1-13: Impact of Clouds on the Radiation Balance

Unit Objectives

• To examine how clouds lead to both warming and cooling in the climate system.
• To review the satellite observations of cloud cover and global patterns of heating and cooling.

Preparation for Online and Class Discussion

1. Read the summary information.

   Class images

2. Print for offline review.

3. Take the summary information quiz. Quizzes are available from your portfolio.

4. Additional Related Reading. Although you are not ultimately responsible for content listed in this section, we encourage you to fully explore the links in order to provide a better scope on the upcoming class discussion.

   Latest cloud montage

Online and Class Discussion

• Social Comments
• Knowledge-building Comments
• Ethical Comments

• Unit Problem to Ponder
• Unit exam questions from previous years
• Unit discussions from previous years

Unit Wrap-up Materials

• Group Discussion Summary:

  Goup members preparing summary: Wes Sass, Noor Salem

  Unit Summary:

  Clouds impact the earth's radiation balance by creating large areas of variability in energy radiated. Cloud cover inhibits the release of infrared, or long wave radiation, and also reflects incoming short wave radiation making for relatively cooler days, and warmer nights. Clouds have temperatures generally equal to the surrounding air temperature. With the knowledge that temperature decreases with height, the higher the cloud is, the cooler its temperature, thus the lower its radiated energy will be. The Stefan-Boltzmann equation explains the amount of energy radiated quantitatively (refer to Figure 1 in the notes).

  Using these concepts we can generalize the impact of clouds on radiation balance over the planet. Tropical regions receive the most radiation, so they also have the highest outgoing radiation. However, along the Inter-Tropical Convergence Zone near the equator, deep convection takes place and creates cloud cover causing a local minimum in emitted radiation. It is low enough to be compared to some polar regions. Looking at the daily impact on radiation balance, dry desert areas have the largest diurnal variation because there are few clouds to block incoming radiation during the day, and few clouds to block outgoing radiation during the night.

  Clouds have a significant impact on radiation and temperature, but it's extremely hard to model their effect quantitatively since global cloud patterns vary largely from day to day.

  Class Discussion:

  Professor Tackle gave a brief discussion on unit 13. The following facts were told by the Professor during his lecture last week:

  1. Any substance will radiate energy including atmosphere, this is why through satellite images, evidence of radiation through clouds can be seen.
  2. The atmosphere is not at constant temperature. The atmosphere is warmer near the surface of the earth than the upper atmosphere.
  3. Figure 2 shows that polar regions are obviously colder than tropical areas, which are much warmer.
  4. Some regions are cooler than others because of the presence of clouds that reflect radiation. Other regions are hotter (e.g. deserts) because of less clouds and that allows the surface to radiate more energy, these regions also have drier climates.
  5. Figure 3 shows that interannual variability is large and it changes from one year to another.
  6. Figure 3 also shows a changed view of how the planet behaves with time. Also, there might be something peculiar on radiated outgoing energy.
  

  Dialog Summary:

  Students' participation in the class electronic dialog for unit 13 was useful where some of them attempted to clarify and articulate certain points that others didn't understand. The discussion or dialog started with one of the students questioning what properties govern the rate of absorption by radiating bodies and whether temperature has any role. A student replied that at constant temperature the body emits and absorbs energy at an equal rate. If the body is hot, absorption is greater and if it's cold then emittance is greater. Also, thickness of the radiating body is another property that affects absorption. Thick layered bodies absorb more radiation than thin layers which reflect most of radiation.

  Other students also mentioned the fact that they actually calculated the difference in outgoing radiation for deserts in the subtropical zones that have the largest diurnal variation. Students mentioned that they used Boltzmann's equation to compare the results with a value given in the topic summary. The students then indicated that they got a totally different value but concluded that perhaps the whole idea seemed to help demonstrate that in the absence of clouds, the diurnal variation of outgoing radiation can be very large.

  The discussion then shifted towards how some clouds cause global cooling and others cause global warming. One student explained that a cloud's ability to reflect sunlight depends on the cloud's thickness (amount of liquid water). The more liquid water in a cloud and the thicker it is, the more sunlight it reflects. Increased cloudiness can seal in warmth by trapping heat near the earth's surface to cause global warming. Another student pointed out that the location of clouds in the atmosphere can determine if they cause cooling or warming. If a cloud is high in the atmosphere, it reflects radiation and temperatures are cooler. Low clouds trap heat in their vapor, therefore warming the planet.

  Other causes of temperatures cooling are volcanic eruptions, one student pointed out based on literature that Mount Pinatubo in the Philippines erupted and continual observations showed that there was a decrease of average surface temperature of the northern hemisphere by as much as 1 degree C (1.8 F).