The radiation reaching the ground is a fraction of that at the top of the atmosphere, because of reflection of some back to space (chiefly from the clouds), and because of the absorption of some by clouds, by the air's gases and by the fine dust particles and tiny droplets which together are called aerosols. The diameters of aerosols are mostly less than 1 m and therefore they are so light that they remain airborne indefinitely (Note 2.G).
The intensity of the Sun's rays on the ground is less at higher latitudes for two reasons (Figure 2.9). First, each ray is spread over a larger area because of increasing obliqueness to the ground. Second, the rays have to traverse a longer path through the atmosphere, so that more radiation is absorbed and scattered by the air.
In the case of the absorption by gases, particular wavelengths are affected especially, as shown in Figure 2.2. Oxygen and ozone absorb radiation with wavelengths below about 0.3 pm, while water vapour and carbon dioxide deplete that with wavelengths above 0.7 pm. Water vapour absorbs radiation in several broad spectral bands with wavelengths up to about 8 pm, and carbon dioxide absorbs almost all the IR radiation above 14 pm. Methane (CH4) and nitrous oxide (N2O) absorb longwave radiation between 7.7-8.3 pm.
The combined effect is to create a 'window around the visible part of the spectrum; most shortwave radiation penetrates the atmosphere through this window (Note 2.H). However, longwave radiation passes through the atmosphere only by means of a window between 8-14 pm, which is split by an ozone absorption band of 9.4-10.0 pm. This window is less transparent, so most longwave radiation from the Earth's surface is absorbed by air molecules and then re-emitted before it reaches outer space.
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