An important characteristic of the Earth's atmosphere is its airpressure, which determines wind and weather patterns across the globe. Gravity exerts a pull on the planet's atmosphere just as it keeps us tethered to its surface. This gravitational force causes the atmosphere to push against everything it surrounds, the pressure rising and falling as Earth turns.
What Is Air Pressure?
By definition, atmospheric or air pressure is the force per unit of area exerted on the Earth’s surface by the weight of the air above the surface. The force exerted by an air mass is created by the molecules that make it up and their size, motion, and number present in the air. These factors are important because they determine the temperature and density of the air and, thus, its pressure.
The number of air molecules above a surface determines air pressure. As the number of molecules increases, they exert more pressure on a surface, and the total atmospheric pressure increases. By contrast, if the number of molecules decreases, so too does the air pressure.
How Do You Measure It?
Air pressure is measured with mercury or aneroid barometers. Mercury barometers measure the height of a mercury column in a vertical glass tube. As air pressure changes, the height of the mercury column does as well, much like a thermometer. Meteorologists measure air pressure in units called atmospheres (atm). One atmosphere is equal to 1,013 millibars (MB) at sea level, which translates into 760 millimeters of quicksilver when measured on a mercury barometer.
An aneroid barometer uses a coil of tubing, with most of the air removed. The coil then bends inward when pressure rises and bows out when pressure drops. Aneroid barometers use the same units of measurement and produce the same readings as mercury barometers, but they don't contain any of the element.
Air pressure is not uniform across the planet, however. The normal range of the Earth's air pressure is from 970 MBto 1,050 MB. These differences are the result of low and high air pressure systems, which are caused by unequal heating across the Earth's surface and thepressure gradient force.
The highest barometric pressure on record was 1,083.8MB(adjusted to sea level), measured in Agata, Siberia, on December 31, 1968. The lowest pressure ever measured was 870MB, recorded as Typhoon Tip struck the western Pacific Ocean on October 12, 1979.
Low-Pressure Systems
A low-pressure system, also called a depression, is an area where the atmospheric pressure is lower than that of the area surrounding it. Lows are usually associated with high winds, warm air, and atmospheric lifting. Under these conditions, lows normally produce clouds, precipitation, and other turbulent weather, such as tropical storms and cyclones.
Areas prone to low pressure do not have extreme diurnal (day versus night) nor extreme seasonal temperatures because the clouds present over such areas reflect incoming solar radiation back into the atmosphere. As a result,they cannot warm as much during the day (or in the summer), and at night, they act as a blanket, trapping heat below.
High-Pressure Systems
Ahigh-pressure system, sometimes called an anticyclone, is an area where the atmospheric pressure is greater than that of the surrounding area. These systems move clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere due to the Coriolis Effect.
High-pressure areas are normally caused by a phenomenon called subsidence, meaning that as the air in the highs cools, it becomes denser and moves toward the ground. Pressure increases here because more air fills the space left from the low. Subsidence also evaporates most of the atmosphere's water vapor, so high-pressure systems are usually associated with clear skies and calm weather.
Unlike areas of low pressure, the absence of clouds means that areas prone to high-pressure experience extremes in diurnal and seasonal temperatures since there are no clouds to block incoming solar radiation or trap outgoing longwave radiation at night.
Atmospheric Regions
Across the globe, there are several regionswhere the air pressure is remarkably consistent. This can result in extremely predictable weather patterns in regions like the tropics or the poles.
- Equatorial low-pressure trough: This area is in the Earth's equatorial region (0 to 10 degrees north and south) and is composed of warm, light, ascending, and converging air. Because the converging air is wet and full of excess energy, it expands and cools as it rises, creating the clouds and heavy rainfall that are prominent throughout the area. This low-pressure zone trough also forms the Inter-Tropical Convergence Zone(ITCZ) and trade winds.
- Subtropical high-pressure cells: Located at 30 degrees north/south, this is a zone of hot, dry air that forms as the warm air descending from the tropics becomes hotter. Because hot air can hold more water vapor, it is relatively dry. The heavy rain along the equator also removes most of the excess moisture. The dominant winds in the subtropical high are called westerlies.
- Subpolar low-pressure cells: This area is at 60 degrees north/south latitude and features cool, wet weather. The Subpolar low is caused by the meeting of cold air masses from higher latitudes and warmer air masses from lower latitudes. In the northern hemisphere, their meeting forms the polar front, which produces the low-pressure cyclonic storms responsible for precipitation in the Pacific Northwest and much of Europe. In the southern hemisphere, severe storms develop along these fronts and cause high winds and snowfall in Antarctica.
- Polar high-pressure cells: These are located at 90 degrees north/south and are extremely cold and dry. With these systems, winds move away from the poles in an anticyclone, which descends and diverges to form the polar easterlies. They are weak, however, because little energy is available in the poles to make the systems strong. The Antarctic high is stronger, though, because it is able to form over the cold landmass instead of the warmer sea.
By studying these highs and lows, scientists are better able to understand the Earth's circulation patterns and predict the weather for use in daily life, navigation, shipping, and other important activities, making air pressure an important component to meteorology and other atmospheric science.
Additional References
- “Atmospheric Pressure.”National Geographic Society,
- “Weather Systems & Patterns.”Weather Systems & Patterns | National Oceanic and Atmospheric Administration,
