The barometer is an instrument for measuring air pressure, or the weight of the atmosphere. On oar everyday barometers, the scale of degrees on the column of mercury is marked in inches. The specialized barometer used by the meteorologist has a scale in millibars. The standard air pressure at sea level is 29.92 inches or 1013.2 millibars. The T4B or millibar scale is based on the decimal system and gives a somewhat finer reading.

The 1TB scale is based on the bar, which is the weatherman's unit of air pressure. One bar is equal to the force exerted by one million dynes of air pressure on one square centimeter. This is the weight or pressure of a column of air reaching from one square centimeter at the earthts surface to the top of the atmosphere. Each millibar on the MB scale is equal to one thousandth part of the bar unit of air pressure.