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How do barometers work?
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Dear
Weather Guy,
How do barometers work? Why do
meteorologists use barometers?
What do barometers do? When were
barometers first invented? How
do you make a model of a
barometer? How accurate are
barometers?
 Surface
air pressure can be measured by
a device called a barometer. A
barometer indicates pressure in
units known as millibars (mbs)
or inches of mercury. Without
getting into the mechanics of
how barometers operate, it's
safe to say that barometers are
the standard for measuring
surface air pressure for weather
purposes. If you live at sea
level near the equator, standard
'normal' sea level air pressure
would be about 1000 mbs. If a
relatively strong high-pressure
system were to move over your
home, the barometer might
indicate the pressure at around
1020 mbs. Conversely, if a
strong low-pressure system were
to move in, the barometer might
register about 970 mbs. (Surface
air pressure tends to be less as
you travel north or go up in
elevation). Of note, both high
and low pressure systems
typically cover at least many
hundreds of miles in area, and
move rather slowly. So you
typically won’t see a change in
pressure from one extreme to the
other in a matter of hours, but
more over a time periods of days
or weeks. And it's highly
doubtful you'll feel the change
in air pressure, but you'll
certainly see the weather
changes associated with pressure
change. Under high pressure,
weather is typically fair, skies
cloudless. But under low
pressure, rain, wind and
inclimate conditions prevail.
The greater the difference in
air pressure between two
competing systems, the greater
the wind. And the greater the
surface area covered by low
pressure, the greater the size
of the area of wind will be.
Since strong wind is typically
associated with storms, and
since storms are characterized
by low pressure, surf
forecasters typically track and
look for developing areas of low
pressure, preferably large ones.
The deeper or lower the
pressure, the stronger the
storm, the stronger the wind,
and the greater the probability
for surf to be generated. The
larger the area of low pressure,
the larger the fetch or ocean
surface that will be affected by
the resulting wind. Of course,
to generate a large swell, the
three requirements we discussed
earlier must be met: High wind
velocity, large fetch area, and
long lasting winds. The ideal
big wave set-up is to have a
massive deep storm surrounded by
extreme high pressure. And the
systems must be positioned so
that the fetch is aimed towards
your beach for a long time. In
reality, this rarely happens,
but rather you get either a
massive storm surrounded by
relatively normal conditions, or
a moderate storm surrounded by
strong high pressure for a day
or two at best.
Make a Homemade
barometer
The air pressure around us
greatly affects our weather.
Notice how your weatherman
always mentions various pressure
systems (low pressure system,
high pressure system, etc.) and
how they will affect tomorrow's
weather. In this experiment, we
will create a tool that lets you
gauge the pressure of the air
around you.
1. Fill the measuring cup or
glass with water and add some
colored dye to it.
2. Flip the empty soda bottle
upside down into the glass
measuring cup.
3. Assure that you use a bottle
that is just the right size. It
should fit snugly in the
measuring cup so that the mouth
of the bottle does not touch the
bottom of the cup.
4. Assure that the level of the
water extends into the neck of
the bottle.
5. Mark a line on the cup to
indicate the water level within
the bottle.
6. Re-examine the bottle in a
few days.
Notice the change in the water
level? The amount of air within
the bottle is fixed and cannot
change since the water extended
into the bottle acts as a
'plug'. Hence, you can consider
the amount of air trapped in the
bottle as an indicator of the
air pressure on the day you
plugged the bottle. The pressure
on the surface of the water
depends on the current air
pressure. When the air pressure
increases (as it does in drier
weather), the pressure on the
surface of the water is greater
and the water is forced up into
the bottle changing the level of
the water. If you see the water
level drop, look for your
umbrella...
------------------------------------
George H. Taylor, CCM
State Climatologist, Oregon
Oregon Climate Service
316 Strand Hall
Oregon State University
Corvallis OR 97331
http://www.ocs.orst.edu
Source:
seps.org
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