WEATHER CORNER

Try Fridays if You Want to Avoid Rain

BY JAN NULL
Special to the Mercury News

SURE, it was overcast much of last weekend. But the few drops that fell after dark Sunday in San Jose didn't constitute measurable rain. And that broke a string of six consecutive weekends of varying amounts of precipitation.

A quick look at the archives shows that until last weekend, San Jose had had measurable rainfall every weekend since mid-January. Statistically, we would expect rain at some point on only three of those seven. Looking at Saturdays and Sundays separately, there were five wet Saturdays, five wet Sundays and four dry days.

Looking for the driest day of the week to do something outdoors? Try Friday. The last time it rained on a Friday was way back on Jan. 15. TGIF.

Oh, and the outlook for next weekend . . . looks like another good chance of rain!!

Q. recently saw a ``weather station'' site that showed something called the heat index. How is the heat index computed? Richard Beal - Mountain View

A. Heat index is the summertime equivalent of wind-chill factor.

It's a measure of what the weather ``feels like'' and is also called the apparent temperature. Instead of cold and wind, the heat index is based on heat and humidity. It is not an actual meteorological calculation, but is based more on physiology.

Because the body cools itself by perspiring, when the air is humid it can't cool itself as effectively and a person feels hotter. (So yes, there is some truth to the saying ``It's not the heat, it's the humidity.''

For example, if the temperature is 90 degrees and the relative humidity is 50 percent, then the heat index is 96 degrees. If the temperature is 100 and the humidity is 50 percent it's 120.

When the heat index is 90 to 105 degrees, sunstroke, heat cramps and heat exhaustion are possible with prolonged exposure or physical activity.

A complete chart and further explanation can be found at www.usatoday.com/weather/wheat3.htm

Q. understand that it takes plenty of knowledge to be a meteorologist.  About how much knowledge and about how much time do you put into education to do what you do? Andy Nunes - Fremont

A. Most meteorologists have at least a bachelor's degree in either meteorology or atmospheric science. To get into these programs requires strong math and science preparation in high school with calculus, physics and chemistry at a minimum.

Typical lower-division college course work is two years of physics and calculus, then extensive upper-division classwork in meteorology. These courses include atmospheric dynamics, thermodynamics, analysis and prediction, forecasting labs, remote sensing and instrumentation. Many
meteorologists then go on to get graduate degrees in an area of specialization.

Nationwide, fewer than 60 schools offer degrees in meteorology. Four are in California. In the Bay Area, San Jose and San Francisco state universities have excellent programs, as does the University of
California-Davis (my alma mater). The fourth California school is UCLA.

Q. am moving to Mendocino and want to protect my antique cars from rusting. They will be in an enclosed garage. I have heard that a light bulb in a basement would keep toy train tracks from rusting. How much do I need to heat the garage to stop the rust? J.A. - Cupertino

A.  By increasing the heat, you are going to change only the relative humidity, not the actual amount of moisture in the air. Relative humidity indicates only the percentage of moisture needed for saturation, not the actual quantity of water vapor. This is why desert air with a temperature of 95 degrees and relative humidity of 16 percent contains more water vapor than polar air with a temperature of 28 degrees and relative humidity of 100 percent.

There are, however, a number of solutions, including dehumidifiers, desiccants and -- believe it or not -- large plastic bags that you can wrap around a car to evacuate all the air.

Q. What evidence is there that the ancient Celts could precisely measure the date of the winter solstice? How did they do it? I can see how they used sunrise/sunsets on the horizon for equinox dates. But wouldn't solstices be much harder? Jared Tinklenberg - Palo Alto

A.The observation of the solstices can be determined relatively easily by watching the northernmost and southernmost points from which the sun rises and sets.

By marking out reference points on Earth (e.g., Stonehenge in approximately 2000 BC) it was possible to tell when subsequent events occurred. This was probably more accurate for the Celts than determining the daily time of sunrise and sunset, as reliable timepieces were still centuries away.

Jan Null, founder of Golden Gate Weather Services, is a retired lead forecaster with the National Weather Service. Send questions to Weather Corner, San Jose Mercury News, 750 Ridder Park Drive, San Jose, Calif. 95190. You also can telephone and fax them at (510) 657-2246 or e-mail them weathercorner@ggweather.com.