Barometers and barographs: weather office tour

Did you know that air has weight? The atmosphere presses down on the surface of the Earth, and a barometer measures how much the atmosphere is pressing down at that location. Although this is a weather instrument, it wasn't pictured in the instrument compound with the others because it is always installed indoors. Barographs are just barometers with pens attached to graph the changes in pressure over time. Changes in atmospheric pressure are one of the best tools a forecaster has to identify how weather systems are moving.

The 2 most common types of barometers are the liquid barometer and the aneroid barometer. (Oddly enough, the literal translation of aneroid is "not water".) Different fluids can be used in liquid barometers, such as mercury, alcohol, or even water. They operate the same way, regardless of the type of fluid.

Liquid barometer

Liquid barometers all have a column of fluid connected to a reservoir of the same fluid. Either the column or the reservoir will be open to the air, but not both. If the reservoir is open to the air, then increasing air pressure will push down on the surface of the fluid, forcing it up higher in the column (meaning higher pressure). Decreasing air pressure will allow fluid to drain back out of the column into the reservoir so that the level in the column is lower (meaning lower pressure). If the column rather than the reservoir is open to the air, as in the model on the left called a Weather Glass, then it will work the opposite way. The column or spout is open to the air, and increasing air pressure will force the fluid lower in the the spout and back into the larger reservoir.

Many home barometers and some of those used by Environment Canada are the aneroid type. To make it easier to see the mechanism, let's look under the cover of an aneroid barograph like the one below left. The aneroid capsule or bellows looks like a corrugated tin can--you can see it at the bottom of the large spring. The bellows is empty of air (a vacuum ) and is only kept from collapsing by tension from the spring connected to its top.

Closeup of aneroid spring

When the atmospheric pressure increases, air will push down harder on the top of the bellows, causing it to collapse a little and stretch the spring. The spring, in turn, pulls the pen arm up higher, tracing a rising line on the chart. When the air pressure falls, the bellows expand and the spring shrinks, allowing the pen to drop back lower and draw a descending line on the chart. Some form of this same mechanism can be found inside most household barometers.

Aneroid barometer

Environment Canada uses primarily digital barometers now, with a display like the one on the left. An enlargement of the pressure reading is shown to the right. If one kilopascal (kPa) is equal to 10 hectopascals (hPa), can you tell what this reading would be in kPa? Ask your teacher if you're not sure.

Vaisala digital barometer Closeup of digital readout
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