Atmospheric pressure, also known as air pressure or barometric pressure, is a critical component of Earth's weather and climate system. Understanding and accurately measuring atmospheric pressure is crucial to almost every facet of today's world, from forecasting weather patterns and tracking storms to improving GPS location accuracy, and even determining the altitude of aircraft and Low Earth Orbit satellites (LEOs).
The ways we have determined air pressure have changed drastically throughout history. Nowadays, we have numerous ways of measuring ambient pressure in a variety of environments.
In this article, we'll focus on the science behind measuring pressure in the atmosphere and the instruments used to do so, with all of the quirks along the way.
What is atmospheric pressure?
Atmospheric pressure is the force exerted by the weight of the air molecules in the Earth's atmosphere on the surface of the Earth. It is the pressure exerted by the weight of the air that surrounds us and is caused by the gravitational pull of the Earth.
Ambient pressure affects the weather, including wind and precipitation patterns, and has a significant impact on many physical and biological processes on Earth.
How is air pressure measured?
Air pressure is commonly measured using a barometer. A barometer is an instrument that measures the pressure of the air in a given area. There are two main types of barometers: aneroid and mercury barometers.
Depending on the type of barometer, atmospheric pressure is measured in a multitude of different units. The most common are atmospheres (atm), millimetres of mercury (mmHg), or kilopascals (kPa). At sea level, the average atmospheric pressure is about 1013 millibars or 1013 hectopascals (hPa). This pressure decreases with increasing altitude, due to the decrease in the number of air molecules present at higher elevations, this is one of the mechanisms that contribute to altitude sickness.
Mercury barometers use a column of mercury (Hg) that rises or falls based on changes in air pressure. Whilst mercury barometers are highly accurate, they have their downsides too - they’re not easily portable, they’re expensive and they also require very careful handling compared to other alternatives out there.
First invented by Evangelista Torricelli in 1643, mercury barometers are the oldest type of barometer. Torricelli discovered this method of meaning atmospheric pressure by taking a glass tube and filling it with mercury (the heaviest liquid available) and dipping the mercury tube into a basin filled with the liquid metal. The column in the tube, otherwise known as the meniscus, rose to 76cm.
From this experiment, Torricelli discovered that 76cm of mercury is equivalent to the atmospheric pressure of air. Thus, the equation for atmospheric pressure was born, named after the ground-breaking physicist and mathematician: 760 torr = 1atm.
Aneroid barometers use a small, flexible metal box called an aneroid cell that expands or contracts based on changes in air pressure. Unlike a mercury barometer, aneroid barometers don’t contain liquid (the term aneroid means ‘without fluid’).
This device instead operates via a metal cell, or a series of cells, containing a tiny amount of air at reduced pressure. An increase in air pressure causes the sides of these cells (or the singular cell) to move closer together, and visa versa, when air pressure drops these cells move further apart. These movements determine the position of a needle which points to a calibrated circular scale so that a clear reading of air pressure can be seen.
The aneroid barometer was invented by Lucien Vidi, a French scientist, in the 1840s. This type of barometer is often lightweight, small, and can be easily adapted to produce a barograph (a continuous record of pressure changes over time marked by a pen attached to a needle). Aneroid barometers, whilst more compact and easy to transport, have to be recalibrated far more often than other ways of measuring atmospheric pressure (every one to two years).
Aneroid barometers that can be bought for the home will often be accompanied by descriptions of predicted weather spanning ‘stormy’ and ‘rainy’ through to ‘fair’ and ‘fine’. Generally speaking, increased pressure is a sign of calm and dry weather whilst lower pressure indicates the presence of rain and wind.
What is the most widely used pressure gauge?
The most commonly used pressure gauge is the Bourdon gauge (it works without any form of electric power!). The gauge is a mechanical device that uses a curved, spring-loaded tube to measure pressure. When pressure is applied to the inside of the tube, it causes the tube to straighten, which in turn drives a pointer along a calibrated scale.
Bourdon gauges are widely used in industrial, marine, and aviation applications because they are simple, reliable, and easy to read. They are also commonly used in pressure-regulating and monitoring systems, as well as in pneumatic and hydraulic systems.
Using atmospheric pressure to measure altitude (GPS)
One particularly interesting way of measuring atmospheric pressure utilizes GPS systems. Atmospheric pressure can be calculated by determining the drag force acting on Low Earth Orbit (LEO) satellites via what’s known as Precise Orbit Determination (POD). POD is the process of tracking the velocity and position of a satellite in orbit using GNSS - a constellation of satellites that transmit positioning and timing data from space to GNSS receivers. By calculating the movement of LEOs via Precise Orbit Determination and pressure sensors, scientists can calculate the atmospheric pressure acting on these objects, to an incredibly high degree of accuracy.
One thing to consider is how air density naturally varies across an LEO’s flight path according to changes in humidity and temperature. In order to calculate readings accurately, a radiosonde (battery-powered telemetry system which uses radio signals to transmit data) is used to measure changes in pressure between one measurement point to another during the GPS systems’ flight.
How does atmospheric pressure cause altitude sickness?
You might have heard people struggling with altitude sickness whilst hiking up mountains. Altitude sickness occurs when a person ascends to a high altitude too quickly and the body struggles to adjust to the environment, this is because atmospheric pressure decreases with increasing altitude, which means there is less air pressure to support the normal exchange of oxygen and carbon dioxide in the lungs. As a result, less oxygen is available for the body to use, leading to the symptoms of altitude sickness.
Altitude sickness air pressure at the top of Kilimanjaro, for instance, is 40% lower than the pressure at sea level, inducing nausea, fatigue, and dizziness that has the potential to be lethal.
Oppositely, scuba divers and elite free divers can experience decompression sickness, which can be equally as dangerous as altitude sickness. Decompression sickness (often referred to as the bends) occurs because the air pressure and nitrogen gas pressure increase the deeper a diver descends into a body of water, causing nitrogen gas to dissolve into the bloodstream. If the diver ascends too quickly, the drop in pressure can cause the nitrogen gas to form bubbles in the bloodstream, leading to symptoms such as joint and muscle pain, skin rashes, and in severe cases, paralysis or death.
There are several other barometer types out there that harness some rather unique methods of measuring atmospheric pressure. One rather unusual barometer is the Shark Oil barometer which only works in a particular temperature range (in warmer climates). There is also the Collins Patent Table Barometer, a glass tubular device, its central spiral filled with distilled water that rises and falls according to changes in air pressure.
Perhaps the most bizarre barometer of them all was the Tempest Prognosticator, designed by Dr Merryweather in 1851. This contraption was powered by leeches. Leeches were known to be skilled at the art of escaping bodies of water during a storm in order to protect themselves. The Tempest Prognosticator contained twelve leeches, individually placed in miniature bottles of liquid. When agitated by an oncoming storm the leeches would wiggle about, their movement would then cause their containers to rattle sufficiently enough to ring a bell placed beside the receptacles. The probability of a storm was therefore determined by how many times the bells rang.
Unsurprisingly, the engineers at Sent into Space don’t use the Tempest Prognosticator to measure the atmospheric pressure on board our spacecraft. If desired, we can mount a barometer to the craft and add visual overlays monitoring changes in air pressure to accompany the raw hero edit captured throughout the craft’s journey. If you have a project you’d like to launch into space and back, barometer on board or not, please get in touch today. We’re always excited for another trip to the skies, come rain or shine.