Atmospheric pressure is measured by a barometer. A mercury barometer measures the pressure by noting the length of mercury which is supported by the weight of the atmosphere. One centimetre of mercury is equal to 13.33 mb, so normal atmospheric pressure can support a column of mercury about 75 cm (or 30 inches) high. An aneroid barometer is a more compact instrument for measuring pressure. It consists of a box of partially exhausted air which expands and contracts as the pressure falls and rises. The box is connected through a system of levers to a pointer which, in conjunction with a dial, indicates the pressure.

Air blows from regions of high atmosphere pressure ("highs" or anticyclones) to regions of low atmospheric pressure. In a high-pressure system, air pressure is greater than the surrounding areas. This difference in air pressure results in wind, or moving air. In a high-pressure area, air is denser than in areas of lower pressure. The result is that air will move from the high-pressure area to an area of lower density, or lower pressure. Conversely, winds tend to blow into low-pressure areas because air moves from areas of higher pressure into areas of lower pressure. As winds blow into a low, the air can be uplifted. This uplift of air can lead to the development of a depression with clouds and rain.