The air around you has weight, and it presses against everything it touches. That pressure is called atmospheric pressure, or air pressure. It is the force exerted on a surface by the air above it as gravity pulls it to Earth.
Atmospheric pressure is commonly measured with a barometer. In a barometer, a column of mercury in a glass tube rises or falls as the weight of the atmosphere changes. Meteorologists describe the atmospheric pressure by how high the mercury rises.
An atmosphere (atm) is a unit of measurement equal to the average air pressure at sea level at a temperature of 15 degrees Celsius (59 degrees Fahrenheit). One atmosphere is 1,013 millibars, or 760 millimeters (29.92 inches) of mercury.
Atmospheric pressure drops as altitude increases. The atmospheric pressure on Denali, Alaska, is about half that of Honolulu, Hawai'i. Honolulu is a city at sea level. Denali, also known as Mount McKinley, is the highest peak in North America.
As the pressure decreases, the amount of oxygen available to breathe also decreases. At very high altitudes, atmospheric pressure and available oxygen get so low that people can become sick and even die.
Mountain climbers use bottled oxygen when they ascend very high peaks. They also take time to get used to the altitude because quickly moving from higher pressure to lower pressure can cause decompression sickness. Decompression sickness, also called "the bends", is also a problem for scuba divers who come to the surface too quickly.
Aircraft create artificial pressure in the cabin so passengers remain comfortable while flying.
Atmospheric pressure is an indicator of weather. When a low-pressure system moves into an area, it usually leads to cloudiness, wind, and precipitation. High-pressure systems usually lead to fair, calm weather.
Vocabulary
Term Part of Speech Definition
air
noun
layer of gases surrounding Earth.
aircraft
noun
vehicle able to travel and operate above the ground.
air pressure
noun
force pressed on an object by air or atmosphere.
altitude
noun
the distance above sea level.
atmosphere (atm)
noun
(atm) unit of measurement equal to air pressure at sea level, about 14.7 pounds per square inch. Also called standard atmospheric pressure.
atmospheric pressure
noun
force per unit area exerted by the mass of the atmosphere as gravity pulls it to Earth.
barometer
noun
an instrument that measures atmospheric pressure.
Celsius scale
noun
scale for measuring surface temperature, used by most of the world, in which the boiling point of water is 100 degrees.
cloud
noun
visible mass of tiny water droplets or ice crystals in Earth's atmosphere.
decompression sickness
noun
serious condition resulting from gases forming tiny bubbles in the bloodstream as a body adjusts to a major change in atmospheric pressure. Also known as DCS, divers disease, and the bends.
exert
verb
to force or pressure.
Fahrenheit scale
noun
scale for measuring surface temperature used by Belize, Liberia, Myanmar, and the United States.
force
noun
power or energy that activates movement.
gravity
noun
physical force by which objects attract, or pull toward, each other.
high-pressure system
noun
weather pattern characterized by high air pressure, usually as a result of cooling. High-pressure systems are usually associated with clear weather.
low-pressure system
noun
weather pattern characterized by low air pressure, usually as a result of warming. Low-pressure systems are often associated with storms.
mercury
noun
chemical element with the symbol Hg.
mercury barometer
noun
tool that determines atmospheric pressure by measuring how much mercury moves in a glass tube.
meteorologist
noun
person who studies patterns and changes in Earth's atmosphere.
millibar
noun
(mbar) unit of pressure equal to .001 bar of atmospheric pressure.
oxygen
noun
chemical element with the symbol O, whose gas form is 21% of the Earth's atmosphere.
precipitation
noun
all forms in which water falls to Earth from the atmosphere.
scuba
noun, adjective
(self-contained underwater breathing apparatus) portable device for breathing underwater.
sea level
noun
base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.
weather
noun
state of the atmosphere, including temperature, atmospheric pressure, wind, humidity, precipitation, and cloudiness.
wind
noun
movement of air (from a high pressure zone to a low pressure zone) caused by the uneven heating of the Earth by the sun.
