Published Tuesday, March 28, 2000, in the San Jose Mercury News


Q.  QThe local forecast on the Weather Channel includes a Doppler radar plot showing where rain is falling in the Bay Area. It is a dynamic plot displaying the changing rain condition over a period of time. If memory serves me, the time period was specified in the past but is no longer. Do you know anything about this? Paul McKiernan - San Jose

A. On the Weather Channel radar loops, the time of each frame is shown in very, very small numbers along the bottom of the
image. The full sequence shown consists of six images covering about two hours. This varies among the different companies that offer radar data on the Internet. During rainy periods, the radar actually produces a new image every six minutes. However, it would require a very long time for the Weather Channel to download data to make a radar loop using all of the six-minute pictures.

Q. In February I saw concentric rainbows in Fremont. The inner rainbow was very vivid, and the outer was fainter. Soon I
noticed a third rainbow that was not concentric with the others. It intersected the outer arc and would have intersected the inner one had it reached down that far. I took pictures. How can that third rainbow be explained? Janet White - Fremont

A. When I first read your question I was dubious of what you described. After seeing the pictures you sent me and doing a little research, however, I found reference to a phenomenon known as a reflection rainbow.

A normal rainbow is seen when the sun or other bright light interacts with falling raindrops. This light is refracted or bent as it enters the drop, then is reflected off the inside of the drop and finally refracted a second time as it leaves the drop. he result of this rather circuitous path is a colored arc of light in the opposite direction of the light source.

And if the light is bright enough, a secondary rainbow can be seen when a second reflection of the light occurs within each droplet. This will be seen as a faint rainbow that parallels the primary rainbow.

For a reflection rainbow to be seen, the light is first reflected off a body of water or other shiny surface and then up into the raindrops. This provides a different angle for the light to enter the drops, and this rare arc you photographed can be seen. The University Center for Atmospheric research has a diagram at showing this phenomenon.

Q. I know that rising air will create clouds, but I never see it happen. How come? Sparky Cohen - San Jose

A. Cloud formation generally happens very slowly, so you have to be patient to see much change. Small cumulus that form over the hills during the afternoon are probably the easiest to see grow. They are usually the result of updrafts along the hills that sweep air higher into the atmosphere, where it cools and condenses.

The rain clouds that we see in the Bay Area normally form way out in the Pacific along fronts or large low-pressure centers. Air near these features is forced strongly upward into the atmosphere where massive condensation takes place up to 30,000 feet or higher. The upper-level winds push these clouds over California.

Q.  Why is the weather often stormy and overcast in areas of low pressure and clear and nice in high-pressure areas? Caitlin Rolla
- Menlo Park

A. First, here's some background about highs and lows. The atmosphere is always trying to reach equilibrium. Air in high-pressure areas flows toward areas of lower pressure. However, because of Earth's rotation, this flow of air -- wind -- veers to the right in the Northern Hemisphere and to the left in the Southern Hemisphere.

As a result, wind blows clockwise around and outward from high-pressure centers and counterclockwise and inward toward lows. Air flowing into low-pressure areas converges near the center and, because it can't go down into the ground, it rises into the atmosphere. The rising air cools and condenses into clouds and sometimes precipitation. The opposite occurs around highs: The air sinks, dries out and gets warmer.

WEATHER EXTREMES: Did you know that the hottest official San Jose temperature was 108 degrees on July 14, 1972?
Compare this with the California and U.S. record 134 degrees in Death Valley and the blistering world record of 136 degrees
in Libya.

Jan Null, founder of Golden Gate Weather Services and Director of Meteorology for, 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 e-mail questions to or telephone and fax them at (510) 315-3015.