By the end of this chapter, you should be able to:
- identify the causes of changing solar radiation on Earth
- calculate properties of the spectrum of solar and earth radiation in terms of the Planck function
- calculate the absorption between you and a light source
- explain why the sky looks blue and hazy in the summer
Atmospheric radiation plays a critical role in life on Earth and in weather. Without solar heating, Earth would be a dead frozen ball hurtling through space. Luckily, the energy that Earth receives from solar radiation is sufficient to produce liquid water on its surface, thus enabling life to thrive. In this chapter, we will look at solar radiation and its changes over time. Radiation is just another form of energy and can be readily converted into other forms, especially thermal energy, which is sometimes called "heat." In this chapter, we will use the word "radiation" to mean all electromagnetic waves, including ultraviolet, visible, and infrared. We will introduce some unfamiliar terms like "radiance" and "irradiance" and will be careful with our language to prevent confusion.
- 6.2: Atmospheric Radiation - Why does it matter?
- Everything radiates—the Sun, the Earth, the atmosphere, and you. The energy provided by the Sun is reused in the Earth system to provide the energy that drives weather and climate. But ultimately, the infrared radiation radiated by Earth into space must balance the solar visible radiation coming into the Earth system. From the point-of-view of the Earth system, we are most concerned about how atmospheric radiation interacts with matter.
- 6.3: Start at the Source - Earth Rotating Around the Sun
- Solar radiation drives the Earth system and makes life possible. Solar radiation is absorbed and then put to use to increase the surface temperature, to change the phase of water, and to fuel atmospheric chemistry. The uneven distribution of solar radiation on Earth’s surface drives atmospheric dynamics.
- 6.5: The Solar Spectrum
- The Sun emits radiation from X-rays to radio waves, but the irradiance of solar radiation peaks in the visible wavelengths (see figure below). Common units of irradiance are Joules per second per m² of surface that is illuminated per nm of wavelength (e.g., between 300 nm and 301 nm), or W m⁻² nm⁻¹. These units are the units of spectral irradiance, which is also simply called irradiance, but as a function of wavelength.
- 6.6: What is the origin of the Planck Function?
- All objects—gas, liquid, or solid—emit radiation. However, photons cannot have continuous values of photon energy; instead, the photon energy is quantized, which means that it can have only discrete energy values (albeit very small amount of energy). When this quantized distribution is assumed, then the distribution of spectral irradiance leaving a unit area of the object’s surface per unit time per unit wavelength interval into a hemisphere is called the Planck Distribution Function.
- 6.9: Kirchhoff’s Law explains why nobody is perfect
- Remember that when radiation encounters matter it may be absorbed or transmitted or scattered (including reflected). For an object acting as a perfect Planck distribution function, it must absorb all radiation completely with no scattering and no transmission. Some objects absorb very well at some wavelengths but not at others. For instance, water vapor absorbs little visible radiation but absorbs infrared radiation at some wavelengths very well.
Thumbnail: Note the two smaller eruptions before the big one. The Sun’s upper atmosphere (corona) is shown here. (CC BY-SA 3.0 Unported; Patrick McCauley/From Quarks to Quasars/SDO via Wikipedia).