Student Features

The Role of Clouds
04.22.04
Research in the last few years has shown that clouds are a major factor in Earth's climate system. The climate is very sensitive to small (10-30 percent) changes in clouds. This sensitivity is very important in trying to figure out what man-made changes will do to Earth's climate. Such changes include increasing carbon dioxide. Scientists want to understand if and how these changes affect climate.

Let's look at how clouds affect Earth's climate:

Diagram showing how clouds affect Earth's climate
Image above: This diagram indicates that all energy comes from the Sun
Credit: NASA


The Earth's Energy Budget chart shows what we know now about the parts of the Earth's temperature controls. All the energy comes from the Sun. An equal amount of energy must go back into space or Earth's temperature will change. The picture shows that clouds reflect more energy than the atmosphere or Earth's surface. Think about how bright clouds can appear -- this is reflected energy. They also radiate more energy. Clouds act like radiators in the atmosphere. They are, however, much colder than a radiator in a building.

One thing that we have learned is that clouds can act to either warm or cool the Earth. High clouds are often thin and do not reflect very much. They let lots of the Sun's warmth in. Low clouds are often quite thick and reflect lots of sunlight back to space.

You can think of clouds as one thermostat that sets Earth's temperature. If, for example, you make the low clouds a little thicker (which makes them more reflective), the Earth's temperature will drop a little. It would be as if you turned down the thermostat a little.

Diagram showing cloud effects on Earth's radiation
Image above: This diagram indicates that high clouds radiate less energy into space than low clouds
Credit: NASA


The main difference between high and low clouds is that the high clouds are colder. This means that they radiate less energy into space than the lower, warmer clouds. Therefore, high clouds work to "trap" more energy than the low clouds.

High clouds are not good at reflecting shortwaves. But, they are very good at blocking longwaves. This also traps heat. Thus, high clouds will cause a warming of the Earth's surface. Low clouds are excellent reflectors. But, they don't stop the longwave energy from escaping to space. Therefore, low clouds help to cool the Earth.

Adapted from The Role of Clouds and S'COOL: Frequently Asked Questions
Langley Research Center