Scattering of radiation

Introduction

Most of the light that reaches our eyes does not come directly from its source, instead it is scattered. We see diffusely scattered sunlight when we look at clouds or at the sky. The land and water surfaces are visible through the light that they scatter. In the atmosphere we see colourful examples of scattering from molecules, aerosols, and clouds containing droplets and ice crystals. Blue sky, white clouds, rainbows, and haloes are optical phenomena due to scattering. Scattering is a fundamental physical process associated with light and its interaction with matter. It occurs at all wavelengths.

This chapter deals with the processes by which radiation is scattered from aerosols that are smaller or larger than the wavelength of the radiation. The chapter also provides the basis for radiative transfer calculations using realistic parameters. For remote sensing techniques it is important to understand these phenomena because many techniques have to correct for the effects of the atmosphere to retrieve specific information.

Aerosol particles are present everywhere in the atmosphere but only on a few occasions can we actually see them. Dust is not always as clearly visible as in the image above where dust has been lifted from the ground. Cloud droplets are also giant aerosol particles. Most aerosols are invisible to the human eye even in very clear air.

All aerosols scatter and absorb incident light (radiation). Less light will reach the ground, even if you are in a minor dust storm. The same phenomenon applies to deep convective clouds.

The back-scattered radiation is received, for example, by a satellite. The METEOSAT VIS image above shows a large dust cloud over the ocean west of tropical West-Africa. The image has been strongly enhanced so that you can distinguish the different VIS intensities. Dust is bright when seen from space but is dark when seen from the ground.