B. The particulate nature of matter / B.2 Greenhouse effect
Emissivity is the ratio of the power emitted per unit area by an object to the power emitted by a perfect black body of the same size and temperature.
Albedo is the ratio of the average energy reflected off a macroscopic system to the energy incident on the system.
The solar constant represents the average intensity of electromagnetic radiation incident on the Earth’s outer atmosphere. This value changes throughout the year which is why we state an average value.
Due to the Earth being a rotating sphere, the mean value of the incoming intensity of sunlight is S4 or 340 W m−2.
The natural greenhouse effect is essential for sustaining life on the Earth. Human activities, such as the combustion of fossil fuels, increase the concentration of greenhouse gases in the atmosphere, leading to the enhanced greenhouse effect.
The greenhouse gases are carbon dioxide, nitrous oxide, methane and water vapour. Their concentrations in the Earth’s atmosphere are affected by natural processes and human activity.
The greenhouse gases absorb infrared radiation because its frequency is the same as the natural frequency of the greenhouse gas molecules. The radiation is absorbed and resonance occurs.
Another explanation for why the greenhouse gases absorb infrared radiation is that the difference in the atoms’ energy levels is equal to the energy carried by a photon of infrared radiation, and the photon is absorbed.
The law of conservation of energy can be applied to the Earth surface–atmosphere system to explain why its thermal equilibrium is at risk.