Due to its high surface temperature of around 5,700 ° C, the sun shines mainly in the short-wave, visible wavelength range. This energy is radiated evenly in all directions. How much of it arrives at a particular planet depends on its distance from the sun. At the upper edge of the earth's atmosphere, the input of solar energy corresponds to the solar constant with an average value of 1,367 W / m². The solar constant is subject to long-term and short-term variations due to the variable solar activity.

Solar radiation as an energy source

In fact, only a quarter of the energy coming from the sun gets to the edge of the atmosphere. First, at any point in time, half of the globe is facing away from the sun. Second, due to the flattening of the globe towards the poles, the radiation only hits the equator with full intensity. This results in the 342 W / m² of solar energy shown in Figure 1, which reaches the upper edge of the atmosphere.

The earth's radiation balance is responsible for whether the earth warms up or cools down. If, in total, less energy is radiated into space than it reaches the earth (positive radiation or energy balance), this causes the earth to warm up. A negative energy balance consequently has a cooling effect as a consequence. All figures given in Figure 1 are global mean values ​​of the individual energy flows and describe an overall balanced energy balance. The individual energy flows provided with numbers are intended to give a quantitative insight into the most important components of the earth's energy balance. However, there are major regional differences, which are related to many factors that determine the earth's climate: inclination of the earth's axis, distribution of water surfaces, atmospheric and oceanic circulation, etc.

Fig. 1: Estimated mean annual and global energy balance of the earth (values ​​in W / m²) (Solomon et al. 2007 revised).

How is radiation implemented on earth?

Of the incident 342 W / m², an average of 107 W / m² is reflected by aerosols in the atmosphere, water droplets in the clouds and the surface of the earth and thus get back into space. Responsible for this is the reflectivity of the earth, the planetary albedo, which assumes a mean value of 31%. Of the remaining 235 W / m², 67 W / m² are absorbed by water vapor and other particles in the atmosphere and heat them directly. Around half of the solar radiation, 168 W / m², reaches the earth's surface and is absorbed by it (-> warming of the earth's surface). The energy absorbed by the earth is reintroduced into the atmosphere in the form of convection and long-wave thermal radiation, or it is used to convert water into another phase state (latent energy / heat).

Short and long wave part of the radiation balance

The incident energy belongs to the short-wave part of the radiation balance. The radiation coming from the sun with a temperature of 5,500 ° C is called short-wave with a wavelength of 0.2 to 3 μm (micrometers), some of which humans perceive as light. The earth's surface and atmosphere, which is around 15 ° C warm, radiate energy in the form of long-wave thermal radiation (wavelength 3-60 μm) into space. This long-wave heat radiation is not visible to the human eye and is at the other end of the spectrum like the incident short-wave radiation from the sun.

Vital greenhouse effect

The earth's surface radiates 390 W / m² in the form of long-wave heat radiation. Only about 40 W / m² can escape unhindered through the atmosphere into space. The remaining 350 W / m² are absorbed due to the absorption properties of the atmosphere and the natural greenhouse gases it contains. The property of greenhouse gases to allow short-wave radiation to pass through unhindered but to absorb long-wave radiation results in the much-cited greenhouse effect. The atmospheric counter-radiation caused by this is 324 W / m², which is radiated back towards the earth's surface. This naturally caused greenhouse effect is a decisive prerequisite for the life-friendly, climatic conditions on earth. Without the natural greenhouse effect, there would be no pleasant average temperature of 15 ° C on earth, but an icy -18 ° C!
As previously stated, the long-wave radiation from the earth's surface (390 W / m²) emits 324 W / m² back towards the earth's surface due to the atmospheric counter-radiation. As shown in Figure 1, 24 W / m² of this is lost due to atmospheric convection and 78 W / m² due to the latent heat required for evaporation. The sum of these proportions results in a total of 168 W / m², which are radiated into space. With the 168 W / m² incident on the earth's surface, the assumption made that the energy balance is balanced is fulfilled.

In principle, every body strives for an energetic balance with its surroundings. The many individual processes and their complex relationships to one another result in a highly dynamic behavior of the temperature of the earth, which fluctuates around the state of equilibrium. Trenerth et al. (2009) calculated a slightly positive energy balance (~ + 1W / m²) for the earth for the period 2000-2004.


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