Pincus, R., and S. A. Ackerman, 2003: Radiation in the atmosphere: Foundations. In Handbook of Weather, Climate, and Water: Dynamics, Climate, Physical Meteorology, Weather Systems, and Measurements, T. D. Potter and B. R. Colman (Eds.), Wiley, 301-342.
"What is the weather like?" Meteorologists answer this question in thousands of locations every day when we make observations to determine the state of the atmosphere. The answer is summarized in a few carefully chosen quantities, typically wind speed and direction, relative humidity, and temperature. In the language of physics these roughly correspond to momentum, mass (of water), and energy. Weather forecasting is the science and craft of predicting how these interrelated quantities will change with time.
Energy is transferred through the atmosphere via five processes: convection, advection, conduction, phase change, and radiation. The first two processes involve the movement of mass from one place to another; conduction occurs when two bodies at different temperatures are in contact; phase changes release latent head. Radiation is fundamentally different because it allows energy to be transferred between two locations without any intervening material.
Why Study Radiation?
Because radiation can transport energy even without a medium, it is the only way in which Earth interacts with the rest of the universe. It is radiation, in fact, that determines Earth's climate, since in the long term the planet must shed as much energy as it absorbs. Even within the Earth-atmosphere system radiation can be a powerful player in determining the local energy budget. Radiation is why it is usually warmer during the day, when the sun shines, than at night, and why the surface air temperature is higher on cloudy nights than clear ones.
Radiation is also the basis for remote sensing, the ability to measure the state of the atmosphere from a remote location (usually the ground or outer space). Remote sensing includes everything from simple cloud imagery, through radar estimates of precipitation, to ground-based sounding. Understanding the capabilities and limits of remote-sensing measurements requires learning something about radiation.
Earth's atmosphere is primarily made of gases, but it also contains liquids and solids in the form of aerosols and clouds. Radiation interacts in fundamentally different ways with gases and condensed materials. Gases, as we will see, interact with radiation in just a few ways but have complicated spectral structure. Clouds and aerosols, on the other hand, affect the radiation in each spectral region in about the same way but make the mathematics much more complicated.
Nature of Radiation
In the atmospheric sciences, the word radiation means "electromagnetic radiation," which is different than the particle radiation emitted from radioactive decay. Visible light is one kind of electromagnetic radiation, as are gamma- and x-rays, ultraviolet, infrared, and microwave radiation, and radio waves.
When radiation is measured using very sensitive instruments at extremely low light levels, it is observed that the energy does not arrive continuously but rather in small, finite amounts. These and other observations described in physics texts, including the photoelectric and Compton effects, suggest that radiation can be thought of as a collection of photons, tiny but discrete packets of energy traveling at the speed of light. This is the particle view of radiation.
We can also describe radiation as an electromagnetic phenomenon. The interactions among electric and magnetic fields, matter, charges, and currents are described by Maxwell's equations. The only nonzero solution to these equations in empty space is a traveling wave. Constants in Maxwell's equations predict the wave velocity, which is exactly the speed of light. Light behaves as a wave in many circumstances, too, diffracting when passed through a slit and reflecting from discontinuities in the medium. These observations are the motivation to describe radiation in purely electromagnetic terms.
How do we reconcile these two views? It is tempting to say that light can be both wave and particle, but this is not quite accurate; rather, there are circumstances in which light behaves like a wave and others in which it behaves like a particle. In this and the following chapter we will primarily use the wave model, which is usually more useful in the context of meteorology.
Radiation is also the single aspect of atmospheric science in which quantum mechanics plays a role. This theory, developed in the first few decades of the twentieth century, is based on the idea that the world is not continuous at very small scales, but is divided up ("quantized") into discrete elements: An electron's angular momentum, for example, can take on only certain values. As we will see, a complete description of radiation requires us to invoke ideas from quantum mechanics several times.