WEATHER CORNER

BY JAN NULL
Special to the Mercury News

HURRICANE Dennis in review:

No, Virginia (and North Carolina), Dennis is not the longest-lasting hurricane. Dennis became a tropical storm on Aug. 24 and lasted as a named storm for 13 days. However, in 1994, Hurricane John formed off the coast of Mexico on Aug. 11 and did not dissipate until 32 days -- and about 6,000 miles -- later after trekking westward across much of the Pacific.

Q  We expect temperature to go down as we go up in altitude. But it doesn't seem to be as simple as that.  We have a house near the 4,000-foot elevation in the Sierra. I compare our temperatures with those in Jackson, nearly visible from our house but more than 1,000 feet below us in altitude. Sometimes we get inversions when it is warmer here than Jackson. But more common -- and the subject of my question -- is that it is often, say, 10 degrees cooler during the day here but about the same temperature at night.

More puzzling is when it is, say, 10 degrees cooler during the day and maybe 5 degrees warmer at night. What is happening?
Al Slensky - Sunnyvale

A. The 10-degree difference in daytime readings is easiest to explain, as the atmosphere cools by about 3.5 degrees Fahrenheit per 1,000 feet in elevation. The nighttime situation is more problematic, but one scenario has the cooler, and heavier, air flowing downhill to the lower elevation. A second scenario is that a low-level inversion forms overnight and the lower elevation cools off faster than the higher one.

Q.  We were in Hong Kong when a typhoon hit. It was alleged to have Force 8 winds. Can you tell me what that means in
mph? E.C. Woodward - San Jose

A. Force 8 winds range from 39 to 46 mph. These are based on the Beaufort Scale, which ranks winds from Force 0 to Force
12.  During the time of sailing ships in the early 19th century, British naval Cmdr. Francis Beaufort devised his wind force scale, which estimated wind speed based on the condition of the ocean.

A Force 0 wind is calm, while a Force 12 has hurricane-force winds of at least 74 mph. A fine accounting of the Beaufort Scale is at www.crh.noaa.gov/lot/webpage/beaufort/ .

Q. When the weather forecasts rain as a percentage (40 percent chance of rain), what does the percentage relate to -- time, area or chance? Jim Brunton - San Jose

A. The use of probabilities of precipitation is one of the most misunderstood elements in weather forecasts. The concept of using percentages is to provide additional information for the user to make a risk-benefit decision. For example, a contractor might decide to pour cement if the chance of rain is only 20 percent, but if it's 40 percent or higher, he wouldn't.

KPIX meteorologist Brian Sussman tells about his first visit to a National Weather Service office. There were four forecasters
on duty, and only one thought it would rain, making it a 25 percent chance of rain. If it were only that easy!

Actually, the usage of probabilities of precipitation is first dependent on whether there will be widespread rain or just localized showers.  In the case of rain, the probability depends on what the forecaster thinks the chance of getting measurable rain (i.e., at least .01 inches) is at any given point in the area. Using computer models and professional experience, a determination is made of how often the meteorologist thinks rain will occur from a similar meteorological situation. For example, if an approaching weather system is weak, the forecaster might expect rain only two times out of 10 from similar systems, and give the probability as 20 percent.

However, with showery weather the percentages relate to area coverage. Thus, a 50 percent probability for showers would mean that measurable precipitation is expected over half of the area.

Finally, the National Weather Service has specific words that relate to a given probability of precipitation for rainy or showery situations. See chart at right for an explanation of their formal usage.

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.