How Is Atmospheric Pressure Measured?

Measuring Instruments

Atmospheric pressure is determined via the use of a barometer. Barometers are either mercury-filled or aneroid. The mercury variety consists of a tube, the top of which is closed and the bottom open. The tube is generally labeled with units of measurement. The bottom is placed in an open container of mercury, allowing the liquid to rise and fall with the ambient air pressure. Mercury in liquid form is heavy, making for small changes in height; thus, a reasonably sized tube can be used.

Aneroid barometers function using a depressurized metal box, often referred to as an aneroid cell. This cell has a spring enclosed within it. As pressure rises, the cell expands, as it lowers, the cell contracts. Most modern types have a lever connected to them that points to the corresponding unit of measurement on the face of the instrument. A barograph aneroid model will have a stack of cells connected to an arm with a pen on the end that makes a line on a sheet of paper that rotates underneath it. This results in a record of pressure changes with time.

The unit most often used by meteorologists is the millibar(mb). Particularly for reporting in the U.S., atmospheric pressure is shown as inches of mercury (inHg). The basis for this unit is that a change of 1 inch corresponds to mercury rising or falling an inch within the tube. Kilopascals (kPa) are sometimes used by scientists. These are millibars divided by 10. An increase in elevation often means a decrease in pressure. This demands a conversion to standard sea-level pressure before meaningful meteorological comparisons can be made between two stations at different elevations. The National Weather Service points out that standard sea-level pressure is 29.92inHg, or 1013.2mb's.

Upper air charts show atmospheric pressure in terms of how far up from the surface a particular millibar level is. Generally, warmer air aloft means lower pressure and higher heights, and vice versa. Lines connecting these height levels are referred to as isohypses. A useful depiction for meteorologists is the 500mb chart, which portrays isohypses of 500mb. The average height of the 500mb level is about 5.5km from the surface. The primary manner in which these levels are obtained is through the use of weather balloons with radiosondes. Radiosondes are containers filled with weather sensors attached to the balloons that transmit data to a receiving station. Barometers are one of the sensors aboard these containers. Upper air pressure measurements can also be obtained by placing barometers on aircraft, or they can be inferred from temperature data obtained by satellite.