Published Tuesday, January 14, 2003 in the San Jose Mercury News

Weather Corner
Reflections on the nature of rainbows

By Jan Null
Special to the Mercury News

The timing of several recent storms resulted in some brilliant rainbows around the Bay Area, and a number of questions about their appearance rained upon the Weather Corner.

Rainbows have always been one of those showstopping spectacles of nature that are surrounded by myths and have graced artwork for thousands of years. But what are they, and how are they formed?

To observe a rainbow is to see a circular arc of several colors. Optically, a rainbow is just the distorted image of the sun that has had its colors rearranged by raindrops through refraction and reflection. Raindrops act as imperfect one-way mirrors that allow most of the sun's light to pass through. A small fraction of the light, however, takes a more circuitous route to provide a brilliant display of color.

As light enters a raindrop, it is refracted in the same way a prism separates light into its component colors. But this is just the beginning of the light's path through the drop. Next, the spread of colors is reflected off the inside back of the drop. Finally, the light is refracted again as it leaves the drop.

The angle of the sun to the raindrops and the observer is very particular. This is why the primary rainbow is always an arc with a 42-degree radius as the observer's back is to the sun. When the light is strong enough, a secondary rainbow is visible at a radius of 51 degrees and is formed by a second internal reflection of light inside the drop. Between these two bows the light is less intense because it has undergone either one or two reflections inside the surrounding drops, and this dark area is called Alexander's dark band.

The best time to see rainbows is following a period of rain when the sun is coming out. This is usually in the afternoon when the rain has moved to the east and the sun pokes out in the west.

To find out more about the optics of a rainbow, go to

Q Those of us who live in the South Bay are blessed with a weather indicator as big as all outdoors -- San Jose airport. For about 360 days of the year, the planes take off toward the north and land from the south. Whenever there is likely to be inclement weather, however, the pattern is reversed. If a storm is on the horizon, planes land from the north, much to the delight of us who live in that area. Is this as good as a barometer in predicting the weather? Henry Tunis - Sunnyvale

A According to the Federal Aviation Administration, Mineta San Jose International Airport switches to ``south plan'' about 15 percent of the time. As you note, this is often prior to a front moving into the area, when winds are from the south and aircraft take off into the wind. There are also a number of occasions throughout the year when cool, heavier air in southern Santa Clara Valley spills toward the bay during morning hours with enough velocity that the runway configuration also will be switched to the south.

Q Your column on a storm intensity index was interesting reading, as my impression, like that of many others, was that the storm front of mid-December was as strong as any I remembered in 30-plus years in the Bay Area. Given that more intense storms are but one ``feature'' forecast for the future of a warming globe, I wonder if there is discussion within the weather and climate-tracking professions about climate change and such strong storms. Whether manmade influences are a part of the climate change scenario would appear to be a secondary matter to be debated by policymakers. If the climate is going through a warming trend (as the consensus data seems to suggest), such storms as those we have recently experienced would appear to be part of our future, and something we should prepare for down the road. Or is that not the case? Kevin Gregory - Sunnyvale

A First, I think it is impossible to tie any single storm to either a long-term event like global warming or even a shorter-term event like El Niņo. Second, the amount of warming we are talking about is not significant as far as individual storms are concerned. The sorts of changes that we might see in the future would include a greater frequency of storms, especially tropical storms and hurricanes, which derive their strength from the temperature of the atmosphere and oceans. But it may be decades before we can separate any increase from the usual variation in the number of storms.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o WeatherCorner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone questions at (510) 657-2246, fax them to (510) 315-3015, e-mail them to or fill out a form online at Please indicate in your e-mail what city you live in.<