|Published Tuesday, September 14, 1999, in the San Jose
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
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
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
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
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 firstname.lastname@example.org.