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 Changes
in atmospheric pressure are probably the most important indications
of changes in the weather and, in particular, the rate of
change is critical to sailors as the pressure gradient determines
the strength of the wind.
It is extremely important
when navigating in areas which are prone to tropical revolving
storms like the Caribbean and the China Sea where a definite,
unusually steep and regular fall of pressure is an indication
of the approach of such a storm.
For meteorological
purposes atmospheric pressure is measured in inches or millibars.
In a traditional barometer it will support about thirty inches
or roughly seven hundred and fifty millimetres of mercury
which for ease of use is referred to as one thousand millibars.
Depending on weather
conditions, the pressure will vary either side of 1000 millibars
(29.53 inches). It also varies with height and latitude.
 Most
domestic barometers are of the aneroid type invented in 1843
by Lucien Vidie, a French scientist.
These consist of corrugated
capsules evacuated of air with the faces kept apart by springs
or the rigidity of the metal.
Changes in pressure
cause the capsules to expand and contract and movement is
recorded by a needle. The instrument is calibrated by using
an adjustment screw usually found on the back of its case.
 A
sister to the barometer is the barograph which is an instrument
that connects a barometer to a pen which records the pressure
on a chart fixed to a cylinder which revolves once in a week.
A trace of pressure is recorded across the chart which to
many people is more valuable than a single instantaneous reading.
Lines joining places
of equal pressure on weather charts are called isobars from
the Greek Iso, meaning equal. Synoptic charts which show atmospheric
pressure at sea level are included in television forecasts
and in many national and local newspapers.
Related link
Setting a Barometer
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