A barometer measures the air pressure. The air pressure is a function of how much air is pressing over an area. Air pressure will change rapidly with a change in altitude. Because of this rapid change with altitude, reported pressure is usually adjusted to sea level. Once all locations are adjusted to sea level then the air pressure can be compared between two places that even have different altitudes. Maps that show pressure adjusted to sea level are called isobaric maps. The isobars connect points of equal pressure. Where the isobars are close together the wind is stronger.

The English units of air pressure are inches of mercury. The metric version is millibars. The average sea level pressure is 29.92 inches of mercury and 1013 millibars. Using these average values it can be determined whether the pressure is above, significantly above, below or significantly below the average value. A few sea level pressure benchmark values follow:

1086 mb (32.08 inches of mercury): Highest Ever Recorded
1030 mb (30.42 inches of mercury): Strong High Pressure System
1013 mb (29.92 inches of mercury): Average Sea Level Pressure
1000 mb (29.54 inches of mercury): Typical Low Pressure System
980 mb (28.95 inches of mercury): CAT 1 Hurricane or a very intense mid-latitude cyclone
950 mb (28.06 inches of mercury): CAT 3 Hurricane
870 mb (25.70 inches of mercury): Lowest Ever Recorded (not including tornadoes)

The change in air pressure over time has important forecasting implications. As pressure lowers over time, especially if it is rapid, that is an indication that a low pressure system or front is approaching. This lowering pressure indicates an increasing likeliness of precipitation. If the air pressure rises significantly or stays well above average for a long period of time, that is an indication precipitation is less likely.

Much of the general public does not understand how air pressure is used in forecasting. Thus it is important to point out when appropriate the importance of air pressure changes over time and the relationship between air pressure and whether a high pressure or a low pressure system is influencing the area. The general public does have a good handle of understanding that low pressure systems tend to bring in precipitation and high pressure systems tend to bring fair weather. Thus, if you can relate the air pressure value to a pressure system then the general public may grasp the significance of the air pressure value especially if they have an understanding of benchmark values.