7.2: Applications of Atmospheric Radiation

Let’s use what you learned in Lesson 6 to examine two applications of atmospheric radiation. The first application involves the role of atmospheric radiation and greenhouse gases in Earth’s climate. The second application is the interpretation of upwelling infrared radiation spectra measured by satellite instruments in space with an eye on improving weather forecasting. These two applications use the principles of atmospheric radiation in quite different ways, but understanding both is critical to your becoming a competent meteorologist or atmospheric scientist.

Earth’s atmosphere is essentially always in radiative energy balance, which is also called radiative equilibrium. By this, I mean that, when averaged over the whole Earth, the total amount of solar radiation energy per second that is absorbed by the Earth’s surface and atmosphere is about equal to the total amount of infrared radiation energy per second that leaves the Earth’s surface and atmosphere to go into space. There can be periods when this balance is not exact because changes in atmospheric or surface composition can alter the absorption or scattering of radiation in the Earth system. It can take a little while for all of the temperatures of all of the Earth system's parts to adjust, but if the changes stop, the Earth system will adjust its temperatures to come back into balance. Right now we are in a period where atmospheric CO₂concentrations are increasing due to industrialization, the outgoing infrared radiation is slightly less than the incoming absorbed solar radiation, and the Earth system's temperatures are adjusting (by increasing) to try to bring the outgoing infrared radiation into balance with the incoming absorbed solar radiation. For most of the following discussion, we will use this concept of radiative equilibrium even though the current balance is not exact.

Always keep in mind that atmospheric radiation moves at the speed of light and that all objects are always radiating. Moreover, as soon as an object absorbs radiation and increases its temperature, its emitted radiation will increase. Thus energy is not “trapped” in the atmosphere and greenhouse gases do not “trap heat.” We will see instead that greenhouse gases act like another radiation energy source for Earth’s surface.

Before we do any calculations, let's summarize how different parts of the Earth system affect visible and infrared radiation (Table 1). Earth's surface either absorbs or scatters both visible and infrared radiation, while the atmosphere mostly transmits the visible radiation, with a little scattering; and the atmosphere mostly absorbs infrared radiation, with a little transmission. Clouds, an important part of the Earth system, strongly absorb infrared radiation and both scatter and absorb visible radiation.

Table 1: Absorptivity, Emissivity, Scattering, and Transmissivity of the Earth System

	Earth’s surface		atmosphere		clouds
	visible	visible	visible	IR	visible	IR
absorptivity	large	opaque	tiny	large	large	opaque
emissivity	large	large	tiny	large	large	large
scattering (reflectivity)	large	large	moderate	none	large	small
transmissivity	none	none	large	small	none	none

Watch this video (52 seconds) to learn more:

Absorption Scattering Transmitting

Click here for transcript of the Absorption Scattering Transmitting video.

Table 7-1 gives the absorptivity and thus, emissivity, as well as the scattering and transmissivity for the visible and infrared. Remember that the fractions of absorptivity, scattering, and transmissivity of radiants must add up to one when radiants encounters matter. I want to point out two features in the table. First, the atmosphere has little absorptivity and moderate scattering in the visible wavelengths while the atmosphere has large absorptivity, small transmissivity, and essentially no scattering in the infrared. Second, note that clouds behave a lot like Earth's surface in all aspects, except that scattering in the infrared can be large at Earth's surface while it is small for clouds.

Image of Earth in the visible. Clouds are scattering radiation at all visible wavelengths back out to space, and thus appear to be white, while Earth's surface selectively scatters visible radiation at only some wavelengths and absorbs the rest. Credit: NASA

Image showing outgoing longwave (infrared) radiation emitted by the Earth and atmosphere during the European heatwave of 2003. The blue and white colors are clouds, which are radiating at the lower temperatures of the upper troposphere, while the yellows are the Earth's surface and the lower troposphere, which are radiating at higher temperatures. Credit: NASA

Extra Credit Reminder!

Here is another chance to earn 0.2 points of extra credit: Picture of the Week!

1. 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.
2. Add a short description of the processes that you think are causing your observation.
3. Upload it to the Picture of the Week Discussion and add your description in the text box.
4. The TA and I will be the sole judges of the weekly winners. A student can win up to five times.
5. Entries not chosen one week will be considered in subsequent weeks.