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the cell to expand or contract. This expansion and contraction drives mechanical levers such that the
tiny movements of the capsule are amplified and displayed on the face of the aneroid barometer. Many
models include a manually set needle which is used to mark the current measurement so a change can
be seen. It was invented by Blaise Pascal.
4.2
Reading the barometer
It is highly advisable to lightly tap the glass near the center brass knob with your fingers before taking
a barometer reading. The light tap will overcome any friction that may affect accurate hand readings,
especially during periods of slow atmospheric changes. The Coast Guard has informed us that tapping
the barometer is even required on the most expensive aneroid barometers, because the mechanism is
made deliberately “stiff”.
The ability if the barometer to indicate changes in barometric pressure makes it a useful instrument in
weather forecasting.
The weather forecast or pressure tendency is based on the rate of change of barometric pressure. In
general, when the pressure increases, the weather improves (sunny to partly cloudy) and when the
pressure decreases, the weather degrades (cloudy to rain).
The weather forecast is an estimation or generalization of weather changes in the next 24 to 48 hours,
and varies from location to location. The tendency is simply a tool for projecting weather conditions
and is never to be relied upon as an accurate method to predict the weather.
The barometer includes a manually set needle, which is used to mark the current measurement so a
change can be seen. Barometric readings should be taken daily. Remember that the rate of change of
barometric pressure is important in determining weather changes. You may want to take multiple
readings each day during periods of unstable weather conditions.
The following basic rule of thumb will hold true in using the barometer to predict weather conditions.
•
A fast rise in barometric pressure means goo weather of short duration.
•
A rapid drop in barometric pressure means disturbances nearby, showers of short duration.
•
Regular elevation in barometric pressure usually will indicate a clear, dry weather conditions
(cold and dry in the winter).
•
A slow but continuous drop in barometric pressure will indicate persistent, bad weather.
•
Slow drops of 2-3 tenths mbar per 24 hours a depression of some distance away.
•
Drops of 1-2 tenths mbar per hour means disturbances nearby of short duration.
•
Steep drops of 6-10 tenths mbar within 4-5 hours period indicates coming rain and/or storm
with strong winds.
4.3
Absolute vs. Relative Pressure
To compare pressure conditions from one location to another, meteorologists correct pressure to
sea-level conditions. Because the air pressure decreases as you rise in altitude, the sea-level corrected
pressure (the pressure your location would be at if located at sea-level) is generally higher than your
measured pressure.
Thus, your absolute pressure (measured at your location) may read 28.62 inHg (969 mb) at an altitude
of 1000 feet (305 m), but the relative pressure (sea-level) is 30.00 inHg (1016 mb).
The standard sea-level pressure is 29.92 in Hg (1013 mb). This is the average sea-level pressure
around the world. Relative pressure measurements greater than 29.92 inHg (1013 mb) are
considered high pressure and relative pressure measurements less than 29.92 inHg are considered low
