WEATHER CORNER

Q.  Where does lightning strike Earth during a thunderstorm? In storms like those we had over the past few weeks, lightning bolts flashed within a mile or so of me, but I never heard about the damage they cause on the ground. I imagine some could simply be passing between clouds, but surely there must be earthly evidence of their powerful touchdowns, right? How come I don't read in the next day's paper about the oaks split into flaming kindling, car computers blown, houses torched, etc.? Robert Howard - Palo Alto

A.  Yes, some lightning discharges are cloud to cloud, but most do strike the ground. Many harmlessly strike trees and rock outcroppings in the higher terrain around the Bay Area.

In California, the greatest lightning danger is summertime wildland fires sparked by strikes. When grounded structures such as high-tension power poles are hit, they carry the current harmlessly to the ground. But lightning strikes around the Bay Area do damage structures, trees and smaller, non-grounded power poles whose transformers appear quite vulnerable. The intense lightning storm in September caused a number of tree and structure fires and numerous power outages.

Q.  I understand the terms ``cloudy'' and ``partly cloudy,'' but not ``variably cloudy.'' Does it refer to the mixture of types of clouds (cumulus, stratus, cirrus, nimbus), or varying percentages of cloudiness over the course of a day (clear in the morning but cloudy in the afternoon, for instance)?  S. Nolan - San Jose

A.  There is no precise answer, but when I was writing forecasts for the National Weather Service, the most common use of the
term variable cloudiness was when the amount of cloud cover would vary through the day. This would often occur as a weather front approached and the clouds would gradually change from high cirrus to midlevel altostratus and then to lower stratus.

Q.  What do you think of ``The Coming Global Superstorm'' by Art Bell and Whitley Strieber? Could you recommend another book on the same subject, the effects of global warming? Helen St. Cyr - San Jose

A.  I have not read the book, but I know enough about it to say it is not very good science. ``The Coming Global Superstorm''
appears to extract the worst-case scenario from a number of theories about global warming and present them as a fait accompli. The authors have no expertise or credentials in global climates, and their main claim to fame are books and talk shows about UFOs.

I don't know of a single source that has all the answers about global warming, because all the answers are not known. We still don't know and understand everything about our atmosphere and its interaction with Earth.

The bottom line is, yes, we have seen increases in global temperatures in the past decade, but are these a function of natural variability or man's influences, or most likely a combination of both?

For a fairly balanced overview, a good starting point is the Environmental Protection Agency's www.epa.gov/globalwarming/ Web site.

Q.  Can you supply a source for the daily barometer setting? The weather page in the Mercury News doesn't carry it. Is it an
obsolete measurement? George Godlewsk - Saratoga

A.  No, the barometer is not obsolete. Barometer readings of the atmospheric pressure can provide valuable information about changes in the weather. However, you can't count on the markings along the side that say Fair, Rain or Stormy. These are just pointers toward possible trends. More important than the absolute pressure reading is the trend, or what the pressure has done over the past few days. Has it fallen, risen, or is it steady?

It's also important to remember that the pressure will change if you move to a higher elevation because a barometer basically
measures the pressure exerted by the air around us. At higher elevation in the hills, the pressure is slightly less than at sea
level. For example, if the pressure in downtown San Jose is 30 inches of mercury, it would be about 29.52 inches where you
live at 300 feet elevation in Saratoga, and 25.77 inches at 4,200 feet on Mount Hamilton.

There are a number of sources of hourly barometric pressure readings. One of the most convenient is NOAA Weather Radio, which broadcasts continuous weather information from the National Weather Service. Weather radios can be bought for
as little as $15 and are available from electronics stores. Hourly weather data also is at www.wrh.noaa.gov/afos/SFO/SWR/SFOSWRCA on the Web.

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 weathercorner@ggweather.com or telephone and fax them at (510) 657-2246.