Clouds and Reflectance

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Clouds and Reflectance


Clouds are composed of frozen droplets or crystals of water vapor. When gathered together in a cloud formation, the billions of objects can efficiently reflect incoming sunlight back into space. This natural shielding provided by clouds can create cooler surface temperatures.

Scientists are still trying to understand how the increasing warming of Earth's atmosphere will affect the types of clouds that form and the extent of cloud cover. The lack of scientific consensus is hampering efforts to develop climate models to understand the climate changes that could occur in the twenty-first century.

Historical Background and Scientific Foundations

The ice crystals that make up a cloud form a structure that has plate-like sides. Each side acts like a mirror. Depending on the position of the crystals in the cloud, incoming sunlight will be reflected straight back if a surface is horizontal, or at an angle if the encountered surfaces are not quite horizontally positioned.

Nephologists (the term given to scientists who study clouds) have determined that on a global scale, clouds reflect about 17% of incoming sunlight back into space. The efficiency of reflectance depends on the depth of the cloud. For example, deep clouds such as the stratocumulus type can reflect up to 90% of the incoming sunlight. The

base of these types of clouds is dark because of the low level of light that has penetrated fully through the cloud. In contrast, cirrus clouds, which are thin and whispy, reflect less sunlight.

Although it is clear that the global atmosphere is warming, climatologists do not agree on how this will influence cloud formation, and, therefore, the reflectance of sunlight.

It has been predicted that warmer temperatures will increase the evaporation of water from the surface and, therefore, will cause more numerous and thicker clouds to form. The result would be increased reflectance of incoming sunlight, which would cool Earth's surface. However, studies of various areas around the globe where the atmosphere has been warming have revealed that clouds tend to be thinner, since the bottom of the cloud rises higher while the top of the cloud stays at the same altitude. So, it may be that a warming atmosphere will produce clouds that are less capable of reflecting sunlight. In that case, the increased heat would be absorbed by the accumulated greenhouse gases, warming the atmosphere still further.

Impacts and Issues

According to the 2007 Assessment Report from the Intergovernmental Panel on Climate Change (IPCC), the years 1995 to 2006 ranked among the 12 warmest years since people began keeping records in 1850. Although the IPCC has stated that “warming of the climate system is unequivocal,” the lack of agreement on the behavior and reflectance capability of clouds in a warming atmosphere hampers efforts to model the climate change in the twenty-first century.

The IPCC report reaffirmed this lack of consensus. A warming atmosphere likely means that rainfall and snowfall will be more frequent in northern Europe, Canada, United States, and the far north. But, increased droughts forecast for equatorial regions points to fewer clouds in those regions of the globe. But current climate models do not all offer the same long-term outlook.


CIRRUS CLOUD: Thin clouds of tiny ice crystals that form at 20,000 ft (6 km) or higher. Cirrus clouds cover 20–25% of the globe, including up to 70% of tropical regions. Because they both reflect sunlight from Earth and reflect infrared (heat) radiation back at the ground, they can influence climate.

CLIMATE MODEL: A quantitative way of representing the interactions of the atmosphere, oceans, land surface, and ice. Models can range from relatively simple to quite comprehensive.

GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.

NEPHOLOGISTS: Scientists who specialize in the study of clouds.

STRATOCUMULUS CLOUDS: Type of cloud that has rounded shapes and forms at low altitudes, i.e., below about 8,000 ft (2,400 m).

WATER VAPOR: The most abundant greenhouse gas, it is the water present in the atmosphere in gaseous form. Water vapor is an important part of the natural greenhouse effect. Although humans are not significantly increasing its concentration, it contributes to the enhanced greenhouse effect because the warming influence of greenhouse gases leads to a positive water vapor feedback. In addition to its role as a natural greenhouse gas, water vapor plays an important role in regulating the temperature of the planet because clouds form when excess water vapor in the atmosphere condenses to form ice and water droplets and precipitation.

Indeed, if the observations that warmer temperatures produce thinner and less light reflective clouds holds true, then the current estimates for atmospheric warming may be low. In equatorial regions, the increased intensity of sunlight could add to the predicted hardship due to water shortages. The IPCC has predicted that African countries could experience 50% less rainfall by 2020, with water shortages affecting up to 250 million people. The consequences for agriculture, economies, and survival could be extreme.

See Also Atmospheric Structure; Greenhouse Effect; Insolation.



Day, John A. The Book of Clouds. New York: Sterling,2005.

Houghton, John. Global Warming: The Complete Briefing. Cambridge: Cambridge University Press, 2004.

Trefil, Calvo. Earth's Atmosphere. Geneva IL: McDougal Littell, 2005.