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Earth from Space – Crepuscular rays

Crepuscular rays seen from space

Rays of light and dark appear to emanate from clouds in this image taken by an astronaut aboard the International Space Station. They are called crepuscular rays.

CREPUSCULAR RAYS OVER THE OCEAN near India are featured in this image photographed by an Expedition 29 crewmember on the International Space Station.

The sight of shafts of light, beaming down from the heavens through a layer of clouds, has provided many an artist, scientist, and philosopher with inspiration throughout the centuries.

Atmospheric scientists refer to this phenomenon as “crepuscular rays“, referring to the typical observation times of either sunrise or sunset.

Shadowed areas bounding the rays are formed by obstructions in the solar (or lunar) illumination pathway such as clouds or mountaintops; however this alone is not sufficient to create the phenomenon. The light must also be scattered—by airborne dust, aerosols, water droplets, or molecules of the air itself—to provide the visible contrast between the shadowed and illuminated parts of the sky.

Crepuscular rays seen from the ground.

A ground-based image of crepuscular rays shows them appear to radiate from a central point, a perspective effect. The orbital image above shows them in fact to be parallel.

When observed from the ground, crepuscular rays appear to radiate outwards from the source of illumination due to the effects of distance and perspective; however the rays are actually parallel.

This photograph from the Space Station provides an unusual viewing perspective from above the rays. The sun was setting to the west on the Indian subcontinent at the time the image was taken, and cumulonimbus cloud towers provide the shadowing obstructions.

The rays are being projected onto a layer of haze below the cloud towers. The image clearly illustrates the true parallel nature of the crepuscular rays.

See the full-size image here.

Orbital image courtesy NASA. Ground-based image of crepuscular rays from Wikipedia, courtesy Chevy111, posted under the Creative Commons Attribution-ShareAlike 3.0 License.

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Gravity waves on the water

Atmospheric gravity waves over the Persian Gulf

Rollercoaster airflow produces smooth and rough patches on the waters of the Persian Gulf. The effect is produced by "gravity waves".

When the Sun reflects off the surface of the ocean at the same angle that a satellite sensor is viewing the surface, a phenomenon called sunglint occurs.

In the affected area of the image, smooth ocean water becomes a silvery mirror, while rougher surface waters appear dark.

Sometimes the sunglint region reveals interesting ocean or atmospheric features that the sensor does not usually record.

The image above shows a large, overlapping wave pattern in the sunglint off the Persian Gulf.

The pattern is not from large ocean waves, however. It is the “impression” of atmospheric gravity waves on the surface of the ocean.

As the name implies, atmospheric gravity waves form when buoyancy pushes air up, and gravity pulls it back down, a bit like a rollercoaster. On its descent into the low-point of the wave (the trough), the air touches the surface of the ocean, roughening the water.

The long, vertical dark lines show where the troughs of gravity waves have roughened the surface. The brighter regions show the crests of the atmospheric waves.

Beneath the crests, the water is calm and reflects light directly back towards the sensor.

The image was captured by the MODIS instrument aboard NASA’s Terra satellite on June 14, 2010.

Adapted from information issued by Jeff Schmaltz, MODIS Land Rapid Response Team, NASA GSFC.

Swirling clouds

Swirling clouds over the Canary Islands in the Atlantic Ocean.

Swirling clouds over the Canary Islands in the Atlantic Ocean.

This false-colour Envisat satellite image, acquired on June 6, 2010, shows a unique cloud formation south of the Canary Island archipelago, some 95 kilometres from the northwest coast of Africa (right) in the Atlantic Ocean. The swirls near the bottom of the image are known as Karman vortices.

Seven larger islands and a few smaller ones make up the Canaries; the larger islands are (left to right): El Hierro, La Palma, La Gomera, Tenerife, Gran Canaria, Fuerteventura and Lanzarote.

Adapted from information issued by ESA.

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Earth was wet in its youth

Earth seen from space

Extreme greenhouse concentrations weren't needed to keep Earth's oceans from freezing billions of years ago.

Four billion years ago, our then stripling Sun radiated only 70 to 75 percent as much energy as it does today. Other things on Earth being equal, with so little energy reaching the planet’s surface, all water on the planet should been have frozen.

But ancient rocks hold ample evidence that the early Earth was awash in liquid water – a planetary ocean of it. So something must have compensated for the reduced solar output and kept Earth’s water wet.

To explain this apparent paradox, a popular theory holds there must have been higher concentrations of greenhouse gases in the atmosphere, most likely carbon dioxide, which would have helped retain a greater proportion of the solar energy that arrived.

But a team of scientists including researchers from Stanford have analysed the mineral content of 3.8-billion-year-old marine rocks from Greenland and concluded otherwise.

Swirls of cloud over the ocean

Swirls of cloud over the ocean

“There is no geologic evidence in these rocks for really high concentrations of a greenhouse gas like carbon dioxide,” said Dennis Bird, professor of geological and environmental sciences.

Instead, the team proposes that the vast global ocean of early Earth absorbed a greater percentage of the incoming solar energy than today’s oceans, enough to ward off a frozen planet.

Earth was a water world

Because the first landmasses that formed on Earth were small – mere islands in the planetary sea – a far greater proportion of the surface was covered with water than today.

The crux of the theory is that because oceans are darker than continents, particularly before plants and soils covered landmasses, seas absorb more sunlight.

“It’s the same phenomenon you will experience if you drive to Wal-Mart on a hot day and step out of your car onto the asphalt,” Bird said. “It’s really hot walking across the blacktop until you get onto the white concrete sidewalk.”

Another key component of the theory is in the clouds. “Not all clouds are the same,” Bird said.

Clouds reflect sunlight back into space to a degree, cooling Earth, but how effective they are depends on the number of tiny particles available to serve as nuclei around which the water droplets can condense. An abundance of nuclei means more droplets of a smaller size, which makes for a denser cloud and a greater reflectivity, or albedo, on the part of the cloud.

The edge of Earth's atmosphere

The edge of Earth's atmosphere

Most nuclei today are generated by plants or algae and promote the formation of numerous small droplets. But plants and algae didn’t flourish until much later in Earth’s history, so their contribution of potential nuclei to the early atmosphere circa 4 billion years ago would have been minimal. The few nuclei that might have been available would likely have come from erosion of rock on the small, rare landmasses of the day and would have caused larger droplets that were essentially transparent to the solar energy that came in to Earth, according to Bird.

“We put together some models that demonstrate, with the slow continental growth and with a limited amount of clouds, you could keep water above freezing throughout geologic history,” Bird said.

Adapted from information issued by Stanford University.