Thanks to everyone who took the time to do a weather-information survey mentioned in an earlier column.

Let me reiterate that this was an unscientific survey. In addition to people responding to the survey after references in the Mercury News, the survey was also posted on my Web site and distributed electronically to others in meteorology-related fields. Consequently, some of the responses are from people with more than a casual interest in the weather who had access to the Internet, which undoubtedly skews results. But overall, the survey did a good job of capturing a sense of people's use of weather information.

So where do most people get their weather data? According to the survey, 57 percent of responders use the Internet as their primary data source, 25 percent list television, 10 percent use newspapers, 7 percent use radio and 1 percent utilize the National Oceanic and Atmospheric Agency's Weather Radio system.

There were more than 250 respondents from the Bay Area, California, the United States, Canada and even Poland. During the next couple of weeks, in addition to answering your regular questions, I'll address some of the results from the survey and some of the interesting comments from respondents.

If you want to learn more about the weather, consider going back to school. For the fall 2002 semester, I will be teaching ``Weather & Climate'' (Meteorology 10, section 7) at San Jose State University. The class will be held Tuesday evenings from 6 to 8:45 p.m. If you would like more information, contact the Admissions Office at SJSU or send me a message at weathercorner@ggweather .com.

Q.  What percentage of lightning strikes end up reaching the ground? In all my 35-plus years of living in the Bay Area, I can't recall any advertisements regarding lightning rods for sale. We do happen to live in a part of the United States where the least number of thunderstorm days occur annually on average -- two to three -- but one would think that more local hardware stores would sell lightning rods. Paul Locher - Cupertino

A.  In regard to the lightning strikes that hit the ground, over the continental 48 states, an average of 20 million cloud-to-ground flashes have been detected every year since the lightning detection network, covering all of the continental United States, was finished in 1989. In addition, about half of all flashes have more than one ground strike point, so at least 30 million points on the ground are struck on average each year in the United States. Besides cloud-to-ground flashes, there are roughly five to 10 times as many cloud-to-cloud flashes.

I'm not aware of local suppliers, but a lightning rod could probably be special-ordered through a national chain like Lowe's or Home Depot, which probably stock them in other areas of the country. They may not stock them here because of the very low demand, and the rods are not required here in local building codes. Like many things, the cost of a lightning rod has to be factored into the overall risk in an area.

Q. Please describe the ``medieval drought'' and other prehistoric California droughts that allowed trees to colonize the now perennially submerged areas of Lake Tahoe and other Sierra lakes, evidenced by numerous examples of tree stumps under more than 20 feet of water. Doug Rischbieter - Arnold

A. Recent evidence actually points toward two periods of severe drought in California, between about 1100 AD and 1350 AD. This was during a period called the Medieval Climatic Optimum when global temperatures were significantly warmer and immediately preceded the Little Ice Age that lasted from 1400 to about 1900.

This was reflected by very low lake levels in the Sierra Nevada, which have been determined by the fact that stumps of Jeffrey pine trees are still rooted as much as 70 feet below the current water level. Using carbon dating, scientists found these trees to have grown on what was then dry land 900 and 650 years ago. Besides Lake Tahoe, similar forests have been found in Tenaya Lake in the high reaches of Yosemite and Walker Lake on the east side of the Sierra.

Q.  When a long-range forecast predicts above-average precipitation, does it mean an above-average number of storms with varying amounts of precipitation, or above-average amounts of total rainfall? Dan Carlentine - San Jose

A.  In reality, the long-range forecasts from the National Weather Service only give the probability of whether above- or below-normal precipitation is expected. They don't speak to either the number of storms or quantify how much above or below normal it will be.