The Basics of Air Pressure

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Air pressure, atmospheric pressure, or barometric pressure, is the pressure exerted over a surface by the weight of an air mass (and its molecules) above it.

How Heavy Is Air?

Air pressure is a difficult concept. How can something invisible have mass and weight? Air has mass because it is made up of a mixture of gases that have mass. Add up the weight of all these gases that compose dry air (oxygen, nitrogen, carbon dioxide, hydrogen, and others) and you get the weight of dry air.

The molecular weight, or molar mass, of dry air is 28.97 grams per mole. While that isn't very much, a typical air mass is made up of an incredibly large number of air molecules. As such, you can begin to see how air can have considerable weight when the masses of all the molecules are added together.

High and Low Air Pressure

So what's the connection between molecules and air pressure? If the number of air molecules above an area increases, there are more molecules to exert pressure on that area and its total atmospheric pressure increases. This is what we call high pressure. Likewise, if there are less air molecules above an area, the atmospheric pressure decreases. This is known as low pressure.

Air pressure isn't uniform across the Earth. It ranges from 980 to 1050 millibars and changes with altitude. The higher the altitude, the lower the air pressure. This is because the number of air molecules decreases at higher altitudes, thus decreasing air density and air pressure. Air pressure is highest at sea level, where air density is greatest.

Air Pressure Basics

There are 5 basics about air pressure:

  • It increases as air density increases and lowers as air density lowers.
  • It increases as temperatures increase and lowers as temperatures cool.
  • It increases at lower altitudes and decreases at higher altitudes.
  • Air moves from high pressure to low pressure.
  • Air pressure is measured with a weather instrument known as a barometer. (This is why it's also sometimes called "barometric pressure.")

Measuring Air Pressure

Aneroid Barometer
This is a close up of the needle at the 'Change' indication of an aneroid barometer used to measure air pressure. Gannet77/E+/Getty Images

A barometer is used to measure atmospheric pressure in units called atmospheres or milibars. The oldest type of barometer is the mercury barometer. This instrument measures mercury as it rises or lowers in the glass tube of the barometer. Since atmospheric pressure is basically the weight of air in the atmosphere above the reservoir, the level of mercury in the barometer will continue to change until the weight of mercury in the glass tube is exactly equal to the weight of air above the reservoir. Once the two have stopped moving and are balanced, the pressure is recorded by "reading" the value at the mercury's height in the vertical column.

If the weight of mercury is less than the atmospheric pressure, the mercury level in the glass tube will rise (high pressure). In areas of high pressure, air is sinking toward the surface of the earth more quickly than it can flow out to surrounding areas. Since the number of air molecules above the surface increases, there are more molecules to exert a force on that surface. With an increased weight of air above the reservoir, the mercury level rises to a higher level.

If the weight of mercury is greater than the atmospheric pressure, the mercury level will fall (low pressure). In areas of low pressure, air is rising away from the surface of the Earth more quickly than it can be replaced by air flowing in from surrounding areas. Since the number of air molecules above the area decreases, there are less molecules to exert a force on that surface. With a reduced weight of air above the reservoir, the mercury level drops to a lower level.

Other types of barometers include aneroid and digital barometers. Aneroid barometers do not contain mercury or any other liquid, but they have a sealed and air-tight metallic chamber. The chamber expands or contracts in response to pressure changes and a pointer on a dial is used to indicate pressure readings. Modern barometers are digital and are able to measure atmospheric pressure accurately and quickly. These electronic instruments display current atmospheric pressure readings across a display screen.

Low and High Pressure Systems

Atmospheric pressure is impacted by daytime heating from the sun. This heating does not occur evenly across the Earth as some areas are heated more than others. As air is warmed, it rises and can result in a low pressure system.

The pressure at the center of a low pressure system is lower than air in the surrounding area. Winds blow toward the area of low pressure causing air in the atmosphere to rise. Water vapor in the rising air condenses forming clouds and, in many cases, precipitation. Due to the Coriolis Effect, a result of the Earth's rotation, winds in a low pressure system circulate counter-clockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Low pressure systems can produce unstable weather and storms such as cyclones, hurricanes, and typhoons. As a general rule of thumb, lows have a pressure of around 1000 millibars (29.54 inches of mercury). As of 2016, the lowest pressure ever recorded on Earth was 870 mb (25.69 inHg) in the eye of Typhoon Tip over the Pacific Ocean on October 12, 1979.

In high pressure systems, air at the center of the system is at a higher pressure than air in the surrounding area. Air in this system sinks and blows away from the high pressure. This descending air reduces water vapor and cloud formation resulting in light winds and stable weather. Air flow in a high pressure system is opposite that of a low pressure system. Air circulates clockwise in the Northern Hemisphere and counter-clockwise in the Southern Hemisphere.

Article edited by Regina Bailey

Sources

  • Britannica, The Editors of Encyclopaedia. "Atmospheric Pressure." Encyclopædia Britannica, Encyclopædia Britannica, Inc., 5 Mar. 2018, www.britannica.com/science/atmospheric-pressure.
  • National Geographic Society. "Barometer." National Geographic Society, 9 Oct. 2012, www.nationalgeographic.org/encyclopedia/barometer/.
  • "The Highs and Lows of Air Pressure." Winter Weather Safety | UCAR Center for Science Education, scied.ucar.edu/shortcontent/highs-and-lows-air-pressure.