Every Cloud Has a Silver Lining: Albedo & Clouds

One of the most important components of the weather system, clouds, actually has a very important effect on the world's albedo. A lot of research is being done on different types of clouds - darker clouds have a lower albedo than white ones - how clouds are formed, and what the effects of global warming will be.

Night clouds 

(Source: National Geographic)

Generally, clouds have a higher albedo than the surface underneath them, so they reflect more incoming sunlight than if they would be absent. In this sense, they have a cooling effect on the climate system. More clouds, more sunlight reflected, more cooling - or not?

The radiation coming from the sun is mostly in the same wavelength order as visual light, and is called shortwave radiation. The albedo of a given surface is the fraction of shortwave light that is being refracted. Nevertheless, not all of the light is reflected (as you'll remember from the first post). Some of the solar radiation is absorbed by the Earth, warming up the planet in the same way you get warmer from sitting in the sun. The Earth emits some of that energy in the form of longwave radiation, or heat.

In this image, you can see how clouds affect the distribution of shortwave and longwave radiation (Source: NASA)

Low and high clouds have different effects on the radiation budget. High clouds are usually thin and have low albedos. All things emit heat, even clouds, but because high clouds are quite cold, they emit less heat back into space. These two effects combined mean that high clouds have a net warming effect (also see NASA).

Low clouds do the opposite. They are thicker and have a higher albedo, and send more incoming shortwave radiation back to space before it can even reach the Earth's surface. They are closer to the Earth's surface, and their temperature is almost the same. This means that they radiate almost the same amount of longwave radiation as the planet. Even though they trap a great deal of longwave radiation as well, their net effect is cooling.

The image below is a compilation of cloud fraction cover as observed by NASA satellites. I recommend checking out the links below for further reading; these are also the links I used to do the research for this entry. Next entry I will look at the effect of pollution and aerosols on clouds and climate.

This animation shows the cloud fraction from 2005 to 2012

(Source: NASA Earth Observations)

Further Reading & References

• NASA's Earth Observatory - On clouds, cloud albedo and the effect on our climate.
• IPCC's Fourth Assessment Report - On the cloud-albedo feedback
• NOAA (National Oceanic and Atmospheric Administration) - On clouds