Atmospheric pressure

Atmospheric pressure, also called air pressure or barometric pressure, is the pressure inside the atmosphere of Earth. It is the force that air puts on us and everything around us. At sea level, this pressure is about one atmosphere, or atm for short.

Atmospheric pressure comes mostly from the weight of the air above us. The higher we go, the less air is above, so the pressure gets lower. This is why it is harder to breathe on tall mountains. Pressure measures how much force is acting on a certain area.

A small column of air just one square centimeter wide, from sea level to the top of the atmosphere, weighs about one kilogram. This weight creates the pressure we feel every day. Understanding atmospheric pressure helps us learn about weather and how airplanes fly.

Mechanism

Atmospheric pressure happens because Earth's gravity pulls on the gases in the air above us. This pressure depends on the size of the planet, how much air there is, and what kinds of gases make up the air. Things like how fast the planet spins and local weather can also change the pressure a little bit.

Mean sea-level pressure

The mean sea-level pressure (MSLP) is the pressure of the air at mean sea level. This is the pressure that weather reports usually talk about on the radio, television, newspapers, or on the Internet.

In flying, pilots use something called the altimeter setting to adjust their instruments for the air pressure.

On average, the pressure at sea level is 1,013.25 hPa. Weather reports around the world share this pressure using hectopascals or millibars. In the United States, Canada, and Japan, they sometimes use inches of mercury instead.

The highest pressure at sea level happens in places like Siberia. The lowest pressure is found in big storms like tropical cyclones and tornadoes.

Surface pressure

Surface pressure is the air pressure at a place on Earth, like on land or in the water. It changes based on how much air is above that spot.

On average, the surface pressure on Earth is about 985 hPa. This is a bit lower than the pressure at sea level, which is usually around 1,013.25 hPa, or 1 atmosphere. Air pressure depends on the weight of the air above a certain area.

Altitude variation

See also: Blaise Pascal § First atmospheric pressure vs. altitude experiment

Further information: Barometric formula and Vertical pressure variation

The air pressure on Earth changes depending on how high you are above the ground. On mountains, the air pressure is usually lower than it is at sea level. As you go higher, the air pressure gets lower.

Temperature and humidity also affect air pressure. For every 100 metres you go up from sea level, the pressure drops by about 1.2 kPa. There are special equations that help us figure out the air pressure at any height above sea level.

Local variation

Air pressure changes a lot on Earth, and these changes help us understand weather and climate. Some of these changes happen in regular patterns. One reason for these changes is something called atmospheric tides. These tides are strongest near the equator, where they can change pressure a little, but they are almost nothing near the poles. Near the equator, these pressure changes follow two main rhythms: one that repeats every 24 hours and another that repeats every 12 hours.

Records

The highest air pressure ever recorded on Earth above 750 meters was 1,084.8 hPa. This happened in Tosontsengel, Mongolia on December 19, 2001. Another high pressure record of 1,083.8 hPa was set at Agata, in Evenk Autonomous Okrug, Russia, on December 31, 1968.

The Dead Sea, the lowest place on Earth, usually has high air pressure because it is very low. A record pressure of 1,081.8 hPa was set there on February 21, 1961.

The lowest air pressure ever measured was 870 hPa. This was recorded on October 12, 1979, during Typhoon Tip in the western Pacific Ocean, from an aircraft observation.

Measurement based on the depth of water

One atmosphere of pressure is the same as the pressure from a column of freshwater that is about 10.3 meters high. This means a diver who is 10.3 meters underwater feels a pressure of about two atmospheres — one from the air and one from the water.

Low pressures, like in natural gas lines, are sometimes measured in inches of water. For example, a common home gas appliance can handle up to about half a pound per square inch of pressure.

Boiling point of liquids

Pure water boils at 100 °C (212 °F) when the air pressure is like it is at sea level on Earth. The boiling point is when a liquid turns to steam and the steam pressure matches the air pressure around it. Because of this, liquids boil at lower temperatures when the air pressure is lower and at higher temperatures when the air pressure is higher. This is why cooking at high altitudes needs special adjustments or tools like a pressure cooker. In the past, explorers used the boiling temperature of water to guess how high they were. To make a liquid turn into vapor at a lower temperature, such as in distillation, the air pressure can be reduced using a vacuum pump, like in a rotary evaporator.

Measurement and maps

Knowing that air pressure changes with height helped scientists measure how tall hills and mountains are. In 1774, a scientist named Nevil Maskelyne tested ideas about gravity on a mountain in Scotland called Schiehallion. He needed to know the exact heights on the mountain’s sides. Another scientist, William Roy, used air pressure to check Maskelyne’s measurements, and they matched very closely. This method is still used today to make maps and surveys.