|Published Tuesday, March 28, 2000, in the San Jose
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 www.unidata.ucar.edu/staff/blynds/rnbw8.gif
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
Jan Null, founder of Golden Gate Weather
Services and Director of Meteorology for Planetweather.com,
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 email@example.com
or telephone and fax them at (510) 315-3015.