How to Compare a Barometer Reading

In normal conditions, a barometer reading will be high when the air is dry and mild and low when the air is warm and wet. A drop in pressure is thought to indicate a coming storm, and the faster the drop, the nearer the storm. A consistent reading indicates consistent weather.

Types

Mercury Barometers measure atmospheric pressure by the amount of mercury pushed through a glass column, and measurements are taken in millimeters. They are the most accurate barometers and are used by a majority of professional weather stations. Aneroid barometers, which are easier to read, gauge atmospheric pressure by the expansion and contraction of an evacuated metal chamber. Measurements for aneroid devices are taken in millibars, also known as hectopascals.

Adjustments

For both types of barometers, adjustments may need to be made to compensate for altitude, temperature and latitude at the time of the reading. To see if your barometer needs calibration, compare your readings to those of the National Weather Service, available on the radio, online, on TV and in most newspapers.

Comparing

Standard surface pressure for sea level is 1,013.2 millibars. As a conversion aid, 1 millibar is equal to 0.7501 millimeters of mercury and represents 100 newtons per square meter of atmospheric pressure. Pressures above the standard after adjustments are called high, and those below are called low.

References

• Colby: Barometer
• Australian Government Bureau of Meteorology: The Aneroid Barometer and How To Use It
• University of Wisconsin Stevens Point: Air Pressure: Pressure Variations Across the Earth
• The Weather Channel: Weather Glossary: Millibar