WEATHER CORNER

BY JAN NULL
Special to the Mercury News

TODAY, we'll answer some questions Weather Corner often gets about the various colors in the sky.

Q. Why is the sky blue?

A.  The energy from the sun reaches Earth's atmosphere across a broad range of the electromagnetic spectrum, but 44 percent arrives as visible light. Our atmosphere is made up of mostly very small nitrogen and oxygen molecules. As this sunlight reaches the atmosphere the shorter wavelengths (i.e., violet, blue and green) are selectively scattered in all directions. When we look at the sky, these scattered wavelengths are viewed together from all angles, and our eyes, which are particularly sensitive to blue light, see the sky as blue.

Q. How about when the sky sometimes looks a milky white?

A.  This occurs when small particles, such as fine dust and salt, are suspended in the air. Even though these are also very small, they are bigger than the air molecules, and are non-selective scatterers. Consequently, all the colors are delineated, and our eyes see their combination, which appears a milky white.

Q. What makes the sun appear red at sunrise and sunset?

A. When the sun is low on the horizon, its light must pass through the greatest depth of the atmosphere. When it is within a few degrees of the horizon, the sun passes through 12 times the thickness of the atmosphere than when it is directly overhead. By the time the sunlight has penetrated this amount of air, most of the shorter waves of light (i.e., blue) have been scattered by air molecules. This leaves mostly longer wavelengths, which our eyes translate into beautiful yellow, orange and red sunrises and sunsets. Other natural events such as fires and volcanic eruptions add further particles to scatter the light and only the longest red light waves reach your eye.

Q. And how about rainbows?

A. The rainbow, is the confluence of light and water, though unfortunately there is no pot of gold. The magic that makes rainbows involves the optical effects of the refraction and reflection of light as it enters and then leaves water droplets.

Rainbows occur when rain is falling in one part of the sky and the sun is shining in another. The geometry necessary for this process to be seen has sunlight coming from behind the observer. As it enters a raindrop the light is refracted, then it is reflected off the inside of the drop, which bends it back toward the observer. Finally, it is refracted again as it leaves the drop. This process separates light into its component colors, which we see with red at the top and violet at the bottom. Rainbows also appear in the spray from a waterfall or sprinkler.

The arc of the rainbow is 42 degrees. On rarer occasions, a less intense secondary rainbow is visible with an arc of 50 degrees. These happen when the light is sufficiently strong to produce two refractions and two internal reflections.

The colors are reversed on a secondary rainbow with violet being on the outside and red on the inside. We see only a partial arc of a rainbow because Earth gets in the way and blocks the light. From an airplane, a rainbow appears as a full circle.

Because most weather moves from west to east, the appearance of a rainbow during the late afternoon may indicate improving weather as the sun becomes visible. Check http://redbaron.bishops.ntc.nf.ca/science/physics/papers/Rainbow.htm on the Web.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.