Atmospheric pressure

Atmospheric pressure is the force exerted by the weight of the air in Earth's atmosphere. It is also known as air pressure or barometric pressure. This pressure is a vital part of our planet's weather and climate systems.

Scientists often use a standard unit called an atmosphere, or atm. One standard atmosphere is defined as 1,013.25 millibars. This is also equal to 760 torr or about 14.696 pounds per square inch (psi). This value represents the average pressure found at mean sea level.

The mechanism behind this pressure is the gravitational attraction of the planet. Gravity pulls on the various atmospheric gases, holding them close to the Earth. The total pressure is a function of the planet's mass and its radius. It also depends on the amount and composition of the gases. The vertical distribution of these gases determines how much weight sits above a specific point. This pressure is often described as hydrostatic pressure. This means it is caused by the weight of the air column above a measurement point.

Atmospheric pressure changes significantly based on altitude. As you move higher away from the Earth's surface, the pressure decreases. This happens because there is less overlying atmospheric mass above you. At low altitudes, the pressure drops by about 12 pascals for every 100 meters. The pressure varies smoothly from the surface to the top of the mesosphere.

Temperature and humidity also play a role in these changes. Pressure is proportional to temperature and is inversely related to humidity. This means that as humidity increases, the pressure tends to decrease.

There are different ways to measure and report pressure depending on the location. Mean sea-level pressure (MSLP) is the pressure at mean sea level. Meteorologists use this value in weather reports on television and the internet. In aviation, pilots use an altimeter setting to adjust for pressure. This helps them determine their height accurately. In the United States and Canada, this is often reported in inches of mercury. Other regions use hectopascals (hPa) or millibars. One hectopascal is exactly equal to one millibar.

History shows us how pressure measurements helped science grow. In 1774, Nevil Maskelyne used pressure to test Newton's theory of gravitation. He conducted the Schiehallion experiment on a mountain in Scotland. He needed to measure the mountain's elevation accurately for his work. William Roy also used barometric pressure to confirm these height measurements. Their results were incredibly close, with an agreement within one meter. This successful use of pressure tools became vital for survey work and map making.

Extreme pressure records show the power of Earth's weather. The highest sea-level pressure occurs in Siberia during the Siberian High. This can reach levels above 1,080 hPa. The lowest measurable pressures are found in the centers of tornadoes and tropical cyclones. A record low was set in 1979 during Typhoon Tip in the western Pacific. An aircraft measured this extremely low pressure from the air.

Conversely, the Dead Sea is a very low point on land. Because it is 430 meters below sea level, it has a high typical pressure of 1,065 hPa.

Atmospheric pressure even affects the physical properties of liquids. For example, it changes the boiling point of water. Pure water boils at 100°C at standard atmospheric pressure.

However, the boiling point is the temperature where vapor pressure equals atmospheric pressure. At lower pressures, such as on a high mountain, water boils at a lower temperature. This is why cooks must adjust recipes at high altitudes. In science, a vacuum pump can lower the pressure to evaporate liquids at lower temperatures. This connects the study of air pressure to the study of thermodynamics and chemistry.