Barometers and Barometric Pressure

Barometric pressure (also known as atmospheric pressure) is the weight of air in the atmosphere above as a pressure. The name barometric pressure comes from the instrument most commonly used for measuring atmospheric pressure: a barometer.

There are two main types of barometer; the most widely available and reliable type is the ‘Mercury Barometer’, or the newer, digital friendly version is called an ‘Aneroid Barometer’.

How does a Barometer Work?

A mercury barometer uses the principal that the pressure of any points positioned on the same horizontal plane of a liquid are equal. This means that if you drew a horizontal line through a glass of water (or any liquid), the pressure pushing on the line would be the same at both ends of the line, but if you drew a diagonal line, the pressure pushing on each end of the line would be different – higher at the top and lower at the bottom.

The mercury barometer usually consists of a glass tube about 3 feet long and filled with mercury that is inverted into another container, which also contains mercury, called a reservoir. When the glass tube is inverted, it causes the mercury level in the tube to fall and create a vacuum at the top.

Because there is a vacuum in the glass tube but not in the reservoir, the amount of pressure in the air pushing down against the reservoir balances the weight of the mercury in the glass tube. However, because the pressures of any two points on the same horizontal plane must be equal (as explained above), the mercury must move up or down inside the glass tube to become equal to the pressure in the reservoir.

Low barometric pressure places a low amount of force on the reservoir, so the mercury in the glass tube will drop to a lower level. In contrast, high barometric pressure places more force on the reservoir, and results in the mercury in the glass tube to rising to a higher level.

Modern Technology and Barometric Pressure

The majority of modern weather instruments – such as weather stations can measure barometric pressure, but use a digital barometer (or aneroid barometer) that uses electrical charges to measure the air pressure. This enables them to take multiple accurate recordings of the pressure and produce more accurate weather forecasts.

How does the Atmospheric Pressure relate to the Weather?

The atmospheric pressure has such a significant impact on the weather that it is one of the most common measurements used to forecast the weather. You will have seen weather maps that indicate highs and lows over an area – these are the same high and low pressure indicated by a barometer.

By carefully watching the pressure on a barometer, you can forecast local weather using these simple guidelines. As a general rule, decreasing barometric pressure indicates storms, rain and windy weather, and rising barometric pressure indicates good, dry, and colder weather.

For more in-depth tips on how to use barometric pressure to forecast the weather in your area, visit our Glossary .

How else does Barometric Pressure effect us?

Research has indicated that changes in the air pressure may affect mental activity in some people. It has been attributed to causing changes in concentration, reaction times and short term memory. There are also many people, particularly those with arthritis, who believe that these changes in pressure are the cause of their aches and pains. Click here to read more about the weather and its effects on human health.

Barometers are a great tool for forecasting your local weather and also make a great learning tool for teaching kids about the weather – and they also make a great gift! Check out our Weather Station Store to find more great tools to help you forecast and monitor your neighbourhood weather.