Weather Journal

Tap That Barometer

Tap, Tap, Tap. Let's see. The atmospheric pressure is currently 1014 millibars. That's up from this morning so we should be in for improving weather conditions.

Have you every wondered why you need to tap the barometer before taking a reading? The answer is friction. to see why, let's look more closely at such a barometer.

The one hanging on my wall is a traditional aneroid barometer. Aneroid means without fluid and refers to the detection cell in these barometers. The cell, known as a sylphon cell is a hollow capsule made of thin corrugated metal from which the air has been removed.

When the air pressure outside the cell changes, the cell surface bends inward or outward depending on how the air pressure is changing. This motion is transmitted to the scale dial of the barometer by a series of springs, levers and gears that amplify the changes in the sylphon cell so that they can be easily seen.

Depending on the construction of the barometer, there will be a number of linkage points where friction can resist the mechanical movement. When we tap the barometer, we release the built- up frictional resistance, thereby allowing the indicator dial to show the air pressure change.

There is another type of barometer used in weather offices and research facilities: the mercury barometer. This type does not have this series of mechanical points to store friction. But they too must be gently tapped to overcome the "sticking" of the mercury at the top of the column to the glass tube. By tapping the barometer tube, a proper meniscus is formed by the mercury.

So next time you walk by, give that barometer a tap.

Learn More From These Relevant Books
Chosen by The Weather Doctor

• Williams, Jack: The Weather Book, 1997, Vintage Books, ISBN 0-679-77665-6.
• Burt, Christopher C.: Extreme Weather: A Guide and Record Book, 2004 (pb), W. W. Norton & Company, ISBN 039333015X.

Written by
Keith C. Heidorn, PhD, THE WEATHER DOCTOR,
June 1, 2000

The Weather Doctor's Journal Tap That Barometer
©2000, Keith C. Heidorn, PhD. All Rights Reserved.
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