Chapter 1 - The Atmosphere

An Introduction to the Atmosphere

Atmosphere: An Overview

An atmosphere (Figure 1) is a single layer or a set of multiple layers containing mixed gases that surround a planetary body.¹ Any planet that has an atmosphere will have weather of some kind; weather isn't exclusive to Earth.² For instance, it rains sulfuric acid on Venus, dust storms sweep across the surface of Mars, and Jupiter's Great Red Spot has been theorized to be part of a large storm.²

Figure 1. The atmosphere and the moon seen in July 2011. Photo credit: ISS team / Johnson Space Center.

Unlike other planets, Earth's atmosphere is the only one in our solar system that is able to sustain life.³ Earth's atmosphere, like any planetary atmosphere, is a set of layers that surrounds the planet, and is composed of various gases, which include nitrogen (about 78%), oxygen (about 21%), argon (about 0.93%), carbon dioxide (about 0.04%), water vapor (anywhere from 0% to 4%), and numerous other trace gases (Figure 2).¹ ² ³ ⁴ ⁵ ⁶ ⁷

Figure 2. Atmospheric composition.

Aside from the fact that the atmosphere provides living organisms with air to breathe, it also makes life possible in other ways such as shielding us from harmful solar ultraviolet (UV) radiation, warms Earth's surface by about 59°F (32.8°C) through heat retention, otherwise known as the greenhouse effect, and prevents extreme diurnal temperature variation.⁴ ⁵ ⁶ ⁷

While Earth's atmosphere is approximately 300 miles (482.8 kilometers) thick, the bulk of it is concentrated within the initial 10 miles (16.1 kilometers) of the surface.³ As a result of this, both air density and air pressure typically decrease with altitude.³ At sea level, air pressure is 14.7 pounds per square inch (psi) and decreases to 10 pounds per square inch (psi) at 10,000 feet.³ ⁷ Earth's gravitational pull is what retains much of the atmosphere near the surface, causing it to become thinner with altitude.¹ ⁴ ⁵ ⁷ ⁸ ⁹ ¹⁰

How Does the Atmosphere Stay in Place?

In his law of gravitation, Sir Issac Newton stated that every particle attracts every other particle due to a force that's directly proportional to the product of the particles' masses and inversely proportional to the square of the distance between them.⁸ In a mathematical equation, it's written as followed: F = (G(m1)(m2)) / r², where G is the gravitational constant, the m's are the masses of two separate particles, and r is the distance between the two particles.⁸ The gravitational constant, G is exceptionally small, as it's 0.00000000006672 Nm² / kg².⁸ As a consequence of the gravitational constant being so small, the gravitational attraction between two small objects is also incredibly small.⁸ The larger an object is, the more gravitational force it'll have.⁸ Because of Earth's massive size and thus large mass as compared to the atmosphere above it, it's gravitational force is strong enough to hold the atmosphere in place.⁸ ⁹ ¹⁰ To put it into perspective, Earth's atmosphere is equivalent to the skin of an apple or a piece of paper laid over a beach ball!² ⁹ Any object that's considerably smaller than Earth will be pulled towards Earth's core.⁸ ¹⁰ 

If Earth's size and mass were to be substantially less, the gravitational force would be too weak to keep the current atmosphere in place.⁸ ⁹ Mars is a prime example of this. Mars has a mass that is less then half of Earth's and its atmosphere only contains six millibars (mb) of pressure, as compared to Earth's atmosphere, which can hold 1000 millibars (mb).⁸ ⁹

The larger a planet is, the more mass it has, which creates a higher gravitational force.⁸ The higher a planet's gravitational force is, the higher the escape velocities are.⁸ An escape velocity is basically the minimum speed that an object or particle must have in order to escape a planet's gravity.⁸ On Earth, the bulk of the gases that compose the atmosphere do not possess a speed high enough to escape the gravitational force.⁸ The end result is that the atmosphere's mass and composition stay fairly constant over long periods of time.⁸ Gravity pulls all particles and gases towards Earth's core, and obviously gases and particles can't enter Earth's crust, thus they collect around the Earth's surface.⁸ This is what causes most of the atmosphere to be contained within the first 10 miles (16.1 kilometers) of Earth's surface and air density to decrease with altitude.³ ⁸

However, this is only part of the reason that Earth's atmosphere stays in place.

The air closer to the Earth's surface is typically denser than the air near the top.⁸ ⁹ ¹¹ As a result, atmospheric pressure decreases with altitude.⁸ ⁹ ¹¹ This is just like a fluid; as in a column of fluid, the pressure at the bottom is higher than the pressure at the top.¹⁰ Fluids move in response to pressure differences, as they move from areas of higher pressure to areas of lower pressure.¹⁰ This is the pressure-gradient force (PGF) and it works both horizontally and vertically in the atmosphere.⁸ ¹⁰ ¹¹ Because air pressure is lower near the top of the atmosphere than at the bottom, gases are pulled upwards by the pressure-gradient force (PGF).⁸ ¹⁰ ¹¹

With both gravity and the upward-directed pressure-gradient force (PGF) into play, their opposing forces － being downward and upward, respectively － come into balance.⁸ ¹⁰ ¹¹ This is more commonly referred to as "hydrostatic balance" or "hydrostatic equilibrium."⁸ ¹⁰ ¹¹ All in all, hydrostatic balance (Figure 3) is what keeps Earth's atmosphere in place.⁸ ¹⁰ ¹¹ 

Figure 3. Hydrostatic balance.

The Atmosphere's Layers

The atmosphere is divided into five different layers.² These layers are each separated by a "-pause" and are divided based on factors like temperature changes, chemical composition, and air density.⁴

In short, from bottom to top, these layers are the:

• Troposphere
• Stratosphere
• Mesosphere
• Thermosphere, and
• Exosphere² ⁵

Because the atmosphere thins with altitude, there is no definite boundary between it and outer space.⁴ Most scientists regard the Kármán Line － which is at an altitude of 62 miles (99.8 kilometers) － as the boundary between the atmosphere and space.⁴

While there are five main layers in Earth's atmosphere, in meteorology, our main area of focus is the troposphere simply because it's the layer where most weather occurs.² ⁵ 

What is Weather? 🠈 Previous     Next 🠊 Atmospheric Composition

REFERENCES

[1] "Atmosphere." Wikipedia. June 17, 2019. Accessed July 6, 2019. https://en.wikipedia.org/wiki/Atmosphere.

[2] Heidorn, Keith C. "The Atmosphere Home of the Weather." The Weather Doctor. August 15, 2001. Accessed July 6, 2019. http://www.islandnet.com/~see/weather/elements/atmosphr.htm.

[3] Sharp, Tim. "Earth's Atmosphere: Composition, Climate & Weather." Space.com - NASA, Space Exploration and Astronomy News. October 13, 2017. Accessed July 6, 2019. https://www.space.com/17683-earth-atmosphere.html.

[4] "Atmosphere of Earth." Wikipedia. June 13, 2019. Accessed July 6, 2019. https://en.wikipedia.org/wiki/Atmosphere_of_Earth.

[5] "Atmosphere." Met Office: weather and climate change. Accessed July 6, 2019. https://www.metoffice.gov.uk/weather/learn-about/weather/atmosphere.

[6] "Earth's Atmosphere." UCAR Center For Science Education. 2015. Accessed July 6, 2019. https://scied.ucar.edu/shortcontent/earths-atmosphere.

[7] "Introduction to the Atmosphere." National Weather Service. Accessed July 6, 2019. https://www.weather.gov/jetstream/atmos_intro.

[8] Haby, Jeff. "Some Perspective on Gravity." Weather Prediction Education. Accessed July 6, 2019. http://www.theweatherprediction.com/habyhints/142/.

[9] "Why Does the Atmosphere Not Drift off Into Space?" NOAA SciJinks － All About Weather. Accessed July 6, 2019. https://scijinks.gov/pressure/.

[10] Ackerman, Steve and Martin, Jonathan. "How is the atmosphere retained above the Earth?" The Weather Guys. December 3, 2018. Accessed July 6, 2019. http://wxguys.ssec.wisc.edu/2018/12/03/atmosphere/.

[11] Haby, Jeff. "Habytime Mini Lecture 28: Hydrostatic Balance." Weather Prediction Education. Accessed July 6, 2019. https://www.theweatherprediction.com/habyhints2/611/.