Clouds and Earth’s Radiation Budget

Clouds and Visible Light to Near-Infrared

Low versus High Clouds

Cirrus consists of tiny ice crystals, so they form at high altitudes where is it frigid.  Thin cirrus clouds reflect less sunlight than liquid water clouds but absorb mid- to far-infrared emitted from the surface.  Due to their low temperatures, cirrus clouds emit less far-infrared to space.  Coupled with their low reflectivity of sunlight, cirrus clouds have a net warming effect during the day.  At night, the thermal radiation emitted from the cirrus toward the Earth’s surface has a net warming effect.

Liquid-water clouds form lower in the atmosphere, and they are typically thicker than cirrus clouds.  They reflect a great deal of sunlight, and being warmer, they emit significant amounts of far-infrared toward Earth and space.  The increased thermal radiation at the surface negates some of the cooling effects during the day, but there is still a net cooling, particularly by stratocumulus clouds.

Clouds Around the Globe

Weather satellites have been monitoring Earth’s atmosphere since April 1, 1960. As sensor and communication technologies have advanced, we see finer and finer detail in cloud cover.  Compare the first weather satellite image (Tiros) to that of the geostationary satellite, GOES, taken in 2017.

With well over 50 years of cloud data, meteorologists have studied the seasonal aerial distributions of clouds.  At any given moment, clouds cover two-thirds of the Earth, so they have a considerable impact on Earth’s radiation budget.

Because of the extensive and complex role of clouds, internationally coordinated research started in 1983, and research continues to improve our understanding of clouds’ impact on climate change.

The first weather satellite, TIROS, sent its first image of Earth’s cloud cover on April 1, 1960.  The grainy black and white image helped meteorologists see patterns of cloud cover over the ocean, helping to provide a better understanding of weather patterns over what were previously very remote to human observations. Image from NASA.

Satellites carry sensors that detect radiation in a variety of wavelengths.  Weather satellites monitor clouds using wavelengths ranging from visible light to microwaves (a subset of radio waves).

Thermal infrared sensors allow meteorologists to view clouds during the night and day, whereas visible light images need sunlight to illuminate the clouds.  Thermal infrared images indicate the altitude of the cloud top too.

There are two primary types of weather satellites: polar-orbiting and geostationary.  Polar-orbiting satellites are closer to the Earth’s surface, and they see a strip of Earth as they travel between the poles.  Geostationary satellites orbit farther from Earth, so it travels more slowly.  These satellites complete a revolution at the same rate as Earth, so they remain over the exact location, allowing us to study how clouds change shape and move over time.  To see movies of clouds associated with different weather events, visit the GOES pages at NASA.

The visible-light image of Earth was taken by a GOES satellite on January 15, 2017. Clouds appear white in the image. Image from NASA. 

Clouds and Seasons

The Earth’s tilt of its axis of rotation from its plane of orbit, or its obliquity, creates our seasons.  When the northern hemisphere is more illuminated during its spring and summer, the southern hemisphere is less bright.  The changing illumination of Earth creates the tropical zone of maximum heating to move north and south of the equator, which in turn alters global wind circulations. Clouds depend on the availability of water vapor.  Since it is readily available over the oceans, the oceans are typically 90% covered by clouds, whereas, over land where there generally is less water vapor, there is 70% cloud cover.  Since there is more land in the northern hemisphere, there is a seasonal change in global cover due to the distribution of land between the hemispheres.

The diagram illustrates the variations in total global cloud cover between 1987 and 2010. The x-axis labels indicate day/month/year. The uneven distribution of land and oceans between the northern and southern hemispheres creates the seasonal variation of the cloud cover — Diagram from the ‘Climate + Clouds’ section of the Climate4You website.

Clouds and Net Radiation

Net radiation is the rate of incoming radiation minus the outgoing radiation.  When inbound is higher than outgoing, the positive net radiation leads to heating.  When outgoing exceeds incoming, the region cools.

As discussed above, clouds have a complicated role in Earth’s radiation budget.  Day or night time, liquid water droplets versus ice crystals, seasonal patterns, and clouds over land or water are just the staring set of variables.  But understanding clouds’ roles in warming and cooling the planet are critically important to analyze and predict the impact of human-driven climate change.

Scientists have been studying the impacts of clouds through international efforts such as the International Satellite Cloud Climatology Project (ISCCP) since 1982.  And the research continues to improve our ability to model the formation of clouds to finer spatial resolutions in long-range climate models, such as those used by the International Panel on Climate Change (IPCC). Clouds remain one of the leading contributors to uncertainty in analyzing and predicting climate change.