It's about time to flush one of the most prevalent meteorological myths -- and a question I have been asked in just about every meteorology class that I have ever taught. In the lands Down Under, water does NOT swirl down the drain in a different direction than it does in the Northern Hemisphere.

Many of us were taught this myth about the Coriolis force and the different ways toilets and sinks drain on either side of the equator, and the story has even been perpetrated by PBS and an occasional college textbook. But it just ain't so.

The Coriolis force is the result of the rotation of the Earth, and it is what causes the air to rotate counterclockwise around low-pressure centers in the Northern Hemisphere and clockwise around lows south of the equator. These motions are very visible when you see the clouds swirling around a hurricane or typhoon.

So why doesn't it affect sinks and toilets? It's all a matter of scale, both in distance and time.

The Coriolis force comes from the Earth's rotation, which is very small with just a single rotation per day. It is also spread over very large distances on the order of hundreds or thousands of miles.

But the rotation rate of water draining from a sink or toilet is on the order of several rotations per second, and it's over a very small distance. The Coriolis force doesn't even affect the rotation of tornadoes or dust devils because their rotation is too rapid and over too small a distance. Consequently, the Coriolis force is much too small a factor to impart rotation either one way or the other.

The factors that do affect how a sink or toilet might drain are much more the result of engineering and imperfections in design and construction. For example, if a drain is not perfectly circular, then the flow will be affected, as is the case with a drain that has any obstruction in it, or even air flowing across the top of a water surface.

METEOROLOGY CLASS:  Do you want to learn more about the weather? For the fall semester, I will be teaching Weather & Climate (Meteorology 10, section 7) at San Jose State University. The class will be Tuesday evenings from 6 until 8:45 p.m. If you would like more information, contact the Admissions Office at SJSU or drop me a note at weathercorner@ggweather.com.

Q. I have always been a big fan of thunderstorms in the Sierra during summer. What causes the thunderstorms to form? Is there a forecast I should look for that would signify the coming of thunderstorms? Is there an area of the Sierra that would be more likely to offer a spectacular show?  Jim Weller - San Jose

A. Summertime thunderstorms in the Sierra are typically the result of two factors. The first is an influx of moist air from the south, and the second is the lift to this air provided by the mountains.

This often happens when there is high pressure aloft centered near the Four Corners region where Arizona, New Mexico, Utah and Colorado meet. The clockwise flow around a Four Corners high-pressure zone brings moisture that often originates over northern Mexico or the Pacific off of Baja California and northward into California. As this moist air rises over the Sierra Nevada, it cools and condensation occurs, resulting in clouds and sometimes thunderstorms.

Q. Weather forecasters, on the radio in particular, refer to areas such as ``inland,'' ``well inland,'' ``inland valleys'' and -- a favorite -- ``central bay.'' What do these terms mean? Is San Jose inland or well inland? Is Concord an inland valley or not? Maybe that means Livermore and Gilroy? Central bay sounds as if it is referring to an island in the middle of the water. Why don't they simply refer to towns we know?

Additionally, why use fuzzy terms that are meaningless such as saying ``inland highs in the 80s''? That covers a wide, wide range, even if I understand what they mean by ``inland.''  Gordon Kass - Los Gatos

A. I can only make some generalizations about what other forecasters mean when they use vague terminology. The overriding factor in the Bay Area is the number of microclimates that we have and how variable our weather can be over a relatively short distance.

In general, ``well inland'' encompasses the valleys in the Bay Area that see more extreme temperatures because of their distance from the bay or ocean. These include Concord, Livermore and Gilroy. The ``central bay'' is probably best described as between the Bay Bridge and the San Mateo Bridge. However, many of the other terms are even more subjective, and there are really no clear-cut definitions.

While there are some specific city forecasts available in most of the local forecasts, there are too many small cities for everyone to get mentioned, and larger areas that are an umbrella for all cities are used. The problem with this methodology is that the temperature ranges can get pretty large. The forecast you cite of ``in the 80s'' might be too general for one specific locale, and thus the individual has to have an idea whether the temperature is usually at the upper or lower end of such a spectrum.