On average any single location in the Bay Area will hear a clap or two of thunder about a half-dozen times a year, and experience widespread thunderstorms about once a year. Compare this to an average of about 50 days of thunderstorms per year in the Plains and the Midwest, and more than 90 in Florida. Worldwide, there are approximately 2,000 thunderstorms occurring at any single time.

The electrical discharge is the visible effect, from the development of a certain type of cloud called cumulonimbus, which develops when rising and descending air, ice particles and rain droplets separate into positive and negative charges. The interaction of these charged particles produces an electrical field within the cloud, with the icy upper levels of the cloud gaining a positive charge and the lower portions becoming negatively charged.

As a thunderstorm passes over the ground, the negatively charged cloud base produces a positive charge on the ground below. Lightning occurs when the difference between the positive and negative charges becomes great enough to overcome the resistance of the insulating air. On average, lightning produces 15 million volts of electricity, although up to 100 million volts can be generated.

Lightning does not strike in a single stroke or ``bolt.'' Rather, it is a stream of charged particles called a leader that moves toward the Earth in a series of steps that give a jagged appearance. These steps are approximately 150 feet long with a pause between steps of about 50 microseconds. As the leader approaches the Earth, there is a surge of charged or ionized particles back toward the cloud in the channel opened by the leader. This occurs so quickly that it produces the optical illusion of lightning flashing downward from a cloud.

Despite what you may have heard, the accompanying thunder is not the ``angels bowling.'' It is the sound made by the explosive expansion and shock wave as air is super-heated by a lightning discharge. A lightning discharge has a temperature of more than 50,000 degrees Fahrenheit, hotter than the surface of the sun.

Typically, lightning strokes extend from a cloud to the ground. But there are also discharges within clouds, from one cloud to another and from a cloud into the air. This type of streak lightning is the most common, with its familiar appearance of a jagged line. It can also ``fork,'' with several lightning strokes occurring in the same area simultaneously. When cloudy skies diffuse or partially obscure lightning, there is the effect of sheet-lightning.

There are many myths associated with thunderstorms. In some regions of the country there is said to be ``heat lightning,'' but this is nothing more than a storm too distant to have distinct characteristics. It is also said that lightning never strikes the same place twice, but a recent study by the National Aeronautics and Space Administration found that some locales like mountain peaks and skyscrapers are struck numerous times.

Lightning is also one of the most deadly weather phenomena. Between 1973 and 2002, an average of 69 people died annually in the United States after being struck by lightning. During the same period, there were 66 tornado fatalities and 45 hurricane deaths.

Q Recently we had a beautiful lightning show. On television, a meteorologist stated that we had 500 strikes. How does one measure how many times lightning strikes? Karen Kirtley - San Jose

A They are detected by the National Lightning Detection Network, which monitors the 20 million cloud-to-ground lightning flashes in the 48 contiguous United States each year. The network is made up of approximately 100 ground-based sensors that detect the electromagnetic signals given off when the Earth's surface is hit by lightning. The exact position is triangulated from multiple sensors and then transmitted via satellite to the NLDN control center in Tucson. The strikes are plotted and transmitted to users around the nation within minutes of occurring.

Q Last month, a friend and I were in Death Valley, braving the heat for a walk from Badwater to the top of Mount Whitney. Would you believe that instead of dying of heatstroke, we almost drowned? A thunderstorm unleashed a flash flood that flooded part of the road near Golden Canyon in Badwater Basin. Since we had to cover 25-plus miles that day, we ignored the rangers' warnings and made our way through a river of calf-deep fast-moving muddy water filled with rocks. Would you please comment on this flash flood phenomenon? And, even though we did use trekking poles to assist us, please advise your readers not to do what we did, on foot or in a car. Dinesh Desai - Los Altos

A Thank you, but you have already expressed it in a much more eloquent manner than I could. But to reiterate, never try to cross moving water. Even ankle-deep water has the energy to knock people off their feet and carry them downstream. The bottom line is get to high ground.