Barometers Make Forecasting the Weather a Breeze!
Take the mystery out of understanding "how weather works" with the instrument that measures air pressure. With simple readings from a barometer, anyone can learn to forecast upcoming conditions. Whether you seek this information as a pilot, navigator, or sportsman for safety; as a gardener, outdoor enthusiast, or professional for optimum performance; or as a person who values the benefits of being prepared, air-pressure measurements from a barometer provide critical clues on the weather patterns that affect your environment.

Learn More About Air Pressure and Barometers

Barometers measure atmospheric pressure, the most difficult weather element to sense without an instrument. There are two types of barometers used in meteorology: liquid (usually mercury) and aneroid (without liquid).

Barometric pressure is denoted in millibars, kilopascals, inches of mercury (Hg), or millimeters of Hg. In the United States, inches of Hg is the most common unit of measurement. To have a consistent system for comparison, pressure measurements are corrected to sea level before recording. Typically, pressure ranges from 29.0" to 30.5" Hg. In general, falling or low pressure indicates foul weather, rising or high pressure indicates fair weather.

In an era when forecasting the weather was an art relying on clues from nature, the water barometer was one of the few tools based on scientific principle. Also known as a "storm glass" or "thunder glass", this instrument dates back to early 17th Century Netherlands. Changes in air pressure cause the water to rise and fall in the spout. Low water level indicates high pressure and fair weather. The water rises (and may spill over) in severe weather as the pressure falls. Historians believe Pilgrims brought thunder glasses to the colonies in the 1620's.

The mercury barometer was invented by Torricelli, a student of Galileo, in 1643. He filled a glass tube (closed at one end) with mercury and inverted it into a small cistern also containing mercury. The mercury in the tube fell to a level where the weight of the air pressing down on the surface of the mercury in the cistern exactly balanced the weight of the mercury in the tube. This height, about 30 inches at sea level, is called barometric pressure.

The Frenchman Vidie, 200 years later, is credited with developing the aneroid barometer, a more compact and rugged instrument. It consisted of an evacuated metal diaphragm linked mechanically to an indicating needle. As atmospheric pressure increased or decreased, the diaphragm compressed or expanded, causing the indicating needle to show the change in pressure.

The modern electronic barometer contains a sensor with electrical properties (resistance or capacitance) that change as the atmospheric pressure changes. Additional electronic circuitry converts the sensor output into a digital display.