The Impact of the Atmosphere on Solar Radiation

Watch: Instructor's Lecture

The atmosphere is the envelope of gases that surrounds Earth and is held in place by gravity. It provides oxygen for life on Earth and protects us from the most damaging radiation from the Sun. Gamma rays, X rays, and the shortest wavelength of UV radiation don't make it to the surface of the Earth because of our atmosphere. The fraction of the solar energy that does intercept the Earth's surface is called solar insolation. Our atmosphere has the following impacts on incoming solar radiation:

Transmission: occurs when electromagnetic energy makes it through the atmosphere to the surface of the Earth uninterrupted.
Reflection: occurs when solar radiation strikes a gas or particle in the atmosphere and is returned back to space in the direction it came from and therefore, plays no part in heating our planet or its atmosphere
Scattering: occurs when solar radiation is redirected in random directions. Our atmosphere is particularly effective at scattering the shortest wavelengths of visible light which are violet and blue. This is why we have a blue sky.
Absorption: occurs when gases or particles absorb incoming solar energy, raising that gas or particle's temperature

The Composition of the Atmosphere

Most of what we call weather occurs within 7 to 15 miles from the surface of the Earth. From the Earth's surface to about 35 miles above the surface, the composition of the Earth's atmosphere is fairly stable. Some gases within the Earth's atmosphere vary from location to location and even day to day. Others are stable and see little variation. These are the"permanent" gases in the atmosphere.

The largest component of the atmosphere is nitrogen, comprising 78% of the atmosphere. While humans require nitrogen and we are surrounded by it, we can not utilize it directly from the atmosphere. It must be "fixed" by bacteria that live in the roots of legumes and in the soil. Animals consume the plants (and each other) providing the nitrogen they (and we) need. Nitrogen and argon (less than 1% of the atmosphere), are both gases that don't combine easily with other substances. Argon is an inert, noble gas. Neither play an important role in the processes we are talking about.

Oxygen is 21% of the Earth's atmosphere and is an important player in natural processes. It combines easily with other molecules through the process of oxidation. Certain

composition of the Earth's atmosphere

forms of weathering rock and the burning of fossil fuels involve oxidation. It's major impact on solar radiation is its role in the formation of ozone, a variable gas we will discuss below. The remaining permanent gases (neon, helium, hydrogen, and xenon) are very small components of the atmosphere and do not play an important role in the atmosphere's interaction with solar energy.

While small in proportion, the "variable" gases play an extremely important role in what happens to the Sun's energy. There are many different variable gases but we will focus on those that absorb and emit heat. Carbon dioxide is perhaps the most important of these even though it is 0.041% of Earth's atmosphere. It is able to absorb both short and long wave radiation. In the context of studying weather, its most important role is as a potent "greenhouse" gas absorbing long wave radiation from the Earth's surface. In addition, plants use CO2 in photosynthesis. Other variable gases include: methane, water vapor, nitrous oxide, and ozone. Water Vapor and Ozone along with carbon dioxide are the most important of these.

Instructor's Video - Ozone

Another important variable constituent of the atmosphere is ozone. Ozone in the lower part of the atmosphere acts as a "greenhouse" gas and is a major pollutant. Higher up, in the stratosphere, the ozone layer absorbs ultra-violet radiation and protects us from the harmful effects of excessive UV radiation which can destroy plankton, cause skin cancer and eye damage. The use by humans of chlorofluorocarbons (CFC's) used in refrigerants, aerosol sprays, foams and packing materials damaged the cycle of ozone formation creating the "ozone hole". The Montreal Protocol signed by 196 nations in 1987 (including the US) phased out the use of chemicals proven to impact the ozone layer. Some recovery is starting to be seen but real improvement is expected by the middle of this century.

NASA photos of the ozone hole through time

Water vapor is yet another small but important player. This is the gaseous form of water and its concentration in the atmosphere varies greatly. Usually, it is less than 1 % but under very warm moist conditions, as much as 4% of the atmosphere can be water vapor. Water vapor is what we are talking about when talk about humidity. It tends to concentrate in the lower part of the atmosphere and like CO2 its a good absorber of heat radiation. It is also responsible for formation of clouds and precipitation.

Lastly, there is particulate matter; dust, ice, salt, pollen, smoke, emissions from burning fossil fuels. They play an important role in cloud formation and precipitation. They produce colorful sunsets and sunrises and they absorb and reflect incoming solar radiation.

There are many additional variable gases in the atmosphere, many of which human activities have added to the atmosphere. They are the "greenhouse gases" and they too impact the Energy Budget.

The Energy Budget

Many factors come together to determine how much solar energy is transmitted, absorbed, reflected or scattered. Clouds, for example, can greatly increase reflection. They can reflect 30% to 60% of incoming solar radiation. Clouds also absorb between 5% and 20% of incoming solar radiation. The exact nature of the surface on Earth that solar energy strikes can also have a major effect on radiation. Of particular important is how reflective a surface is. This property is important because it determines how quickly a surface heats when exposed to insolation. The reflectivity of a surface is called its albedo. A surface with a very high albedo is highly reflective. For example, snow can reflect 45% to 85% of the solar radiation it receives, therefore, heating slowly. The albedo of water is only about 8% but it its highly dependent on the sun angle striking it.

We can simplify what is a very complex process and describe a "budget" for what happens to solar radiation. If you look at 6 different textbooks you will see six different but similar sets of numbers. So, we will agree to use these numbers for this class. Although the Earth's atmosphere is very transparent to incoming solar radiation only about 45% is transmitted to the Earth's surface. The fraction of the total radiation from the sun that is reflected back into space is about 33%. This energy is reflected by molecules in the Earth's atmosphere, by clouds and by the surface of the Earth. This leaves about 22% which is absorbed by the Earth's atmosphere.

**The Earth's Energy Budget**
45% is transmitted to the surface of the Earth
33% is reflected off of the Earth's surface and its atmosphere - the Earth has an albedo of 33%
22% is absorbed by the Earth's atmosphere

It may appear that our atmosphere is heated primarily by the 22% of incoming solar radiation which it absorbs, but that, in fact, is not the case. The Earth is heated primarily from long wave, terrestrial radiation, radiating off the surface of the Earth. As the Earth's surface heats up, it begins to radiate energy which as a much cooler surface than the Sun is primarily long wave, terrestrial radiation. The Earth's atmosphere is not heated primarily from the Sun!

I find the best analogy for explaining how the Earth is heated is that of a blanket. A regular blanket doesn't have any intrinsic energy source. Instead a blanket works by trapping your own body heat. Put a heavier blanket on your bed and it will trap more of your body heat. Our atmosphere is pretty transparent to short wave, solar radiation allowing 45% to be transmitted to the surface of the Earth. The atmosphere is less transparent to outgoing, long wave radiation which the Earth emits as it is warmed by solar energy. Our atmosphere traps our own terrestrial long wave radiation, heating our atmosphere just as a blanket traps your body heat. Until the Industrial Revolution, we had that illusive "perfect weight" blanket. The planet was neither getting warmer or cooler overall. But we have since added the "greenhouse" gases; carbon dioxide, methane, nitrous oxide and a whole slew of gases; which are all particularly efficient at absorbing terrestrial, long wave radiation. So our perfect weight blanket has been replaced by an all too efficient blanket that is trapping more long wave radiation, driving the temperature of the Earth up.

We have learned about the composition of the atmosphere from many sources. Satellites, weather balloons, rock records all have contributed to our knowledge. But my very favorite source of information came from some amazing feats accomplished by a man named Joseph Kittinger. While his primary purpose, was to test flight suits for high altitude bail outs from airplanes, he also added to our knowledge about the atmosphere and its structure. Please read about his accomplishments and watch this video of his jump. Red Bull sponsored Felix Baumgartner in an attempt to beat Kittinger's records. Kittinger helped him do it. You can watch the video of his jump.