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. Matter is simply molecules and atoms and the structures that they build, such as the air, the clouds, the Earth, and the Sun.
When radiation encounters matter, three things can happen. The radiation can be transmitted through matter; it can be absorbed by the matter; it can be scattered by the matter. One of these three things must happen, so we can sum them up to one:
where τ is the transmissivity, the fraction transmitted; a is the absorptivity, the fraction absorbed; and s is the reflectivity, the fraction that is scattered or reflected.
Scattering and reflecting are related but are different because reflection is scattering in a particular direction whereas scattering tends to go in a range of directions.
What can happen when radiation meets matter. The sum of radiant energy that is scattered, absorbed, and transmitted must equal the amount of incoming radiant energy.
Credit: W. Brune
Extra Credit Reminder!
Here is another chance to earn one point of extra credit: Picture of the Week!
- You take a picture of some atmospheric phenomenon—a cloud, wind-blown dust, precipitation, haze, winds blowing different directions—anything that strikes you as interesting.
- Add a short description of the processes that you think are causing your observation. A Word file is a good format for submission.
- Use your name as the name of the file. Upload it to the Picture of the Week Dropbox in this week's lesson module. To be eligible for the week, your picture must be submitted by 23:59 UT on Sunday of each week.
- I will be the sole judge of the weekly winners. A student can win up to three times.
- There will be a Picture of the Week dropbox each week through Lesson 11. Keep submitting entries!