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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.

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Figure 6

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