Second oldest working barometer in the US

2011-08-20 21:00:27.000 – Roger Pushor,  Weather Observer/IT Specialist

Second oldest working barometer in the US

A barometer is a scientific instrument used in meteorology to measure atmospheric pressure and only at the Mount Washington Observatory will you find the second oldest working mercury barometer in the United States with the oldest one being at the Blue Hill Observatory and Science Center located at the summit of Great Blue on the Blue Hills Reservation.

The mercury barometer continues to be used at the Mount Washington Observatory because of its reliability, simplicity and accuracy. The mercury barometer is an instrument that can work without any power source. At the Observatory we use this instrument to calibrate the barograph which is a recording aneroid barometer allowing us to see air pressure trends or ‘tendency’ as it produces a paper graph. Pressure tendency can forecast short term changes in the weather. Numerous measurements of air pressure are used within surface weather analysis to help find frontal boundaries, surface troughs and high pressure systems.

The first barometers were developed in the mid 1600 hundreds with Evangelista Torricelli being universally credited with inventing the first barometer.

A mercury barometer has a glass tube of approximately 34 inches in height, closed at one end, with an open mercury-filled reservoir at the base. The weight of the mercury creates a vacuum in the top of the tube. Mercury in the tube adjusts until the weight of the mercury column balances the atmospheric force exerted on the reservoir. High atmospheric pressure places more force on the reservoir, forcing mercury higher in the column. Low pressure allows the mercury to drop to a lower level in the column by lowering the force placed on the reservoir. Since higher temperature at the instrument will reduce the mercury’s density, when reading the height of the mercury must be adjusted to compensate for this effect.

The mercury barometer’s design gives rise to the expression of atmospheric pressure in inches of mercury. The pressure is quoted as the level of the mercury’s height in the vertical column. 1 atmosphere is equivalent to about 29.92 inches of mercury at sea level. Here at the summit at 6288 feet above sea level we experience pressures in the range of 23-24 inches of mercury.

As one of the Observer / IT Specialists at MWO I enjoy latest in digital technology and have a great appreciation of the older analog technologies.


Roger Pushor,  Weather Observer/IT Specialist

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