Introduction
It may surprise you, but the air around you actually has weight, and it presses up against everything that it comes in contact with. It is known as atmospheric pressure and is defined as the force that gets exerted on a surface by the air as it is being pulled to the earth by gravity. This force is generally measured using a barometer, an instrument that has a column of mercury, in a glass tube, which falls or rises as the air pressure changes.
The pressure is measured by meteorologists by how high the mercury rises in this tube. An atmosphere or atm is a unit of measurement that is found to be equal to the average air pressure that is found at sea level, with the temperature being 59 degrees Fahrenheit or 15 degrees Celsius. One atm is 769 millimetres (29.92 inches) or 1,013 millibars of mercury.
Changes in Air Pressure with Altitude
Another feature of air pressure is that it drops as there is an increase in altitude. The atmospheric pressure that you will find at Denali, Alaska, is only about half of what it is in Honolulu, Hawaii. The latter is a city at sea level, whereas the former is the highest peak in North America.
The oxygen that is available for breathing will also decrease as the pressure decreases. At places where the altitudes are very high, the oxygen and atmospheric pressure get so low that people face breathing troubles. Bottled oxygen is used by mountain climbers when they go to very high peaks, and they also take time to prepare themselves to get used to the change in altitude. This is because moving from higher pressure to a lower one can result in decompression sickness, and this is also called ‘the bends’, an issue for scuba divers to come to the surface too soon, as well.
Even in aircraft, care is taken to create artificial pressure so that the passengers remain safe and comfortable as they fly. The pressure is also an indicator of weather as a low-pressure system can lead to precipitation, wind, and cloudiness, whereas high-pressure systems provide weather that is fair and calm.
The density and the depth of the atmosphere are two reasons why there is a change in the air pressure with a change in the altitude. Most of the gas molecules that are found in the atmosphere are pulled to the earth’s surface due to gravity which means that the air is denser near the surface. There are more collisions of particles with more of them in a given volume, and therefore there is greater pressure.
The depth, that is, the distance from the top all the way to the bottom of the atmosphere, is the highest at sea level, and it will go down at higher altitudes. With the depth of the atmosphere being greater, there is more air that gets pressed down from above. This means that the air pressure at sea level is greatest, and it falls as the altitude increases. The air pressure at the top of Mount Everest is only about one-third of the pressure that is at sea level.
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How is Air Pressure Measured?
The air pressure gauge, known as the barometer, is the instrument that the local atmospheric pressure is measured by. This instrument, as mentioned before, uses a column of mercury in a glass tube to measure the changes in the air pressure. There are several different units used to express atmospheric pressure. The air pressure unit is generally measured in millimetres or inches of mercury (Hg). The other units are kilopascals, standard atmospheres, millibars (mb), dynes per square centimetre, or pounds per square inch (psi).
Atmospheric pressure is also called barometric pressure since it is measured through a barometer. When it is rising, it indicates that the atmospheric pressure is rising, and decreasing atmospheric pressure is denoted by a falling barometer reading.
Reading a Barometer
It is quite easy to read a barometer if you are aware of what the different values on it indicate. You can interpret the readings as follows to be able to understand how the atmospheric pressure is changing and to understand the barometer.
High Pressure: The barometric reading is considered high if it is over 30.20 inches Hg, and this means that it is associated with weather that is clear and calm.
Normal Pressure: Normal pressure is associated with weather that is steady, and the reading here is in the range of 29.80 and 30.20 inches Hg.
Low Pressure: Low pressure is associated with rainstorms and warm weather, and here the reading has to be below 29.80 inches Hg to be considered low.
This is a topic that is often taught to school children with the help of different air pressure experiments like the indoor tornado experiment, unspillable water experiment, book blowing experiment, etc.
Effects of Air Pressure
Air pressure has no adverse effects as long as it is within the normal ranges. As mentioned before, this pressure increases at sea level immensely, and this is one of the reasons why it is difficult to explore too deep in the oceans. It is said that the pressure can crumple a man like a can.
On the other hand, low pressure at higher altitudes is also dangerous. Mountain sickness, where the pressure is low, can result in headaches, nausea, dizziness, fatigue, and shortness of breath. It can also result in arthritis being worsened because of how thin the air is with low pressure.
FAQs on Air Pressure
1. What is air pressure and what causes it?
Air pressure, also known as atmospheric pressure, is the force exerted on a surface by the weight of the air above it. This pressure is caused by Earth's gravity pulling the gas molecules in the atmosphere downwards. The air is denser and heavier near the Earth's surface, resulting in higher pressure at lower altitudes.
2. How is air pressure measured and what are its standard units?
Air pressure is measured using an instrument called a barometer. The standard unit of atmospheric pressure at sea level is one atmosphere (atm). Other common units used to express air pressure include:
- Pascals (Pa) or kilopascals (kPa), the SI unit.
- Millibars (mb), commonly used in weather forecasting.
- Millimetres or inches of mercury (mm Hg or inHg), used in barometers.
- Pounds per square inch (psi).
3. Why does air pressure decrease as we go to a higher altitude, like up a mountain?
Air pressure decreases with an increase in altitude for two main reasons. Firstly, as you go higher, the column of air above you becomes shorter, meaning there is less air to press down on you. Secondly, gravity pulls most air molecules closer to the Earth's surface, making the air less dense at higher altitudes. Fewer air molecules in the same volume result in lower pressure.
4. How does a change in air pressure help in forecasting the weather?
Changes in air pressure are a key indicator of weather patterns. A high-pressure system, where air is sinking, usually brings calm, clear, and fair weather. Conversely, a low-pressure system, where air is rising, is associated with unstable conditions like cloud formation, wind, and precipitation like rain or snow.
5. What are some real-world examples that demonstrate the importance of air pressure?
Air pressure is fundamental to many everyday phenomena and technologies. Some important examples include:
- Drinking with a straw: When you suck on a straw, you reduce the air pressure inside it, allowing the higher atmospheric pressure outside to push the liquid up.
- Aeroplane cabin pressurisation: Aircraft cabins are artificially pressurised to maintain a safe and comfortable air pressure for passengers, mimicking the pressure at a lower altitude.
- Suction cups: Pressing a suction cup against a surface forces the air out from underneath. The higher air pressure outside then holds it firmly in place.
6. What are the effects of very high or low air pressure on the human body?
Extreme air pressure can have significant effects on the human body. At very high altitudes where air pressure is low, people can experience altitude sickness, with symptoms like headaches, dizziness, and shortness of breath due to less available oxygen. Conversely, moving too quickly from a high-pressure environment (like deep-sea diving) to a low-pressure one can cause decompression sickness, also known as 'the bends'.
7. Is there a difference between 'air pressure' and 'atmospheric pressure'?
While often used interchangeably in daily life, there is a subtle difference. Atmospheric pressure specifically refers to the pressure exerted by the weight of the Earth's atmosphere. Air pressure is a more general term that can also describe the pressure of air in a contained space, such as inside a car tyre or a balloon. In the context of weather and science lessons on this topic, both terms generally refer to atmospheric pressure.
