Global Cloud Cover Trends

1. Global Cloud Cover Trends Discussion Session Lisha M. Roubert University of Wisconsin-Madison Department of Atmospheric & Oceanic Sciences

2. Clouds • A cloud is a visible mass of condensed droplets or frozen crystals floating in the atmosphere above the surface of the Earth or another planetary body. On Earth the condensing substance is typically water vapor, which forms small droplets or ice crystals.

3. Cloud Formation-Summarized • When the temperature or the pressure decreases, this means that the air cannot physically hold as much water as it can in warmer weather. The lower the temperature is, the more condensation occurs. • Condensation transforms water vapor into water droplets, or (depending how cold the air is) ice particles. The water molecules that are formed through this condensation collect on the surface area of the condensation nuclei and form a cloud.

4. Brief history of Clouds • Clouds haven’t been studied for very long compared to most sciences. • First naming system was created by Jean-Baptiste Lamarck around 1800. It was not very popular. • Twenty years later Luke Howard created the current naming system for clouds.

5. Some Cloud Types

6. Global Cloud Cover

7. Global Cloud Cover

8. Cloud Albedo • Cloud albedo is a measure of the reflectivity of a cloud. High values mean that the cloud can reflect more solar radiation. • Cloud albedo varies from less than 10% to more than 90% and mainly depends on droplet size and liquid water content. The smaller the drops and the greater the liquid water content, the greater the cloud albedo.

9. Cloud Effects on Climate • Low level clouds • They tend to have a net cooling effect. They are very thick and reflect a lot of the incoming shortwave radiation (solar radiation). • They emit large amounts of longwave radiation (heat from earth) towards space and to the higher levels in the atmosphere. • High Level clouds • These tend to have a net warming effect. They are thin and reflect very little shortwave radiation. • They emit a small amount of longwave radiation to space.

10. Optical Properties of Raindrops • When people think of rain and clouds they often think of rainbows. • One of the first people to investigate rainbows was Isaac Newton. • He created some of the first lenses and prisms in order to experience the nature of light. When light passes through a prism, it changes direction (refraction). The angle of refraction is different for each of the colors which make up white light.  So seven distinct colors are seen when light passes through a prism and the light is said to be decomposed.

11. Rainbows • On occasions we can see a secondary rainbow. These are caused by a double reflection of sunlight inside the raindrops. As a result of the second reflection, the colors of a secondary rainbow are inverted compared to the primary bow (blue on the outside and red on the inside).  Our eyes see red light coming from droplets of water higher in the sky, while droplets of water lower in the sky send violet light to our eyes.   Each drop acts as a prism

12. For Next week: • Read the article “Clouds Reveal Climate Trends” that is posted on the website. • Write a paragraph answering these questions and we will have a discussion about it next time: • What factors can affect cloud cover? What did the results of this study show about the effect that these factors have on cloud cover? Explain. • Why are scientists interested in studying high level clouds? Explain. • What does the plot in the article illustrate? Interpret the results it shows.