Virtually all of the energy at the surface of the Earth comes to us from the Sun. This is the energy that, in combination with the greenhouse gases in the atmosphere, maintains our current surface temperature in the range where liquid water is present. In this chapter we will examine solar radiant energy, and its most important interactions with the various constituents of the Earth’s atmosphere. Most of it reaches the lower troposphere, some to be absorbed by water vapour, and some to be scattered by clouds, molecules, or aerosols. The rest (roughly half) reaches the ground. Human activity has the power to alter these inflows, in a number of ways. We devote one section to the potentially disastrous Nuclear Winter scenario, and another to the potentially useful idea, known as Geoengineering, of reflecting some of the incoming sunlight in order to re-balance the Earth’s energy budget.

An overview of optical scattering in the atmosphere includes the sizes and concentration of scatterers, the mathematical formalism of scattering, and definitions of the lidar scattering and extinction coefficients. The Rayleigh, Mie, and geometric scattering regimes are defined by the scattering parameter, and implications of Rayleigh scattering on lidar measurements are elucidated for both signals and background. Molecules store energy in translational, rotational, and vibrational motions, and atoms store energy in electronic excitations. These energy storage mechanisms cause the lidar observables of Doppler shifts, molecular and Raman spectra, and atomic spectra, which, along with Rayleigh scattering, enable lidar measurements of temperatures and winds; water vapor and trace gas concentrations; and aerosol extinction coefficients at altitudes from the surface up to the mesosphere and lower thermosphere. The lidar techniques that exploit all these phenomena operate over a range of wavelengths from the long wave infrared to the ultraviolet and the reasons for the differing wavelengths of the various techniques are explained with a graphic that summarizes the chapter.

The range of atmospheric scattering particle sizes is compared graphically with the range of wavelengths used in lidar to show that common particles are both much smaller and much larger than the wavelengths. Scattering must therefore be considered in all three regimes, so formulations and important results are described for Rayleigh, Mie, and geometric optics scattering. Aerosols in the troposphere and stratosphere are then described, with sources, sinks, and size ranges. The lidar ratio is defined. Other particle types are found in water and ice clouds, polar stratospheric clouds, and noctilucent clouds. Depolarization by nonspherical particles is described with Stokes vectors and Mueller matrices, and algorithms are given for finding the degree of depolarization from lidar measurement data. Particle classifiers are described, with examples, illustrating classification techniques using lidar ratios, depolarization ratios, and color ratios. The theory of sun photometry is then reviewed, and AERONET data products are described, with examples.

Chapter 1 is an introduction, reviewing the current state of instructional texts on atmospheric lidar. We point to the lack of a treatment of light scattering as employed by lidar from fundamental physics, the motivation for writing this book. We then summarize the scattering processes, Rayleigh, Raman, Mie, and fluorescence, that enable us to probe the state of the atmosphere with lidar. We include a description of the structure and content of the chapters that follow.

In Chapter 4, we look at nonresonant scattering, specifically Rayleigh and Raman scattering from linear molecules. We continue with the semiclassical (quantum) treatment, leading to the induced dipole moment and associated differential scattering cross section. Explicitly adding vibrational and rotational manifolds of the ground state, we show the results for all three regimes: Rayleigh, rotational Raman, and vibrational Raman scattering. We then apply these results to nitrogen and oxygen molecules and associate the results with macroscopic quantities, such as the index of refraction of an ensemble, or gas. From this point, we focus specifically on Rayleigh + vibrational Raman spectra of O2 and N2, determining vibrational and rotational constants and the thermal populations of the states, based on their molecular energies, which leads to the spectral strengths of individual lines. We finish this chapter with a description of the Cabannes spectrum and the effect of the density fluctuations on its lineshape, considering the success of theoretical models in reproducing these spectra in Knudson (low-density), kinetic (medium-density) and hydrodynamic (high-density) regimes.

Our initial introduction of surface brightness characterized it as a flux confined within an observed solid angle. But actually the surface brightness is directly related to a more general and fundamental quantity known as the “specific intensity.” The light we see from a star is the result of competition between thermal emission and absorption by material within the star.