Published Tuesday, May 4, 2004 in the San Jose Mercury News

Weather Corner

Flush these meteorology myths down toilet

Special to the Mercury News

There are urban myths and legends involving almost any topic imaginable, and meteorology is no exception. Many are things we have heard all of our lives and take for granted. For example, toilets flush in different directions in the northern and southern hemispheres, or on the equinox you can balance an egg on its end. How about when humid weather is described as 90 degrees with 90 percent relative humidity, or pictures of raindrops are tear-shaped? They are all part of meteorological mythology.

The rotation of the Earth and the resulting Coriolis force has been cited as the cause for toilets and sinks that drain in opposite directions on either side of the equator. Ain't so! The Earth rotates very slowly (once a day), and the resulting Coriolis force is very small and must act across large distances to have any effect. By contrast, the circular flow of water in a sink or toilet is one rotation every few seconds over a very small distance. The bottom line is that imperfections in the design of a drain or toilet are the dominant factor in which way the water flows, not the Coriolis force.

And how many times have you seen someone on television -- usually the science-challenged anchor -- balance an egg on its end on either the spring or autumn equinox? But does he try on the other 363 days of the year? Nope! All you need to balance an egg is a raw egg, a hard, flat surface and a steady hand. It will work any day of the year. A detailed explanation is on the Bad Astronomy Page at

I can't tell you the number of times I have seen or heard a reference to weather being 90 degrees with a relative humidity of 90 percent. These 90-90 conditions do not occur naturally, at least not in the United States.

For this to happen, the dew point -- the temperature that moist air must be cooled to for condensation to occur -- would have to be at least 87 degrees, and the highest that I have ever seen in the United States is 82 degrees. The combination of 90 degrees and a dew point of 82 degrees yields a relative humidity of only 77 percent. Probably the only place where 90-90 could occur is around the Arabian Gulf, the warmest large body of water on the planet, where the dew point occasionally approaches 90 degrees.

Finally, too many artists have taken too much artistic license when depicting the simple raindrop in a teardrop shape. In fact, a small falling raindrop is almost a perfect sphere, and large drops have a flattened bottom because of wind resistance as they fall. The teardrop shape that is frequently drawn is most likely from watching a drop on a window or similar surface where surface tension does tend to stretch out a drop's shape.

Q.  I am wondering if the climate in San Francisco has gotten warmer in the past several years. I looked at the numbers for daily averages and record highs. Also, though I realize that one year will not make a difference statistically, last year seemed particularly hot. Having grown up in the Bay Area, I am almost certain that the weather has become warmer since I was a child. (I am now 33.)

I have an ongoing argument with a friend of mine, who is 60, which I would like to settle. She says the weather is not different, that there always (in her 40 years of residence here) have been stretches of hot days. I say that there are more hot days than 20 years ago, based on my experience as a kid, in which you'd be considered nuts to wear a bathing suit to the beach. I believe global warming is already affecting us here, but I want an expert's opinion. Lisa Giampaoli - San Francisco

A.  There are several important issues at play in your question. First, last year was warm, but nowhere near a record. I did a quick scan of the past 90 years of average temperature records for San Francisco, and 2003 was only the 23rd warmest. That being said, there have been four different locations of the downtown temperature sensor during that period, and some have been warmer than others.

There is also an issue of ``urban heat'' islands, where the radiation of heat from streets and buildings in an urban area increases the recorded temperature reading. To better track long-term records, it is better to monitor rural sites that have not been moved.

Finally, does any of this indicate global warming? Not by itself. It is impossible to use any single event or site to accurately depict what the climate is doing. By definition, climate is a measure of long-term patterns. It smooths out the ``noise'' from a given year or years, and it must be free of the biases of a single station. This is why we usually look at 30-year averages to determine what's ``normal.''

Q.  How does topography affect the weather? Dorinda George - Tallahassee, Fla.

A.  Topography is one of the major contributing factors to any region's climate. Other key elements are latitude, elevations, continental location and ocean currents.

The terrain produces roughness to the wind flow that can change the speed and direction. It also heats unevenly, and that produces rising currents of air as well as regions of higher or lower barometric pressure. The terrain can also act as a barrier. It can restrict the flow of the coastal stratus or rain clouds that dump more precipitation because of the added lift from the terrain.

Q.  Fog dispersal devices were used during the World War II. What exactly did they do, and would a modified version help airports today? Wani Bhatti - Santa Rosa

A.  Efforts were made in France to heat the air along airport runways to evaporate low-lying fog. However, the exhaust plumes from the diesel heaters created their own visibility problems, plus the warm rising air made conditions quite turbulent for landing aircraft.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to him c/o Weather Corner, 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 or fill out a form online at