5.5.2 Atmospheric Pressure

The Earth's atmosphere, a thin layer of air surrounding the planet, plays a vital role in our everyday lives. In this lesson, we will explore atmospheric pressure and its relationship with altitude.

The Earth's Atmosphere

The atmosphere is composed of various gases, primarily nitrogen (approximately 78%) and oxygen (approximately 21%), along with trace amounts of other gases such as carbon dioxide and argon. It extends from the Earth's surface up to thousands of kilometres into space. Although the atmosphere is relatively thin compared to the size of the Earth, its effects are significant.

Atmospheric Pressure and Air Molecules

Air molecules constantly move and collide with surfaces, creating what we call atmospheric pressure. The pressure exerted by the atmosphere is caused by the weight of the air above a particular surface. As you move closer to the surface of the Earth, the number of air molecules and the weight of the air above that surface increase, resulting in higher atmospheric pressure.

Relationship Between Altitude and Atmospheric Pressure

As we ascend to higher altitudes, the density of the atmosphere decreases. This decrease in density means there are fewer air molecules above a given surface compared to lower altitudes. Consequently, the weight of the air decreases, leading to a decrease in atmospheric pressure with increasing height.

Simple Model of Atmospheric Pressure

A simple model to understand atmospheric pressure is to imagine the atmosphere as a stack of thin layers of air. Each layer has its own weight, and collectively, these layers create atmospheric pressure. The weight and density of the layers decrease as we move higher in the atmosphere, resulting in lower atmospheric pressure.

Variation of Atmospheric Pressure with Height

The decrease in atmospheric pressure with increasing height is gradual but significant. For example, at sea level, the atmospheric pressure is approximately 101.3 kilopascals (kPa). However, at higher altitudes, such as at the top of a mountain, the atmospheric pressure is noticeably lower, often measured below 100 kPa.

Conclusion

Understanding atmospheric pressure and its variation with height is essential in comprehending various phenomena related to weather, climate, and even human activities at different altitudes. By recognising that atmospheric pressure is influenced by the weight and density of the air above a surface, we can explain why atmospheric pressure decreases as we ascend to higher altitudes.