The pressure of living on Earth

Published Feb. 26, 2007
My barometer log records the passing of the remnants of Hurricane Katrina through northeast Ohio in August 2005. On Tuesday, Aug. 30, at 8:25 a.m., my barometer recorded a pressure of 29.82 inches, and I made a note that the rain had started at 8:10. By 5:53 p.m. the pressure had dropped to 29.57 inches and rain was falling steadily, at 8:37 p.m. pressure was 29.47 inches and rain was falling harder, and at 10:08 p.m. pressure was 29.40 inches. The next morning at 8:03 a.m. the pressure was at 29.43 inches but the had rain stopped, at 5:59 p.m. the pressure had risen to 29.72 inches and the sky was clearing, and at 10:50 p.m. the pressure was at 29.81 inches, just about where it was before Katrina arrived.
I started a barometer log after reading in two books on volcanoes — “The Day the World Ended,” about the eruption of Mont Pelee on Martinique, and “Krakatoa: The Day the World Exploded” by Simon Winchester — that atmospheric pressure spiked during eruptions. Winchester said the “newfangled habits of the late 19th century included increasingly precise means of forecasting weather,” and he mentioned people “faithfully tapping the glass each day.” (My barometer requires a light tap to make the needle jump to the proper reading.) Winchester described the recording barograph, a mechanized barometer with an ink trace on graph paper wrapped around a clockwork-driven drum, all encased in an elegant wooden case with glass sides. Barographs, priced beyond the means of most households, were found in private clubs and recorded the eruption of Krakatoa.
The barograph, writes Winchester, demonstrates that weather changes slowly and smoothly (my log does the same), but on Aug. 27, 1883, the pen jumped. A sudden rise was followed by two or three minor oscillations, a deep depression, a less steep rise, and more small oscillations. Barographs around the world showed that the shock wave circled the earth seven times.
The first barometer was built by Evangelista Torricelli, a former assistant to Galileo, with the help of Vincenzo Viviani in 1643, made of mercury in a glass tube. Torricelli determined that air exerts pressure by filling a glass tube with mercury, holding a finger over the open end and placing the open end in a vessel into which some of the mercury flowed. Air pressure kept most of the mercury in the tube, and the space at the top was a vacuum, the height of the mercury column indicating the varying pressure, measured in inches. Blaise Pascal, of Rouen, France, about 1647, performed experiments that convinced people of the correctness of Torricelli’s ideas, which at first were disputed because Aristotelian doctrine said that nature abhorred a vacuum.
Pressure, measured in force per area, is the product of the height of the column times the density of mercury times the force of gravity. The force of atmosphere is about 14.7 pounds per square inch or, in the metric system, 1,013,250 dynes per square centimeter or 101.3 kilopascals (kPa). A dyne is the force needed to accelerate one gram one centimeter per second squared (acceleration is measured in squared units of time), and the pascal is a metric unit of force named for the aforementioned Frenchman.
The World Meteorological Organization (www.wmo.ch) executive committee in 1953 adopted the millibar as the standard pressure unit. The millibar equals 1,000 dynes per square cm, and a column of mercury having a scale height of 760 mm yields a pressure of 1,013,250 dynes per square cm, or 1,013.25 mb. Barometers are marked in millibars, centimeters or inches, sometimes having two scales. The height measure is abbreviated mm Hg or cm Hg, the Hg standing for mercury, and mm Hg is also called a torr, after Torricelli. My barometer has inches and millimeters and is an aneroid barometer, containing no mercury but depending rather on the elasticity of a thin, flexible-walled vacuum capsule in combination with a stiff spring. The aneroid barometer was invented in the 1840s by Lucien Vidie.
I have recorded my barometer log in inches for two years, but I started recording both metric and English units this week after studying mapping and deciding to become more familiar with the metric system. I had been vaguely aware of that other set of numbers on the barometer but had mostly ignored it, but now I see the centimeter scale anew, as if it had just been added.
The pressure lines on weather maps are isobars, from Greek “isos,” equal, and “baros,” weight. By connecting points of equal atmospheric pressure they illustrate the location of high- and low-pressure systems. I can easily follow the movement of those systems, but what I really want to see is a volcano spike — the chances are slim, but I can always hope.

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