Everyday Math

Math and the Outdoors

What do the numbers on an aneroid barometer signify?

The non-liquid aneroid barometer measures air pressure. As the atmospheric pressure changes because of storm systems (or lack of storms), the instrument records the changes. (Because atmospheric pressure changes with distance above or below sea level, a barometer can also be used to measure altitude.) The aneroid barometer contains a small capsule that acts like a bellows, but with the air removed. When the air pressure increases, the sides of the capsule are pushed in and the connected needle rises (moves clockwise). When the air pressure decreases or falls, the capsule’s sides puff out and the needle moves in a counterclockwise direction.

The numbers on a common barometer range from about 26 to 31, with divisions of 10 or more in between each number. A needle (actually a hand similar to a clock hand) points to the numbers on the barometer and moves in response to the changing air pressure. These numbers are based on the principle that atmospheric pressure supports 30 inches (76.2 centimeters) of liquid mercury in a tube with one end sealed; and this information is based on the mercury barometer, the first type ever made.

How is an aneroid barometer read? In general, a falling hand on a barometer indicates a low pressure system is on the way with poor weather (usually a storm with snow or rain); a steady barometer means there will be no changes with the ongoing pressure system; and a rising barometer means high pressure and fair weather; an even higher reading, around 31, means an extremely dry atmosphere. The timing of the barometric change is also telling: A change of a degree either way in a few hours means that the weather will change quickly; a slow change of 0.3 or so a day indicates weather arriving in 12 to 24 hours. A quick rise in the barometer also often indicates high winds and unsettled weather.

All barometers work because of our weather systems: Changes in air pressure are caused by differences in air temperature. And this, in turn, creates the wind and weather patterns that carry the high and low pressure systems around the Earth’s lower atmosphere.


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