How to Understand Barometers
Changes in the barometer mean changes in the weather are coming. Most people look at the barometer and know that if the barometer is falling the weather is changing, because the face of the barometer has the words "Fair," "Change" and Stormy," or a similar legend, emblazoned upon it in an arc that spans the face of the instrument. If the barometer's needle did move as far as the face of the barometer's words say, the weather would be turning for a terrific storm indeed, but the small movements are the ones which tell the tale to a knowing sailor.
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Instructions
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Turn the knob in the center of the face of the barometer until the pointer is over the barometer's indicator hand. While knowing that the average barometric pressure at sea level is 29.92 inches of mercury (barometric pressure is expressed in "inches") may impress your sailing companions, it isn't necessarily useful in predicting weather changes unless you are at sea level.
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Record the pressure that the barometer's indicator hand is pointing to. Although you set the pointer at that number, the pointer may--and often, is--accidentally moved by a curious companion. Even if it isn't, keeping a written record means that you won't have to refer to both the barometer's pointer and your barometer log.
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Record the barometer's reading once every 3 hours. Each of the smallest markings on the face of a "working" barometer--as opposed to a decorative barometer--will equal 1/100 of an inch of change in barometric pressure. Note that a change of as much as 3/100 inch may occur at sunrise and sunset and may be disregarded if other weather indicators do not worsen.
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Keep a "weather eye" on the barometer, even if you don't record its reading. A falling barometer--a change greater than 1/10 inch over 3 hours at a time other than sunrise or sunset indicates that the trailing edge of a high pressure front is passing and being replaced by the leading edge of a low pressure front. A barometer rising at the same rate indicates that a high pressure area is settling in, and the wind and clouds should be observed for changes.
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Use the barometer in concert with other weather aids and skills, including weather radio, wind and cloud types and conditions, sky color and wind direction to make the best estimate of weather. Changes in the barometric pressure may not mean that the weather is about to change drastically, but a barometer either rising or falling more than 35/100 inch, coupled with a backing wind (one which moves counterclockwise around the compass) is generally an indication of a significant change from the current conditions.
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Tips & Warnings
The U. S. Coast Guard Auxiliary in your area offers boating safety and weather courses that are well worth the time and effort.
Where weather is concerned, never trust your safety to any single instrument. Learn to use them all, including your eyes, ears, nose and brain.
