Atmospheric Pressure

Vocabulary/new concepts:

Atmosphere – This is the gaseous envelope that surrounds the Earth. It is made mostly of nitrogen (about 78%) and oxygen (about 21%), argon (about 0.9%), carbon dioxide, and other gases in small amounts.

Force (Thrust) – the force with which a body acts on a surface. It is applied perpendicular to the surface.

Pressure – the magnitude of the force acting per unit area.

Pressure = force/unit area

The unit of pressure adopted in the International System of Units (SI) is the Pascal (Pa). One pascal is equal to the force of one newton acting on one square metre (N/m²).

Recall also the meaning of the following terms:

Earth’s gravitational force
evaporation, boiling of liquid
boiling temperature

Atmospheric pressure

The Earth is surrounded by a thick layer of air called the atmosphere. There are about 10 tons of air above every square meter of the Earth’s surface. This air exerts force and creates pressure on all bodies on Earth, which is called atmospheric pressure. The upper layers of air press down on the lower layers. The more air there is over a place, the greater the atmospheric pressure. The air that is closest to the Earth’s surface is the densest and has the most force.

Atmospheric pressure changes constantly. It depends on:

• Altitude – the higher we go, the air becomes thinner and the pressure decreases; at sea level, atmospheric pressure is highest.
• Air temperature – leads to a change in atmospheric pressure during the day; at a higher temperature the earth’s surface heats up strongly, warm air rises upwards and creates a zone of less density and low pressure; cold air is denser and so at lower temperatures the pressure in this zone increases.
• Humidity – high humidity implies lower atmospheric pressure, as water vapour is lighter than air.
• Air pollution – leads to global warming and climate change, so it is important to reduce emissions and keep the atmosphere clean.
• The movement of air masses – winds carry air at different pressures and so affect the pressure distribution in the atmosphere.

Pressure affects many areas of our lives – meteorology, aviation, mountaineering, diving, engineering, and manufacturing processes.

How does atmospheric pressure affect the boiling point of a liquid?

The temperature at which a liquid boils and begins to change from a liquid to a gas aggregate state throughout its volume is called the boiling point. The vapour pressure then equalises with the ambient pressure. This temperature is not a constant quantity but depends on the atmospheric pressure exerted on the surface of the liquid.

The higher the atmospheric pressure at a location, the higher the boiling point. When boiling, bubbles form throughout the volume of the liquid, which reaches the surface only if the pressure within them slightly exceeds the surrounding atmospheric pressure, because the molecules of the liquid need more energy (i.e., higher temperature) to overcome the forces of attraction between them and pass into the gaseous state.

At high altitudes, the atmospheric pressure is lower and water boils at a lower temperature. In this case, the molecules of the liquid need less energy (hence, lower temperature) to separate from its surface, whereby boiling starts at a lower temperature. For example, in the Alps or Himalayas, the boiling point is 85°C, i.e. the liquid boils at a lower temperature. Because of this, dishes at such altitudes take longer to cook and some foods cannot be cooked there.

In the kitchen, we use a pressure cooker. In it, higher than atmospheric pressure is created. The pressure is increased considerably and the water in the pot boils at a higher temperature, up to 115°C, but no energy is expended in turning it into steam, so the food in it is cooked faster and to a better quality.

How is atmospheric pressure measured?

To measure atmospheric pressure, a special device is used – a barometer.

There are different types of barometers:

• Mercury barometer – a classical type of instrument invented in 1644 by the Italian scientist Evangelista Torricelli. It is a glass tube filled with mercury and with a vacuum at the top. The lower end of the tube is open and immersed in a container of mercury. A scale placed next to the mercury tube reads the atmospheric pressure in millimetres of mercury. This instrument is primarily used in laboratories because it is dangerous to operate due to the toxicity of mercury.

• An aneroid barometer (aneroid) – the most commonly used barometer today. Its main part is a hermetically sealed metal vacuum box. The force of the air pressure acts on the flexible walls of the box from the outside. The compressive force of the compressed spring in the instrument acts against it. As a result, the box deforms, the changes are transmitted to the pointer which reads the pressure on the scale.

• Altimeter – used to measure the altitude of a location. It works on the aneroid principle and measures pressure changes. Taking these changes into account, the instrument calculates and displays at what altitude you are. There are different types of altimeters, and they are most commonly used by hikers, in aviation and meteorology, and surveying.

The international unit of atmospheric pressure measurement is the Pascal (denoted by Pa). It is named in honour of the French physicist and mathematician Blaise Pascal. One pascal is equal to the force of one newton per square meter N/m².

A derived, commonly used standard unit of measurement in meteorology and engineering is the hectopascal (hPa):

1 hPa = 100 Pa, and the kilopascal (kPa): 1 kPa = 1000 Pa.

Why is it important to measure atmospheric pressure?

Measuring atmospheric pressure provides valuable information in many areas of human activity and helps us to predict various natural phenomena.

For example:

• To make short- and long-term weather forecasts. Changes in atmospheric pressure are closely related to several weather phenomena – precipitation, clouds, storms, hurricanes, etc. Data analysis enables meteorologists to make timely warnings about dangerous natural phenomena, making it possible to take measures and prevent unwanted consequences.
• To ensure flight safety, by setting the flight altitude to avoid turbulence. Use atmospheric pressure data to determine flight altitudes for navigation and route planning.
• Monitor climate change and analyse the impact of human activity on the atmosphere.
• To plan the activities of the many sectors of industry that depend on weather conditions – agriculture, construction, and energy.

Atmospheric pressure also affects the human body, especially in more sensitive individuals and people with chronic diseases.

Low atmospheric pressure can cause headaches, fatigue and tiredness, muscle and joint pain, breathing difficulties (in people with asthma and other respiratory diseases), sleep problems, and heart problems. It is associated with adverse health effects as it affects blood oxygen levels and individual body functions.

High atmospheric pressure is kinder to people as it is associated with less precipitation and fewer changes in weather. It implies stable atmospheric conditions, which can even have a positive effect on the general state of the body, improving mood, and reducing allergic and asthmatic reactions, as well as joint pain.