Лекция: Effect of the atmosphere

Solar radiation is virtually all in the short-wave­length range, less than 4 µm. About 18 per cent of the incoming energy is absorbed by ozone and water vapour. Ozone absorp­tion is concentrated in three solar spectral bands (0.20-0.31, 0.31-0.35 and 0.45-0.85 µm) while water vapour absorbs to a lesser degree in several bands between 0.9 and 2.1 µm. Solar wavelengths shorter than 0.285 µm scarcely penetrate below 20 km altitude, whereas those >0.295 µm reach the surface. Thus, the 3 mm (equivalent) column of stratospheric ozone attenu­ates ultraviolet radiation almost entirely except for a partial window around 0.20 µm, where radiation reaches the lower stratosphere. About 30 per cent is immediately reflected back into space from the atmosphere, clouds and the earth's surface, leaving about 70 per cent to heat the earth and its atmosphere. Of this, the greater part eventually heats the atmosphere, but much of this heat is received by the atmosphere via the earth's surface. The ultimate retention of this energy by the atmosphere is of prime importance, because if it did not occur the average temperature of the earth's surface would fall by some 40°C, obviously making most life impossible. The surface absorbs almost half of the incoming energy available at the top of the atmosphere and reradiates it outwards as infrared waves of greater than 3 µm. Much of this reradiated long-wave energy can be absorbed by the water vapour, carbon dioxide and ozone in the atmosphere, the rest escaping through atmospheric windows back into outer space, mainly between 8 and 13 µm.

Упражнение 8.

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в тексте:

1. Какие газы наиболее интенсивно поглощают приходящее солнечное излучение?

2. Излучение в каком диапазоне длин волн не проникает ниже
20 км высоты?

3. Какая часть приходящего солнечного излучения отражается атмосферой, облаками и поверхностью земли?

4. Что называется вторичным тепловым излучением земли?

Упражнение 9.

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Упражнение 10

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Almost all energy affecting the earth is derived from solar radiation, which is of shortwave-length (<4 µm) due to the high temperature of the sun (~6,000 K) (i.e. Wien's Law). The solar constant has a value of approximately 1,370 W m-2. The sun and the earth radiate almost as black bodies (Stefan's Law, F = бТ4), whereas the atmospheric gases do not. Terrestrial radia­tion, from an equivalent black body, amounts to only about 270 W m-2 due to its low radiating temperature (263 K), and it is infrared (long­wave) radiation between 4 and 100 µm. Water vapour and carbon dioxide are the major absorb­ing gases for infrared radiation, whereas the atmosphere is largely transparent to solar radia­tion (the greenhouse effect). Trace gas increases are now augmenting the 'natural' greenhouse effect (33 K). Solar radiation is lost by reflection, mainly from clouds, and by absorption (largely by water vapour). The planetary albedo is 31 per cent; 48 per cent of the extraterrestrial radiation reaches the surface. The atmosphere is heated primarily from the surface by the absorption of terrestrial infrared radiation and by turbulent heat transfer. Temperature usually decreases with height at an average rate of about 6.5 °C/km in the troposphere. In the stratosphere and thermosphere, it increases with height due to the pres­ence of radiation absorbing gases.

The excess of net radiation in lower latitudes leads to a poleward energy transport from trop­ical latitudes by ocean currents and by the atmosphere. This is in the form of sensible heat (warm air masses/ocean water) and latent heat (atmospheric water vapour). Air temperature at any point is affected by the incoming solar radi­ation and other vertical energy exchanges, surface properties (slope, albedo, heat capacity), land and sea distribution and elevation, and also by horizontal advection due to air mass move­ments and ocean currents.

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