A4-1 What Is Atmospheric Pressure?

Atmospheric pressure on an object is due to the weight of the atmosphere (i.e. the force due to gravity acting downward on the air particles) pushing down on the surface of that object. Atmospheric pressure decreases as altitude increases simply because there are fewer and fewer air particles at higher elevations. The base unit for measuring pressure is the pascal (Pa). This is a very small unit, however, and most countries in the world instead use the hectopascal (hPa) when measuring air pressure (1 hPa = 100 Pa). The United States, however, still uses an older unit of atmospheric pressure known as the millibar (mb). In practice, however, the actual numbers used in measuring air pressure values are the same since one hectopascal is equal to one millibar (1 hPa = 1 mb).

Try This!

Investigating Air Pressure Around the World

Click on each location name in order to examine how altitude affects air pressure.

Location Altitude Air Pressure (hPa/mb) Notes
       
Singapore 0 m
(0 ft)		 1013.25
  • Breathing is easiest here with 100% oxygen capacity.
  • The human heart rate varies on average between 64 beats per minute (bpm) while resting and up to 170 bpm while exercising.
Vail, Colorado 2,455 m
(8,022 ft)		 713.52
  • Most people feel an impact on their breathing since they only have 75% of the oxygen available at sea level.
  • The human heart rate ranges between 70 and 155 bpm.
La Rinconada, Peru 5,100 m
(16,700 ft)		 488.99
  • This is the highest city in the world with permanent residents. No one can adjust on a year round basis to higher altitudes since only 55% of the oxygen present at sea level is available at this altitude.
  • The human heart rate ranges between 85 and –140 bpm.
Top of Mt. Everest, Nepal 8,848 m
(29,029 ft)		 286.27
  • This the highest point on Earth. Climbers have only 33% of the oxygen available at sea level.
  • The human heart rate ranges between 123 and 140 bpm; the heart rate remains high even at rest.
  • Any altitude above 8,000 m (26,000 ft) is deemed to be in the “death zone.” The amount of oxygen in this zone is not high enough to sustain human life.
Did You Know?

Pressure Extremes

The lowest sea level air pressure in the world, 870 hPa (870 mb), was recorded in the eye of Typhoon Tip on October 12, 1979. The highest air pressure, 1085.7 hPa (1085.7 mb), was recorded in Tonsontsengel, Mongolia on December 19, 2001.

Typhoon Tip on October 12, 1979, the day the world record for lowest air pressure was set.
Math Link

How Pressure Changes With Altitude

You already know that atmospheric pressure decreases as altitude increases simply because there are fewer and fewer air particles at higher elevations. But what is the exact relationship between atmospheric pressure and altitude? Go to Math Link…
History Link

Measuring Air Pressure

A device that measures air pressure is known as a barometer. The first barometer was invented by Evangelista Torricelli (1608–1647) in 1643. Go to History Link…
Word Link

Aneroid: This word was first used in 1848 and is formed by combining the Greek a meaning without with the Greek word neros which means liquid. An aneroid barometer is a device that measures air pressure without the use of a liquid (i.e. mercury).

Barometer: This word dates from 1665 and is formed by combining the Greek word baros which means weight with the Greek word metron which means measure. The word barometer thus refers to a device that measures weight (of air). The invention of this word is generally attributed to the English scientist Robert Boyle (1627–1691).