The Passage of Solar Radiation through the Atmosphere

The atmosphere does not block sunlight; it lets most of it through. But the light that reaches the ground has already been changed — absorbed in part, scattered in part — by the air it travelled through. That journey, from the top of the atmosphere down to the earth's surface, is what this section describes.

The atmosphere is largely transparent to short-wave solar radiation. In plain terms, most of the sun's incoming energy passes straight through the air and reaches the ground. The atmosphere does not heat up much from direct sunlight; it is the earth's surface that absorbs this radiation and warms up.

The real action happens within the troposphere, the lowest layer of the atmosphere. Here, water vapour, ozone and other gases absorb much of the near-infrared radiation. So while visible light sails through, a good chunk of the invisible, heat-carrying part of the spectrum is trapped in the lower atmosphere.

Alongside absorption, there is scattering. Very small suspended particles in the troposphere — dust, smoke, salt particles — scatter the visible spectrum in all directions. Some of this scattered light goes back to space, and some is directed towards the earth's surface. This scattering is not just a physical detail; it is the reason the sky has colour at all. …