Table of Contents
Introduction
The barometric pressure is a measure of the force exerted by the molecules in the air on a unit area below. You might think of pressure as the force that the the atmosphere exerts on your head. Standard sea level pressure is 14.7 pounds per square inch. So, if the surface area of the top of your head measures about 175 square inches (the size of an average human head), then you have 2572.5 pounds of air (more than one ton) weighing down your head all the time!
Barometric Pressure Units
Barometric pressure results from the accumulated force (F) of molecular collisions in the air on a surface (A; see image above). Pressure is a force divided by a unit area. Pounds are a unit of force, and inches are a unit of length. Pounds per square inch may be a convenient unit to conceptualize barometric pressure, but it’s not the best unit to use if you want to convert between pressure and other variables.
What is pressure? There are two kinds of pressure: static pressure and dynamic pressure. Static pressure is the force applied per unit area by the weight of the air above on a surface below. You might recall that weight is equal to mass times gravity. Gravity is an acceleration, not a force…weight is a force. Dynamic pressure is the force applied per unit area to a surface below due to the acceleration of air as it moves vertically. Dynamic pressure is important only in exceptional circumstances, such as thunderstorm downdrafts. I occasionally see students refer to pressure as “wind pressure”. This is a misnomer: pressure applies in the vertical, while wind registers a force in the horizontal direction.
Why did the barometric pressure consider starting a war with mathematicians?
Because it thought, “If I’m just force divided by area, I might as well divide and conquer!”
The standard unit of force used by the scientific community is the Newton (N), named after Isaac Newton, and the standard unit of area is 1 m². A Newton is one kilogram-meter-per-second-squared. Conveniently, we have a name for a unit of pressure that is 1 N per m²: the Pascal (Pa), which is named after Blaise Pascal (1623-1662), a French polymath who conducted the first pressure-versus-altitude experiment.
In meteorology, we use the hectoPascal (hPa), where 1 hPa is equivalent to 100 Pa. Another unit that meteorologists commonly use is the millibar (mb), which is mathematically equivalent to the hectoPascal. Therefore, hectoPascals and millibars are interchangeable. One bar is approximately standard atmospheric pressure at sea level, and a millibar is one thousandth of a bar.
What are common values of barometric pressure at the surface?
The average sea level pressure around the globe is 1013.25 mb or 1013.25 hPa. Sea level pressure usually ranges between 980 and 1045 mb or hPa. The minimum sea level pressure recorded in the instrumental era was 870 mb in Typhoon Tip (October 12, 1979), and the highest was 1083.8 mb in Siberia on December 31, 1968.
Pressure-Measuring Devices
Source: https://www.britannica.com/technology/barometer
The device used to measure barometric pressure is known as a barometer. Two common kinds of barometers are mercury barometers and aneroid barometers. Mercury barometers are much like a liquid thermometer. A thermometer consists of a sealed tube containing a liquid that expands or contracts. In a mercury barometer, at the bottom of the tube, the mercury is exposed to the air. As the air pressure changes, the mercury in the tube expands or contracts, rising higher or falling lower in the tube with the varying barometric pressure. The mercury barometer was invented by Italian physicist Evangelista Torricelli (1608-1647) in 1643.
How tall would a water barometer have to be to measure barometric pressure?
In 1640 Torricelli, along with Galileo Galilei, conducted an experiment to draw water out of a deep well. The highest the water would go was approximately 33-34 feet. This is the height of water in a barometer. Incidentally, it was this experiment that inspired Torricelli to create the mercury barometer.
An aneroid barometer contains a metal cylinder (aneroid cell) that tightens as the pressure increases. The aneroid cell is made from an alloy of the elements beryllium and copper. The aneroid barometer was invented in 1844 by French scientist Lucien Vidie (1805-1866). A barograph (shown below) connects a needle between an aneroid barometer and a rotating drum, tracing the pressure values on paper much like a seismometer records earthquakes.
Source: Wikipedia
Beyond Barometric Pressure
In other posts, you can read about measurements of temperature, wind, and humidity. Together, these four variables are called state variables because they indicate the basic state of the atmosphere at a given time. We measure other variables, though, such as precipitation type and amount, visibility, cloud height, and cloud coverage. Click to find out how we plot these data on maps and graphs.
