Published Tuesday, October 22, 2002 in the San Jose Mercury News

Weather Corner

Courts often need an expert witness on the weather to help settle disputes

Special to the Mercury News

Besides forecasting the weather and writing the Weather Corner, one of the other hats I wear is that of a forensic meteorologist -- or expert witness -- about the weather.

While I don't foresee a television show a la ``CSI: Crime Scene Investigation'' in my future, I am continually amazed and fascinated by some of the interesting issues about which I get to play weather data detective. These have ranged from civil litigation matters, including a divorce case, to a criminal case of murder.

On a fairly regular basis, weather has at least some bearing on matters being argued in court. Even in O.J. Simpson's case, there was testimony about what the temperature was at the time of the murder to determine how long it would have taken for some spilled ice cream to melt.

More typically, forensic meteorology involves how much it rained or how hard the wind blew. With rainfall-related cases, weather information is usually needed to determine whether the rain contributed to a vehicle accident, a fall in which someone slipped, flood damage or a construction delay. Cases involving the wind can range from damage caused by wind to debris and other hazardous material carried by the wind. Other weather elements -- ice, fog, humidity and large ocean waves -- can play a part.

One case involved a person who allegedly had been exposed to asbestos while working near the New Jersey shipyards during World War II. This matter required going through several years of hand-written archives of hourly weather data for the area and calculating the percentage of time the wind was blowing from the shipyards to where this person worked.

In the divorce case, the meteorological issue had to do with Seasonal Affective Disorder, or SAD, in which weather is thought to influence mood -- and the relative climates between two locales. In the murder case, the weather came into play regarding a set of fingerprints, and whether humidity and rainfall might have degraded them and lessened their value as evidence.

Q. I understand that our warm weather in September was caused by high-pressure air ``aloft'' (at a level of about 18,000 feet) that ``fell'' down to lower altitudes and heated up during its descent. How does that mass of high pressure get up at 18,000 feet? Or, conversely, how does the low pressure form down here? Curtis Panasuk - San Carlos

A. When we speak of high pressure aloft, it is not a matter of high pressure moving from the surface into the upper atmosphere. Instead, it is when there is a dome of air that extends from the surface into the upper atmosphere.

Areas of both high and low pressure -- as well as the circulation of air around the globe -- result from two factors. The first is the unequal heating of the polar and equatorial regions of the globe, and the second is the rotation of the Earth. The two cause a movement of cold air masses from the poles toward the equator and warm air masses from the equator toward the poles as the Earth's atmosphere tries to reach equilibrium. The cold air masses have denser air and higher pressures, while the warm air masses have lower pressure.

Q. Where I came from, in Minnesota, a forecast of snow flurries meant a series of intermittent and relatively brief periods of falling snow during a day. Nowadays, I never seem to hear the term ``snow flurry'' or ``snow flurries.'' Instead, it always seems to be ``snow showers.'' Can you please set me straight on the difference, if there is one? Jean Myer - Mountain View

A.  A snow shower is a brief period of snowfall that is variable in intensity and begins and ends rapidly. Just as with rain showers, the intensity can be predominantly light, moderate or heavy. A snow flurry is simply a light snow shower.

Q.  Where in Northern California is the most perfect climate for growing the most perfect garden? I'm looking for something frost-free, with some sunshine, that hardly ever goes above 80 degrees. Marilyn Wilson - San Diego

A.  Without knowing what type of garden you are thinking about and where you actually consider Northern California, I can only make some generalizations about the perfect climate for your perfect garden.

While it is impossible to find a locale that is 100 percent frost-free in Northern California, the likelihood of frost goes down significantly the closer to the coast one goes. This is primarily due to the moderating influence of the relatively warm waters of the Pacific and being closer to sea level. Additionally, the farther south along the coast, the warmer it will be. But keep in mind that these areas also tend to be some of the most desirable and expensive!

Jan Null, founder of Golden Gate Weather Services, 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 telephone questions at (510) 657-2246, fax them to (510) 315-3015, e-mail them to or fill out a form online at Please indicate in your e-mail what city you live in.