11.6 NOTES
How is air pressure measured?
Objective: Explain how a barometer measures air pressure
A mercury barometer is a glass tube filled
with mercury. One end of the tube is open.
The closed end of the tube forms a vacuum.
The open end of the tube sits in a container
of mercury. Air pressure pushes down on
the surface of the mercury pushing it up the
vacuum. Air pressure at sea level can raise
a column of mercury to a height of 760mm.
When air pressure changes, the level of
mercury in the tube will rise or fall.
Another kind of barometer
is called an aneroid
barometer. Aneroid means
“without liquid”. This
barometer is made of an
airtight metal container
with thin sides. When air
pressure increases, the
sides bend in. When air
pressure decreases, the
sides bend out. When the
sides bend, the pointer moves on a scale, measuring air pressure in millimeters of
mercury.
Air pressure at sea level is 760 mm, also called one atmosphere. Air pressure can
also be measured in millibars (mb). Standard air pressure is equal to 1,013.20 mb.
Altitude is measured with a
device called an altimeter. As
you go higher, air pressure
decreases. Pilots, scientists,
surveyors, and mountain
climbers use altimeters.
Mount Everest is the highest
point on Earth, standing at
about 8,900 meters high. The
mercury column here is only
about 250 mm high. Skydivers
and parachuters use aneroid
barometers to monitor their
altitude.