WEATHER CORNER

BY JAN NULL
Special to the Mercury News

Q When is one most likely to encounter hurricanes or violent weather in Hawaii? Fred Friedlander - Saratoga

A. Well, the bad news is that the hurricane season in Hawaii lasts from June 1 through Nov. 30. But the good news is that in the past 50 years, only five hurricanes have caused serious damage to the Hawaiian Islands.

While the peak of the Central Pacific hurricane season is July and August, when the tropical waters that power these storms are at their warmest, those five storms were spread out. There was one each in July, August, September, November and December. The latter, Nina, came after hurricane season and generated 82 mph winds in Honolulu as it passed 120 miles west-southwest of Kauai on Dec. 1, 1957.

On Aug. 5, 1959, Dot swept across Kauai with 103 mph winds. Another late-season storm, Iwa, passed close to Kauai on Nov. 23, 1982, with 92 mph winds. The combination of wind and flooding caused an estimated $234 million in damage.

In 1986, Estelle produced very high surf on Hawaii and Maui, and five to 10 inches of rain inundated parts of Oahu on July 23. By far the most powerful hurricane was Iniki, which made a direct hit on Kauai on Sept. 9, 1992. Sustained winds of 130 mph caused six deaths and $2.3 billion in property damage. Even hurricanes that do not hit the islands directly can create spectacularly large waves and flood beaches.

Q.  If Pacific tides are not predictable on a regular basis (tide clocks apparently work fine for Atlantic tides but not for those in the Pacific), how does the National Weather Service predict the tides and why are they not regular? Russell Hacker -  Los Altos

A. I don't know the relative merits of tide clocks, but the equations used to compute tides along the West Coast are quite accurate. These tables are generally available at marinas and bait shops, as well as on a number of Internet sites. The excellent site at http://tbone.biol.sc.edu/tide/sites_uswest.html covers the West Coast.

To figure the tides at a given location, you must take into account differences in height and time from the nearest reference point.

For example, Redwood City's tides are different from but based on the Golden Gate Bridge's. At Redwood City, the high tide
occurs 58 minutes after high tide at the Golden Gate and is 2.2 feet higher. The low tide at Redwood City is 92 minutes after the low at the Golden Gate and is 0.1 feet higher. Be sure to check whether the tide table uses daylight-saving time. If it does not, add one hour to the table's times when daylight-saving time is in effect.

Q. Are condensation trails an index of increasing moisture aloft? Does the angle of the condensation trail compared to the ground track give an index of wind speed? Emil Kissel, science teacher - S.V. Carden School, San Jose

A. The presence of contrails does show the presence of upper-level moisture. This happens when the temperature is close to the dew point at which condensation would occur and the additional moisture from a jet's exhaust is enough to cause saturation. I don't think the angle compared to the ground track is enough (or easy enough to measure) to determine the speed. However, if you look at the horizontal drift of the contrail relative to a fixed object, you can do the geometry and calculate wind speed. It would be best to track a single cloud element of the contrail and see how far it moved. You would also need to know the altitude of the aircraft, but an estimate of about 33,000 feet would probably cover most cases. It sounds like a good class project. Good luck.

Q. When the weather forecasts call for a percentage chance of rain -- say 40 percent -- what does that relate to -- time, area or chance? Jim Brunton - San Jose

A. The probabilities of precipitation are among the most misunderstood elements in weather forecasts. The idea of percentages is to provide additional information to make a risk-benefit decision. A high probability of rain might help you decide to order tents for an outdoor wedding or to reschedule soccer practice.  How you use the information depends on whether the forecast is for widespread rain or only localized showers.

If the outlook is for rain, the probability is what the forecaster thinks is the chance of getting measurable precipitation -- at least 0.01 inch -- at any point in the forecast area. The forecaster will use computer models as well as professional experience to decide how often rain will occur under similar meteorological conditions. For example, if an approaching weather system is weak, the forecaster might expect rain only four times out of 10 from similar systems and give the probability of precipitation as 40 percent.

But when the outlook is for showery weather instead of widespread rain, a 40 percent chance would mean that measurable precipitation is expected over 40 percent of the area.

Weather Extremes: San Jose's all-time record-low temperature of 17 degrees Fahrenheit occurred Jan. 9 and 10, 1920. However, this is almost balmy compared with the national record low of minus 80 degrees in Prospect Creek, Alaska, and the world record of minus 129 at Vostok, Antarctica. Brrr!

Jan Null, founder of Golden Gate Weather Services and Director of Meteorology for Planetweather.com, 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 e-mail questions to weathercorner@ggweather.com or telephone and fax them at (510) 657-2246.