WEATHER CORNER

Weather Conslting?

Q.  I checked out your Web page (ggweather.com), and it says you're a ``certified consulting meteorologist.'' What the heck does a meteorologist consult about? And how do you get certified? Thanks. Keep up the great column. Tom Robinson - Redwood City

A. First, I want to point out that I am certified and not certifiable!

The American Meteorological Society administers the Certified Consulting Meteorologist (CCM) program, which attests to a meteorologist's qualifications and professionalism. This is done through a series of written and oral exams plus an evaluation of the candidate's experience. This certification is especially important in a field like meteorology, where anyone can get on radio or television and call himself a meteorologist. The actual activities of a consulting meteorologist can be quite varied, ranging from providing expert testimony in court to research to private forecasting.

I'm often asked how weather data might be used in court. Most often weather comes into play as part of a civil action involving property damage. Testimony might be provided about the amount of rain that caused a slide or the wind speed that blew off a roof. A number of times, to counter fraudulent lawsuits, I've also provided data that showed a certain weather event did not occur.

Weather elements also can be critical in traffic accidents where rain, ice or visibility can be a factor. Less often there is a need for meteorological information in criminal cases. Here testimony might be about how the weather affected a piece of evidence or how the visibility affected the identification by a witness. Weather even played a role in the O.J. Simpson trial, when temperature records were introduced to determine how long it would take spilled ice cream to melt.

Weather information from consultants also is used by industry in a variety of ways. Construction firms may need such data to verify the number of ``rain days'' or to determine the best time of year for a particular activity. Consulting meteorologists also do forecasting for individual companies or events, including working with video or movie productions to find the best weather possible for their purposes.

Q.  How do cirrus clouds form? Why do they have a wispy shape? Why do they usually have an end that is bent or swept up? Is that the ``shape'' of the wind or air movement at that altitude? Can you determine the wind direction at high altitudes by the way cirrus clouds are pointing? Why are cirrus  clouds a leading indicator of good weather?   Curtis Panasuk - San Carlos

A.  Cirrus clouds are one of three types of high clouds in the cirroform family that form in the very cold, upper levels of the atmosphere. At these altitudes above 20,000 feet, water vapor in the atmosphere is in the form of ice crystals and can form cirrus, cirrostratus or cirrocumulus clouds.

Cirrus are the thin wispy clouds you ask about. They form when water vapor condenses into ice crystals to make them visible. They are thin and wispy because of strong winds and little water vapor at these altitudes. The tails are actually accumulations of ice crystals that fall below the clouds and move slightly slower because there's less wind at the lower altitude.

If you know the altitude of the clouds (30,000 feet is pretty typical for cirrus over San Jose), it is possible using simple trigonometry to calculate the speed of the winds carrying them along. When directly overhead, an object at 30,000 feet that traverses an arc of 30 degrees of the sky will travel
about 16,000 feet, or about three miles. If it travels this distance in three minutes, the speed would be 60 mph -- typical for winds at 30,000 feet.

Wispy cirrus can indeed be an indicator of fair weather, but they can also precede an approaching warm front, which is indicated by the cirrus thickening into the cirrostratus and then altostratus.

Q. I often see the cloud formations on the TV weather report and wonder if certain patterns seen on the weather map might be recognized with the naked eye when I stand in a place where I can see from horizon to horizon. In other words, say there are three lines of clouds on the satellite weather image, and one line goes right through the Bay Area. Would I be able to see the one line overhead, with a break on either side? The
same question applies for the more detailed green rain patterns on the radar map. Vince- San Jose

A. The actual sensor resolution on the geostationary satellites that provide most of the weather pictures is about one mile. This means the smallest feature that would show up would be about one mile across. And keep in mind that the ``colorized'' pictures on television have even a lower resolution. Consequently, you might actually be able to see a narrow band in the sky, though it might not be visible on a satellite picture.

The resolution of weather radar is better, ranging from about 200 yards near the radar site to about a half-mile 50 miles away. Thus, it's possible to see individual large clouds that are producing rain, though this, too, is reduced somewhat on the lower-resolution displays shown on television. Another factor to keep in mind is that there is sometimes as much an hour delay from the time radar or satellite pictures are taken until they're available for display.

Q. As a longtime resident of South Santa Clara Valley (living in both Gilroy and Morgan Hill) I've always wondered which of those two cities has the hotter summer weather? Paul Knofler - Gilroy and Morgan Hill

A. In general, the afternoon reading for Gilroy reported daily by the National Weather Service is about 2 degrees warmer than Morgan Hill's on most summer days. This is primarily because the normal afternoon sea breeze from the southern San Francisco Bay reaches Morgan Hill about an hour earlier than Gilroy, and stops the afternoon temperature from rising further. The average afternoon high in July is 88 degrees in Gilroy and 86 degrees in Morgan Hill. This occasionally varies when the cool southerly ocean breezes work up the Pajaro River and cool Gilroy more, and sooner, than Morgan Hill.

Jan Null, founder of Golden Gate Weather Services and Adjunct Professor at San Francisco State University, 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 or e-mail them to weathercorner@ggweather.com.