Лекция: Energy transfer within the earth-atmosphere system

The distribution of solar radiation is often described as if it were all available at the earth's surface. This is, of course, an unreal view because of the effect of the atmosphere on energy transfer. Heat energy can be transferred by the three following mechanisms:

1 Radiation: Electromagnetic waves transfer energy (both heat and light) between two bodies, without the necessary aid of an intervening material medium, at a speed of 300 * 106 m s-1 (i.e. the speed of light). This is so with solar energy through space, whereas the earth's atmosphere allows the passage of radiation only at certain wavelengths and restricts that at others.

Radiation entering the atmosphere may be absorbed by atmospheric gases in certain wavelengths but most shortwave radiation is transmitted without absorption. Scattering occurs if the direction of a photon of radiation is changed by interaction with atmospheric gases and aerosols. Two types of scattering are distinguished. For gas molecules smaller than the radiation wavelength (λ), Rayleigh scattering occurs in all directions and is proportional to (1/λ4). As a result, the scattering of blue light (λ@0.4 µm) is an order of magnitude (i.e. x10) greater than that of red light (λ @0.7 µm), thus creating the daytime blue sky. However, when water droplets or aerosol particles, with similar sizes (0.1-0.5 µm radius) to the radiation wavelength, are present, most of the light is scattered forward. This Mie scattering gives the greyish appearance of polluted atmospheres.

Within a cloud, or between low clouds and a snow-covered surface, radiation undergoes multiple scattering. In the latter case, the 'white out' conditions typical of polar regions in summer (and mid-latitude snowstorms) are experienced, when surface features and the horizon become indistinguishable.

2 Conduction: By this mechanism, the heat passes through a substance from point to point by means of the transfer of adjacent molecular motions. Since air is a poor conductor, this type of heat transfer can be virtually neglected in the atmosphere, but it is important in the ground.

3 Convection: This occurs in fluids (including gases), which are able to circulate internally and distribute heated parts of the mass. The low viscosity of air and its consequent ease or motion makes this the chief method of atmospheric heat transfer. It should be noted that forced convection (mechanical turbulence) occurs due to the development of eddies as air flows over uneven surfaces, even when there is no surface heating to set up free (thermal) convection.

Convection transfers energy in two forms. The first is the sensible heat content of the air (called enthalpy by physicists), which is transferred directly by the rising and mixing of warmed air. It is defined as cpT, where T is the temperature and cp (= 1,004 J kg-1 K-1) is the specific heat at constant pressure (the heat absorbed by unit mass for unit temperature increase). Sensible heat is also transferred by conduction. The second form of energy transfer by convection is indirect, involving latent heat. Here, there is a phase change but no temperature change. Whenever water is converted into water vapour by evaporation (or boiling), heat is required. This is referred to as the latent heat of vaporization (L). At 0 °C, L is 2.50 * 106 J kg-1 of water. More generally,

where T is in °C. When water condenses in the atmosphere, the same amount of latent heat is given off as is used for evaporation at the same temperature. Similarly, for melting ice at 0 °C, the latent heat of fusion is required, which is 0.335 * 106 J kg-1. If ice evaporates without melting, the latent heat of this sublimation process is 2.83 * 106 J kg-1 at
0 °C (i.e. the sum of the latent heats of melting and vaporization). In all of these phase changes of water there is an energy transfer.

Упражнение 2.

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