The atmosphere also radiates long-wave radiation according to its own temperature. Through long-wave radiation, heat is exchanged between the ground and the atmosphere, as well as between the gas layers in the atmosphere, and is also dissipated into space. (a) Ground and atmospheric radiation means that both the ground and the atmosphere emit radiant energy according to their respective temperatures. . (2) Characteristics of long-wave radiation on the ground and in the atmosphere 1. The absorption of long-wave radiation by the atmosphere is very strong, and the absorption is not only related to the absorbed substances and their distribution, but also to the temperature and pressure of the atmosphere. The atmospheric components that play an important role in absorbing long-wave radiation are water vapor, liquid water, carbon dioxide and ozone. Their absorption of long-wave radiation is also selective. Figure 2. 12 depicts the emission and transmission spectra of long-wave radiation in the whole atmosphere. As can be seen from the figure, except for the 8- 12 micron section, the atmospheric transmittance of the whole long-wave band is close to zero, that is, the absorption rate is 1.
8- 12μ m has the smallest absorption rate and the largest transparency, so it is called "atmospheric window". The radiation in this band is located in the place with the strongest radiation ability on the ground, so 20% of the energy radiated from the ground is emitted into space through this window. There is a narrow ozone absorption band near 9.6 microns in this window, which can absorb almost all the far infrared radiation above 14 microns of ground radiation, so the band can be regarded as close to a black body. Water vapor is the most significant absorber of long-wave radiation, which can be absorbed in all bands except 8- 12μ m band. The absorption capacity is strongest in the band around 6 microns and above 24 microns, and the absorption of long-wave radiation by liquid water is similar to that of water vapor, but the effect is stronger. The thick cloud surface can be regarded as a blackbody surface. Carbon dioxide has two absorption bands, the centers of which are located at 4.3 microns and 14.7 microns respectively. The first absorption band is located at the end of the radiant energy curve of the absolute blackbody at the temperature of 200-300 K, which has little effect. The second absorption band is from 12.9- 17. 1 micron, which is more important.
2. Characteristics of long-wave radiation in the atmosphere The transmission process of long-wave radiation in the atmosphere is very different from that of solar radiation. First, the direct radiation of solar radiation enters the atmosphere in the form of directional parallel radiation, while the ground and atmospheric radiation are diffuse radiation. Second, when solar radiation propagates in the atmosphere, only the weakening effect of the atmosphere on solar radiation is considered, but the influence of the atmosphere itself is not considered. This is because the atmospheric temperature is low and the short-wave radiation generated is extremely weak. But when considering the propagation of long-wave radiation in the atmosphere, we should not only consider the absorption of long-wave radiation by the atmosphere, but also consider the long-wave radiation of the atmosphere itself. Third, when long-wave radiation propagates in the atmosphere, scattering can be ignored. This is because the scale of gas molecules and dust particles in the atmosphere is much smaller than the wavelength of long-wave radiation, and the scattering effect is very weak.
(3) Atmospheric inverse radiation and ground effective radiation 1. Atmospheric Inverse Radiation and Atmospheric Insulation Effect The part of atmospheric radiation pointing to the ground is called atmospheric inverse radiation. The inverse radiation of the atmosphere compensates the energy lost by radiation on the ground, which shows that the atmosphere has a heat preservation effect on the ground, which is called the heat preservation effect of the atmosphere. According to the calculation, if there is no atmosphere, the average temperature near the ground should be -23℃, but in fact, the average temperature near the ground is 15℃, which means that the existence of atmosphere increases the temperature near the ground by 38℃. 2. Ground effective radiation The difference between the radiation emitted by the ground (for example) and the atmospheric inverse radiation (Δ ea) absorbed by the ground is called ground effective radiation. Expressed as F 0, then F 0 = eg-δea(2.20). In general, the ground temperature is higher than the atmospheric temperature, and the effective radiation on the ground is positive. This means that the surface often loses heat through the radiation and absorption of long-wave radiation.
Only when the inversion temperature in the near-surface layer is strong and the air humidity is high can the effective radiation be negative, and then the ground can gain heat through the exchange of long-wave radiation. The main factors affecting effective radiation are: ground temperature, air temperature, air humidity and cloud conditions. Generally speaking, the effective radiation under hot and humid weather conditions is smaller than that under dry and cold conditions, and it is smaller under cloud cover than under clear sky conditions; When the air turbidity is high, the effective radiation is smaller than that when the air is dry and clean. The effective radiation is small when the wind is strong at night; The effective radiation at high altitude is large, and the near-surface temperature is large when the altitude drops significantly; When there is inversion, the effective radiation is very small, and even negative value can appear. In addition, the effective radiation is also related to the nature of the surface, and the effective radiation of smooth surface is smaller than that of rough surface. The effective radiation under vegetation cover is less than that under bare land. The effective radiation has obvious diurnal and annual variations. Its daily variation is similar to the temperature.
During the day, due to the increase of vertical temperature gradient in the lower atmosphere, the effective radiation value also increases, reaching the maximum at noon 12 ~ 14; At night, due to the ground radiation cooling, the effective radiation value gradually decreases and reaches the minimum value in the early morning. When there are clouds in the sky, the daily variation law of effective radiation will be destroyed. The annual variation of effective radiation is similar to the temperature, with the maximum in summer and the minimum in winter. However, due to the influence of water vapor and clouds, the maximum effective radiation does not necessarily appear in midsummer. The effective radiation in the areas south of Qinling Mountains and Huaihe River in China is the largest in autumn and the smallest in spring. The effective radiation in North China and Northeast China is the largest in spring and the smallest in summer, which is due to the influence of water vapor and cloud conditions.