Mammatus, or bumpy, formations form on the bottom of many types of clouds. Often, mammatus formations on cumulonimbus clouds indicate a severe impending storm. The clouds' formation mechanism is still undetermined, but the formations seem to require a specific temperature, moisture, and wind speed. Newscom/FILE
Double Rainbows - Rainbows are formed when sunlight passes through water suspended in the air; the rays of light are reflected from inside water droplets at many angles. Differing angles of reflection create different wavelengths, and thus different colors. Double rainbows, like these over Baghdad in 2004, occur when sunlight is reflected twice inside water droplets, rather than once. The colors of the second rainbow are inverted, with red on the inside and violet outside. MARWAN NAAMANI/AFP/Getty Images/Newscom/FILE
Light pillars, like this one observed in Minamifurano in Hokkaido, Japan, occur when sunlight reflects on clouds of ice particles known as 'diamond dust.' If the frozen surfaces of the ice particles are vertical, the reflection of light can create a pillar-like effect. Kyodo/Newscom/FILE
Sun Dogs - If viewed from the correct angle, light reflections on ice particles can create 'sun dogs,' bright spots around a solar halo. Newscom/FILE
Penitentes, first described by Charles Darwin, are thin points of ice or snow that point toward the sun. They can be several feet high. Penitentes are formed in climates where the dew point is below freezing, making snow and ice sublimate rather than melt. As sunlight penetrates the ice, a positive feedback mechanism in which radiation becomes trapped by reflecting between the walls forms the penitentes. Mark Sanderson/GNU Free Documentation License/no Invariant Sections, Front-Cover Texts, or Back-Cover Texts
Fire whirls occur when a large fire creates a tornado-like column of rotating air. Often caused by wildfires, fire whirls can make fires much more dangerous. In 1923, an earthquake on the Kanto plain in Japan caused a massive fire and a fire whirl that killed 38,000. Here a firefighter observes a fire whirl caused by a wildfire in Wildomar, Calif. Zuma/Newscom/FILE
Raining animals is a rare and difficult-to-explain meteorological phenomenon. However, reports of fish and frogs falling from the sky can be found from all over the world. According to the BBC, some scientists believe that strong winds can lift and carry these creatures for miles, but this has never been tested or witnessed. 19th century British illustration
Aurora Borealis, also known as the northern lights, are brightly colored lights observed in the sky near the magnetic north pole. The lights are caused by a complex reaction between particles in Earth's magnetosphere and particles in Earth's upper atmosphere. Collisions between these atomic and molecular particles release energy and light in the upper atmosphere, often visible for long distances. Newscom/FILE
Moonbows, like this one at Cumberland Falls State Resort Park in Kentucky, are produced in a similar way to rainbows. The significantly lower light of the moon than the sun makes moonbows rarer and more difficult to see. Newscom/FILE
Ball lightning is an extremely rare and controversial electrical phenomenon. It consists of a glowing spherical object reportedly lasting many seconds. Ball lightning was long believed to be a hoax, but recent laboratory experiments have produced phenomena matching ball lightning's description. This 19th-century engraving depicts the phenomenon.
Red tide, or algal bloom, occurs when algae accumulate rapidly in one area or body of water. The organisms often contain red or brown pigments and can make the water appear stained. Red tides are often associated with deaths of fish, birds, and mammals near the habitat. Some red tides produce toxins, others can deplete oxygen in the water. Newscom/FILE
US health officials announced their estimate after world organizations warned the deadly virus might become a permanent part of west African life.
BySharon Begley and Tom Miles, Reuters
Global experts issued stark new warnings of the scale of West Africa's Ebola outbreak on Tuesday, with the U.S. government estimating between 550,000 and 1.4 million people might be infected in the region by January.