WEATHER CORNER

BY JAN NULL
Special to the Mercury News

QWhat's the deal with summer? First, it's now summer in the Northern
Hemisphere and our warmer season. But is it also summer in the Southern
Hemisphere? Do they have cold summers? Or do they have summer while
we have winter? And given that for the most part, summer weather
usually begins here by early May, why is the solstice called the ``first day
of summer?'' J.P. Thomas - Redwood City

AGood questions, and even I learned some new stuff here. Penny
Ingman, a friend from Australia, said they call their cold season ``winter''
and the warm season ``summer.'' So they celebrate Christmas right after
the beginning of summer. And autumn starts in March, while spring
begins in September.

The solstices and equinoxes are astronomical terms and are used more as
a matter of convenience than for meteorological accuracy. For most
northern hemisphere meteorological uses such as determining an average
summer temperature, the seasonal breakdown is: June, July and August
are summer; September, October and November are fall; December,
January and February are winter. That leaves March, April and May for
spring.

Q Why do hurricanes, toilets and bathroom drains swirl in opposite
directions in the two hemispheres? Aaron Koressel - Santa Cruz 

AThis is definitely true in the case of hurricanes, and other circulations
around areas of low pressure. Wind blows from high pressure toward
low pressure. Because of Earth's rotation, they veer to the right in the
northern hemisphere and to the left in the southern hemisphere.

This force is called the Coriolis force. This means that the circulation
around a hurricane is counterclockwise north of the equator, and
clockwise in the south. 

However, the circulation around drains and toilets is more of a function of
the design of the fixture. The Coriolis force is so small that it plays no
role in the rotational direction of a draining sink or toilet.

An excellent discussion of this topic is available at
http://www.ems.psu.edu/~fraser/Bad/BadCoriolis.html

Q I saw a Web page with atmospheric pressure, but what does the
word really mean? Danitra McPhadden  - Interlachen, Fla. 

AImagine a one-inch column of air directly overhead that extends
to an elevation of about 100,000 feet. Atmospheric pressure is
simply the weight of the air molecules in that column. The average
weight of that column, at sea level, is about 14.5 pounds per square
inch.

As you go to a higher elevation, there are more of the molecules
below and the pressure is less. At 18,000 feet about half the air
molecules are below, and the pressure is only a bit more than seven
pounds per square inch.

Instead of measuring the weight of the air, however, an instrument
called a barometer is used. This measures the force of the air on a
liquid and the distance it is displaced in a closed tube.

Mercury is the most commonly used liquid and has an average
displacement of 29.92 inches. Newer barometers use cylinders that
have a partial vacuum and expand and collapse with changes in
atmospheric pressure.

QI understand that high barometric pressure is usually associated
with fair weather, and low pressure with rainy weather. Because San
Jose is normally sunny during the summer, one would expect high
barometric pressure this time of year. My barometer at home,
however, typically records relatively low pressure -- below 30 inches
of mercury -- during the dry season.

And it reads fairly high -- above 30 inches -- in the winter until a
cold front approaches and produces or threatens rain. Can you
explain this apparent anomaly? Or do I need a new barometer? 
C.M.  -  San Jose 

AYou don't need a new barometer. It's just those pesky markings
of stormy or rain or fair that are often placed around the edges of
barometers. While there is often stormy weather with very low
readings and dry weather with very high pressure, the large range in
between is a barometric no-man's land. It's possible to have rain
with a pressure of 30.20 inches and dry weather with 29.75 inches.

The biggest influence locally is the temperature. If the air is heated
it becomes lighter and the pressure decreases. This is why, on hot
days such as we have recently experienced, we can see barometer
readings below 29.75 inches of mercury.

If you were to watch your barometer for 24 hours you would see it
drop as the temperature rises and then rise again overnight as it
cools. This is called the diurnal cycle.

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.