1. Sunlight Versus Earthglow
The two radiation spectra representing sunlight and terrestrial heat emission (Earthglow) are compared above. Two key points are: 1. the two spectra are separated in wavelength and do not overlap significantly; 2. the area under each of the intensity functions (which approximate blackbody curves) have about the same value. The first point is explained by the fact that the two radiating bodies have very different temperatures: the sun, ~6000 K, and the Earth, ~260 K – which places the peak intensities at ~0.5 m and 10 m, respectively (well apart). The second point is explained by the fact that Earth’s energy source (sun) and sink (terrestrial radiation) must be maintained in a close balance.

2. The Basic Energy Balance of Earth
The Earth adjusts its climate (global temperature) so that just enough heat is emitted to space from the ground and atmosphere to balance the amount of sunlight absorbed by the surface and atmosphere. This is the basic energy balance that defines the climate state of the Earth. The incoming energy (sunlight) occurs in the solar spectrum, while the outgoing energy lies in the Earthglow spectrum.

3. The Seasons
The seasons we experience are caused by two factors. First, the orbit of Earth around the sun is not circular, but is elliptical with the Sun at one foci; hence, the distance between Earth and the sun varies annually between a maximum and a minimum value, and the energy impinging on Earth varies inversely with the square of this distance. Second, the Earth tilts on its axis of rotation, and the angle of the sunlight hitting the ground changes from more direct to more oblique over the course of the year. This second factor actually dominates the changes in the seasons on Earth today.

4. Earth’s Energy “Box”
As noted earlier, the climate of our planet is determined by an energy balance between incoming solar radiation and outgoing heat (or thermal infrared) radiation. This balance can be represented using a “box” model, in which the heat content of the climate system (atmosphere, upper ocean, and land surface) is the quantity in the “box.” The heat content of the box is related to the mean temperature of the climate system.

5. Components of the Radiation Budget
This diagram identifies specific components of the energy balance that affect the overall climate. Details are discussed in the lecture on climate. The salient point here is that roughly one-third of all the solar energy impinging on the Earth is immediately reflected away to space, an this is referred to as the “albedo.”

6. Interactions of Radiation with Air

7. Leonardo’s Experiment–Why is the Sky Blue?
Leonardo da Vinci, fascinated by the question of why the sky is blue in color, devised a very clever experiment using smoke to scatter sunlight, then observing the color of the scattered light, which was of “…an azure hue…” On this basis, he concluded that minute particles of moisture dispersed in the atmosphere scattered the blue light preferentially.

8. Rayleigh Scattering: Why the Sky is Blue
John William Strutt, Lord Rayleigh, much later deduced that air molecules (O2 and N2) actually scatter the sunlight – according to Rayleigh’s law, in which the intensity of the scattered light varies inversely as the fourth power of the wavelength. Thus, blue light, at a wavelength roughly half that of red light, is scattered much more efficiently. The atmosphere observed away from the direction of the sun therefore appears blue in color.

9. Interactions of Radiation with Particles

10. Scattering and Absorption by Particles
The scattering of sunlight by particles suspended in air – referred to as aerosols – contributes substantially to Earth’s albedo. Scattering, absorption, refraction, diffraction and other effects also create a variety of interesting phenomena that we see from time to time. Facing in the direction of the sun, you can observe sunrises and sunsets, twilights, halos and corona. Facing away from the sun, you may see rainbows and glories. These phenomena are caused by Rayleigh scattering and refraction (sunrise, sunset, twilight), refraction in rain drops (rainbows), refraction in ice crystals (halos), diffraction by cloud droplets (corona), and backscatter by cloud droplets (glory). Absorption by paricles also creates color changes in the sky ranging from deep reddening to brown to gray shading.

11. Colors in Air
Several conditions leading to the coloring of the air are indicated in this diagram. Sunlight is usually yellowish white owing to the roughly equal amounts of the three primary colors of light – blue, green and red. In equal amounts, these combine to produce white light (according to our eye-brain sensing system). By diminishing or enhancing one color or another relative to the others, say by Rayleigh scattering, the resulting color combination may range from red to orange to yellow to green to blue to purple. Clouds and large aerosols tend to scatter all the wavelengths (colors) of light about equally, and so the scattered light is whitish. Thus, clouds and fogs take on a bright, white appearance.

12. Sunset Refraction of Sunlight
The geometry of sunset/sunrise involves observation of the sun at the horizon. However, ray paths of light are bent by refraction in passing through the atmosphere. As a result, the image of the sun is apparently displaced upward, and we see the flattened sun sitting on the horizon even after it has actually set.

13. Rainbows
Rainbows are caused by the refraction and internal reflection of sunlight rays in large water droplets. The rays are actually bent back toward the sun, which is why you see the rainbow with your back to the sun. The colors of the rainbow are produced by the difference in the refractive index of water for different colors, resulting in a slightly different angle of deflection for each color.

14. Halos
Halos, by contrast with rainbows, are due to the refraction of solar rays through hexagonal ice crystals floating in the atmosphere. The refraction diverts the rays in the forward direction only, so unlike rainbows, halos are seen looking toward the sun. A similar phenomena occurs in the corona, which involves diffraction of light around water and ice particles, which results in brilliantly colored bands of light encircling the sun.

15. Concluding Remarks
Radiation in the atmosphere is has two major components. Sunlight and Earthglow, or thermal radiation emitted by Earth. Sunlight is the principal source of energy for everything that happens on our planet, and in our lives. Earthglow, on the other hand, is Earth’s heat emission that allows our planet to remain in energy balance, and prevents the climate system from overheating, or freezing. Both sunlight and Earthglow obey several simple physical laws that allow them to be easily characterized for climate analysis. The solar-terrestrial radiation balance is the crux of the climate problem to be discussed later. Changes in the intensity of sunlight reaching the Earth over the course of a year cause the seasons. This variation is mainly due to the tilt in Earth’s axis of rotation, but also the eccentricity of its orbit around the sun. Ultraviolet radiation from the sun is critical from an environmental point of view, and will be revisited. Finally, the interaction of sunlight with air, and with the ubiquitous small particles suspended in the atmosphere, as well as clouds, creates a spectacular palette of color and texture that we can all enjoy on a warm summer’s evening.