Weather Notebook
Bryan Yeaton

29.92 Inches of Mercury
Mon Mar 14, 2005

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When your local meteorologist says that the barometer reads 29.92 inches of mercury, just what is he or she talking about?

Hi! I'm Bryan Yeaton and this is The Weather Notebook.

First of all, you have to realize that—even though you can’t feel it—air has weight. Depending on how tightly packed the air molecules are, the air can be heavier or lighter, or as you have more likely heard, have higher or lower pressure.

We measure changes in air pressure with a barometer, which is basically a glass tube about a yard long, partly filled with mercury. It is sealed on top, but the bottom is open, and that end is stuck in a cup of more mercury. Heavier air will push down on the open container, forcing mercury up the tube. Thus, a rising barometer shows higher pressure. A ruler attached to the barometer shows the level of the mercury, which can be represented in inches, millibars, or any other measurement. Twenty-nine point nine two happens to be the average air pressure at sea level (as is 1013 millibars—they are exactly the same reading). Heavier air—higher pressure, usually means drier, sinking air and nicer weather, while low pressure often brings storms.

So, you may wonder who figured all this out. His name was Evangelista Torricelli, a student of Galileo. By 1643, he had found that the weight of our atmosphere—all 70 miles of it-- could support a column of water in a cylinder, on average, 33 feet high. However, these tubes were pretty difficult to cart around, so Torricelli substituted mercury, a heavy, liquid metal, much denser than water.

The Weather Notebook is underwritten by Subaru, and is a production of the Mount Washington Observatory, where the average air pressure is only 23.66 inches of mercury.

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