Published Tuesday, December 25, 2001 in the San Jose Mercury News


Because of geometry, Bay Area rain often falls below the radar

Special to the Mercury News

``Hey, how come it's raining but nothing is showing up on the weather radar on the Internet or on television?''

I've been asked that a lot recently as a series of storms moved through the Bay Area. The answer is mostly simple geometry and geography -- how the radar beam is aimed compared with the curvature of the Earth.

Lets look at the geometry first.

The one-degree-wide radar beam is tilted about a half-degree above horizontal, so its altitude increases the farther it is transmitted from the antenna. At the same time, the curvature of Earth results in the ground dropping farther below the beam at longer distances.

As a result, 25 miles out, the bottom of the beam is about 1,200 feet above the radar antenna's elevation; at 50 miles it has risen 3,900 feet, and by the time it's 100 miles out the beam has climbed to 9,500 feet.

The cone-shaped beam also grows in width and breadth at longer ranges, spreading out more than 5,000 feet at 100 miles away.

Now, consider the geography.

The radar transmitter is near the top of 3,846-foot Mount Umunhum in the Santa Cruz Mountains. Consequently, the radar sees no lower than about 4,300 feet over San Jose, 7,300 feet over San Francisco, 8,700 feet over San Rafael and a whopping 13,000 feet over Santa Rosa. It can effectively detect rain 5,000 feet higher.

All of this means that weather can sneak under the radar coverage. A couple of weeks ago, as a relatively warm system moved through, the tops of the rain-producing clouds reached only 10,000 feet altitude over Marin County. An inch of rain fell, but none showed up on radar.

A more dramatic event happened Nov. 5-6, 1994, when San Francisco got a record 24-hour rain of 6.76 inches, but the radar indicated less than an inch.

The spring equinox occurs on March 20 or 21, yet the Mercury News weather page showed that the number of daylight hours was exactly 12 on March 16-17. We say that the autumn equinox occurs on Sept. 20 or 21, yet it was on Sept. 26 that the number of daylight hours was exactly 12.

I had assumed that we would go through the equinoxes, and maximum and minimum solar altitudes, at the same time at all latitudes. Am I wrong? Bob Foglesong - Palo Alto

That's a timely question because the winter solstice -- the official beginning of winter -- occurred at 11:22 a.m. Friday. The reason the equinoxes, with equal hours of day and night, and the solstices with the longest and shortest days of the year, don't quite match the sunrise and sunset times has to do with how each is calculated.

Dates and times of equinoxes and solstices are computed based on the center of the sun's disk. However, sunrise occurs when the upper edge of the sun's disk becomes visible on the horizon, while sunset occurs when the upper edge disappears. This slight difference is enough to skew the tables a few days.

Has the number of thunderstorms in the San Francisco Bay Area increased over the last 10 years? I grew up in the Bay Area, and I recall that thunderstorms were few and far between. Now it seems that every time there is a storm, isolated thunderstorms are forecast somewhere in the Bay Area. Is it because of climate changes?  Sal Murillo - Gilroy

From the data that I have seen, the number of thunderstorms has not increased significantly in the Bay Area compared with the long-term averages. However, I think the media and Bay Area meteorologists are more attuned to thunderstorms because we now have better tools with which to track them. These include radar that was installed in 1994, access to a computer network showing lightning strikes, and better satellite imagery.

Are weather balloons released by ships at sea to take soundings? Dudley Warner - Los Gatos

Yes, weather balloons used to be launched at sea, but it is not routinely done any more. In the 1950s and 1960s, Coast Guard cutters on station in the Pacific launched weather balloons daily. In a National Oceanic and Atmospheric Administration project in the 1980s and early 1990s, weather balloons were automatically inflated, launched and tracked from container ships at sea. However, weather satellites now can look down and collect much the same data -- temperature, humidity, pressure and wind.

Why does it seem to rain hardest just before the rain stops? Dudley Warner - Los Gatos

Most rain in the Bay Area is the result of either a cold front or an occluded front passing through. In these types of weather systems, the heaviest rain falls near the back edge, so we often see clearing shortly after this part of the storm goes through.

I see five-day weather forecasts on television and on assorted Web sites, but it's hard to find a good longer-term forecast, say 10 days or 21 days. Are they available? Gordon Kass - Los Gatos

As little as five years ago I would have laughed at the idea of putting much faith in a forecast over five days. Now, with more powerful supercomputers, forecasts go out two weeks. But the accuracy drops off dramatically after five to seven days. Beyond that, the forecast is probably more of a trend rather than specific weather.

One of my favorite Web sites with forecasts out seven days is Just enter your city or ZIP code, and it will give you a seven-day forecast plus current satellite and radar pictures.

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 or e-mail them to Please indicate in your e-mail what city you live in.