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Monitoring the weatherMonitoring air
pressure





Mercury barometer

The aneroid barometer consists of a closed capsule
with flexible sides. Any change in pressure alters the thickness of
the capsule. Levers magnify these changes and cause a pointer to
move on a dial, or numbers to change on a digital readout.

The pressure of the atmosphere at the surface of the earth will balance
the weight of a column of liquid. Mercury is favoured because it is so
heavy compared with other liquids, and the barometer tube can be
relatively short - less than a metre.
Mercury-in-glass barometers, being expensive, cumbersome and difficult
to transport, have largely been replaced by 'aneroid' barometers. These
consist of a flexible metal cell or bellows from which some air has been
evacuated. When the air pressure rises it presses the sides of the cell
together, and when the pressure falls the cell opens up again. The cell is
connected to a pointer on a dial. It can also operate a pen that moves
across a graduated paper chart wound around a drum for either 24-hour or
7-day rotation: this is a barograph.
(below) Aneroid barometer








 








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