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How Barometers Work

Barometers measure air pressure. Before the advent of satellite and mass communication, barometers were an essential device used to predict weather. They were especially important on ships where violent storms could mean life and death for sea voyagers.

Anyone who has watched the weatherman on the evening news has seen the maps with arrows trailing off a big "H" or "L". The "H" represents a high pressure center, while the "L" represents a low pressure center. The entire planet is surrounded in an atmosphere. The atmosphere is basically all the gasses that are held to the planet by gravity. All of these gasses are sort of stacked up on the surface of the Earth. At the bottom of this stack, at sea level, we find that a column of air weighs about 14.7 pounds. Or as scientists would say, it exerts 1013.25 millibars of pressure1.

This pressure is exerted in every direction, not just downward. When the sun shines on all of these gasses it stirs them us, actually making them spread out or become lighter. (This is how a hot air balloon is able to rise from the Earth's surface.) Where there is a column of warm air moving upward the air pressure goes down on the surface. Once this air begins to rise towards the upper atmosphere, it becomes cooler. It also becomes heavier. But it cannot come down directly on top of the warm air coming up beneath it, so it spreads out sideways, until it forms a column of heavier air (the high pressure center). The heavier air then pushes downwards. Meanwhile at the bottom of the column, the heavy air is pushing outwards and begins rushing into the place where the warm air was rising to take its place.

How Barometers Work Image

This whole cycle creates especially turbulent winds and weather near the low pressure center. With a barometer, then, a mariner can predict the coming weather by noting whether the air pressure, via the barometer is rising or falling. But how to measure this air pressure? It turns out that a very simple device invented by Evangelista Torricelli in 16432, could do the trick and the principles which guided it are still in use today.

The first barometer was basically an inverted tube of glass about half-filled with mercury, resting in a pool of mercury. When the air pressure increased, it forced the mercury up the tube. When it decreased, some mercury was let out of the tube and into the reservoir. The fluctuations could be tracked by a graded scale on a placard behind the glass tube. Many barometers today operate on the same principle, only there is a film or screen preventing the mercury or other medium from spilling from the reservoir should the barometer be tipped.

An aneroid barometer works on a similar principle. Only in this case there is a kind of bellows partially filled with a gas attached to a piece of metal that expands or contracts with the relative pressure. As the pressure increases the metal needle rises. As it decreases, the needle falls.

Reading a Barometer

The actual readings of a barometer are based on the inches that a column of mercury will be set in a vacuum tube at sea level. The average is about 30 inches. The actual change in air pressure (often called barometric pressure) will normally only vary about 1/2 an inch above or below this mark in the United States3. Both the speed and direction of change in pressure are important in using a barometer to predict weather.

A sudden extreme drop indicates a short violent storm. A long slow rise indicates consistently improving weather. Other weather changes can be interpolated from these extremes. Actual readings will likely be different based on altitude as this does have an effect on air pressure. Figure about .055 inches difference for every 50 feet above sea level. Temperature can also be a factor.

  1. University of Illinois on Air Pressure
  2. USA Today on Barometers
  3. Charles Edwin on Reading a Barometer

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