Photo: Zaphod

These cumulus clouds are not sunshower material. See a late spring sun-snow shower here!

Almost all of us have noticed a summer rainshower that occurs while the sun is shining on us. In this event, known as a sun-shower, the raindrops fall from a type of cloud called a cumulus cloud.

Cumulus clouds are a manifestation of the process of convection. The atmosphere can induce convection when the surface temperature is much higher than the air temperature some distance above the surface. This can happen when the day heats up in summer or when very cold air moves over an unfrozen lake in winter. This condition can destabilize the atmosphere’s normal stratification, causing cumulus clouds to shoot upward quickly and produce precipitation in a short time. These clouds are very local and so an individual cumulus may be surrounded by clear sky. Precipitation dropped from the base of these clouds takes time to reach the surface.

Sunshowers occur when the precipitation from such an isolated cumulus hits the surface after the cloud has moved away. Though less common in winter, sun-snowshowers do occur and they are a result of the same process. Those of you who live in areas prone to lake effect snows have probably experienced sun-snowshowers more than once. Here in the land of cheese and beer, they are a rarity but keep your eyes peeled – sun-snowshowers do happen!

This flattening of the sun on the horizon is an example of a mirage. Mirages are not illusions; they are images formed as light bends on its passage through the atmosphere.

This bending, called refraction, explains many everyday phenomena. For example, refraction causes objects partly immersed in water (such as a straw in a glass of water) to look bent. Refraction depends on changes in optical properties along a light ray’s path to your eye. Because the optical density for air and water are different, a ray of light moving through air and water will bend.

The optical density of air is a function of temperature. So, as air passes through layers of the atmosphere with different temperatures, it bends the light. The atmosphere, particularly in the early morning, usually has a layered structure; air temperature changes with height. If the sun is on, or just above the horizon, the light travels along a long path through the atmosphere. As the light passes through these different layers, it will refract and deform, flatten or distort the sun’s disk. Light rays coming from the top of the sun bend less than rays coming from the bottom, slightly squashing the sun’s circular shape.

Solar eclipse from space

You’ve seen photos of lunar and solar eclipses, or maybe you’ve even been present for one yourself, but have you ever seen an eclipse of the Earth? Astronaut Bill McArthur and flight engineer Valery Tokarey snapped this photo from aboard the International Space Station on March 29, 2006 during a total eclipse of the sun. That day the shadow of the moon’s umbra crept from Brazil to central Asia, where the eclipse’s path of totality finally ended with a fully-blackened sun sinking below the horizon of western Mongolia.

In this image, the southern shores of the Mediterranean Sea are subsumed by a lunar shadow that extended more than 2,200 miles into North Africa’s Sahara desert. Framing the dark sphere in a halo of progressively lighter shades is the penumbral shadow. Within the shadow observers witnessed a partial eclipse of the sun, whereas those in the darkest center circle experienced a total eclipse.

Image credit: NASA

Saturn in eclipse

Yes, this is a real picture. More accurately, it’s 165 pictures pasted together from NASA’s Cassini spacecraft’s flyby of Saturn as the planet between the probe and the sun. From this unique vantage point, the contrast of light and shadow enabled astronomers to discern new bands of ice and dust — perhaps the remnants of a shattered moon — between the inner and outermost edges of the ring system. This panorama reveals Saturn casting a massive shadow some 75,000 miles long over Cassini’s camera. Though the brightness in this image has been enhanced to reveal detail, the photo’s sharp contrasts owe foremost to the millions of square miles of orbiting ice crystals that ring the planet. The dark gaps between the bands are thought to be the result of gravitational forces exerted by the planet’s many moons, but these forces are not the only cause. The Encke Gap, among the largest of these vacuous rings, is maintained by Saturn’s innermost moon and “ring shepard,” Pan, which plows through the orbiting field of ice and dust.

Image courtesy CICLOPS team.

The leaves of trees and other plants contain three main pigments: carotene, anthocyanin, and the photosynthetic pigment, chlorophyll, which captures the sun’s energy to make food for plants. As the most abundant pigment, chlorophyll is what gives leaves their green hue in spring and summer.

Another chemical in leaves, auxin, controls a special band of cells at the base of each leaf stem, called the abscission layer. During the growing season, auxin prevents this layer from fully developing and blocking the tiny, internal tubes that connect each leaf to the rest of the tree’s circulatory system.

In fall, however, cooler and shorter days trigger an end to auxin production, allowing the abscission layer to grow and cut off the circulation of water, nutrients and sugar to the leaves. When this happens, chlorophyll disintegrates rapidly, letting carotene shine through as the yellow in maple, aspen and birch leaves. Anthocyanin, meanwhile, provides the oranges and reds of maples, sumacs and oaks. When there’s less sun, anthocyanin isn’t as chemically active and leaves are more orange or yellow than red.

The senior scientist and educator with the Space Science and Engineering Center explains that unlike the hard, physical boundary here on Earth, the Sun’s surface is a hot mass of gas that is more or less continuous with its atmosphere.

“The Sun is a star and so we have to look first at what a star is: usually it’s mostly hydrogen gas that is being converted into helium,” Limaye says. If you could travel from the Sun’s outermost regions toward its core, this gas – though very thin at first – would gradually become so thick that you could no longer move.

“But the temperature and pressure is so high, there’s no possibility of anybody standing on what one thinks of as a surface,” he adds.

Still, the Sun does have a surface we can see, called the photosphere. Up close, it resembles water bubbling in a pot on the stove, except that while water boils at 100 degrees Celsius, the photosphere is more than 500 times hotter.

From this roiling surface, says Limaye, streams of particles shoot upward occasionally and then loop back down, forming giant arcs that can be thousands of miles long and contain millions of tons of material.