GLOBAL CLOUD COVER TRENDS

Discussion Session

Lisha M. RoubertUniversity of Wisconsin-Madison

Department of Atmospheric & Oceanic Sciences

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.

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.

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.

Some Cloud Types

Global Cloud Cover

Global Cloud Cover

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.

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.

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.

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).

Each drop acts as a prism 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.  

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:1) 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.

2) Why are scientists interested in studying high level clouds? Explain.

3) What does the plot in the article illustrate? Interpret the results it shows.