Under Pressure

2017-06-27 16:11:48.000 – Taylor Regan, Weather Observer

At the Mount Washington Observatory, there are quite a few instruments we employ during our hourly weather observations to help us interpret and report the conditions on the summit. The barograph is one such tool, which is used to trace out trends in the air pressure. This allows us to visually determine whether the pressure has been rising, falling, steady, or a variety of other combinations, which believe it or not, can be quite useful in monitoring the weather! Why monitor pressure? And what is a barograph exactly?

Air pressure is a useful variable to monitor because changes or trends can be indicative of incoming foul weather or clearing skies. For example, high pressure is indicative of typically light winds, and clearing skies which results from cold air descending to the surface. High pressure in general, is a welcome sign of a settled weather pattern. Conversely, when the pressure falls, it is generally a harbinger of inclement and stormy weather. When a low pressure system enters the region, air rises and cools, allowing water vapor to condense and form clouds and often precipitation.

To measure pressure, weather observers turn to the barometer. There are two main types of barometers: mercury and aneroid. A mercury barometer works by having a glass tube that is closed at the top, but open at the bottom, sit in a pool of mercury. The mercury in the column adjusts to compensate for the force of the atmosphere pressing down on the exposed pool of mercury. An increase in pressure forces the mercury to rise higher in the tube whereas a decrease in the atmospheric pressure causes the mercury to lower in the tube.

The mercury barometer was invented in 1643 by Evangelista Torricelli, after his initial pressure experiments (using a 35 foot tall column of water), led some neighbors to accuse him of practicing witchcraft. Because mercury weighs 14 times more than water, this allowed Torricelli to build a much more inconspicuous pressure measuring device, which became known as the mercury barometer, and is still in use today.

An aneroid barometer on the other hand, works without mercury (aneroid meaning without fluid). In 1844, a French scientist named Lucien Vidi was responsible for the development of the aneroid barometer. A sealed metal chamber, or a series of sealed metal chambers, expands and contract in response to changes in the surrounding atmospheric pressure. These barometers eventually replaced many mercury barometers, as they were much cheaper to produce, easier to use, and ultimately, easier to transport because there was no liquid in them.

All of this brings me to the barograph, which is a special implementation of an aneroid barometer. A barograph uses a particular series of gears and pivots to allow a needle, dipped in ink, to trace out pressure trends on a rotating cylinder to which a paper chart is mounted. The instrument is calibrated such that changes in the needle position correlate to specific changes in pressure, which are recorded over time on the graph paper fixed to the rotating drum. In most cases, the barograph records for a number of days, prior to the chart being removed and a new blank one replacing it. The line that is traced out on the graph paper is called the barogram.

Taylor Regan, Weather Observer