Rainbows, halos and other optical effects

• A USATODAY.com graphic shows why the sky appears blue.
• A USATODAY.com graphic shows what causes rainbows.
• USATODAY.com reports on a stunning halo display at the South Pole.
• Image identifies kinds of halos in South Pole display
• The sky acts on light in many different ways to make rainbows, different kinds of rings around the sun and moon and other effects. The University of Illinois at Urbana-Champaign's Guide to Atmospheric Optics Web site is a good way to begin understanding what's going on.
• Information about rainbows on the University Center for Atmospheric Research Web site goes into more detail about rainbows than the University of Illinois site. It also has links to other sites with detailed information.

The Green flash

• The green flash usually occurs just above the disk of the setting sun, usually when the sky is clear. A good source of information on the green flash is on the Web site of Andrew T. Young of San Diego State University. His extensive Web site goes deep into the science, has several photos of green flashes, links to other Web sites with information, and an extensive biblography that includes books and articles ranging from beginning to highly technical levels. Click here to go to his site.

Sunrise, sunset times, other sky data

• USA TODAY Online has a page of links to information on sunrise and sunset times, moonrise and moon set times, the phases of the moon, and sky information from the U.S. Naval Observatory. Click here to go to it.

All about clouds

• USA TODAY has an index to clouds with links to photos and other information.

Northern lights

• USA TODAY's Online aurora home page has links to several sources of information about the northern lights.

• USA TODAY's links to information about astronomy
• Frequently asked questions about sky color
• Answers to Ask Jack questions about sky color