What is the Atmosphere?
The atmosphere is a mixture of gases that surrounds the planet. On Earth, the atmosphere helps make life possible. Besides providing us with something to breathe, it shields us from most of the harmful ultraviolet (UV) radiation coming from the Sun, warms the surface of our planet by about 33° C (59° F) via the greenhouse effect, and largely prevents extreme differences between daytime and nighttime temperatures. The other planets in our solar system also have an atmosphere, but none of them have the same ratio of gases and layered structure as Earth’s atmosphere.
Gases in Earth’s Atmosphere
Nitrogen and oxygen are by far the most common; dry air is composed of about 78% nitrogen (N2) and about 21% oxygen (O2). Argon, carbon dioxide (CO2), and many other gases are also present in much lower amounts; each makes up less than 1% of the atmosphere’s mixture of gases. The atmosphere also includes water vapor. The amount of water vapor present varies a lot, but on average is around 1%. There are also many small particles – solids and liquids – “floating” in the atmosphere. These particles, which scientists call “aerosols”, include dust, spores and pollen, salt from sea spray, volcanic ash, smoke, and more.
Layers of Earth’s Atmosphere
The atmosphere grows thinner (less dense and lower in pressure) as one moves upward from Earth’s surface. It gradually gives way to the vacuum of outer space. There is no precise “top” of the atmosphere. Air becomes so thin at altitudes between 100 and 120 km (62-75 miles) up that for many purposes that range of heights can be considered the boundary between the atmosphere and space. However, there are very thin but measurable traces of atmospheric gases hundreds of kilometers/miles above Earth’s surface.
There are several different regions or layers in Earth’s atmosphere. Each has characteristic temperatures, pressures, and phenomena. We live in the troposphere, the lowest layer, where most clouds are found and almost all weather occurs. Some jet aircraft fly in the next higher layer, the stratosphere, which contains the jet streams and the ozone layer. Temperatures reacj their lowest in the mesosphere, because the there are almost no air molecules there to absorb heat energy. The sky also changes from blue to black in the mesosphere, because there are so few molecules for light to refract off of there. And fartherst from the surface we have the thermosphere, which is the widest layer of the atmosphere and absorbs much of the harmful radiation that reaches Earth from th Sun. The exosphere represents the transition from Earth’s atmosphere to space.
The troposphere is the lowest layer of our atmosphere. Starting at ground level, it extends upward to about 10 km (6.2 miles or about 33,000 feet) above sea level. We humans live in the troposphere, and nearly all weather occurs in this lowest layer. Most clouds appear here, mainly because 99% of the water vapor in the atmosphere is found in the troposphere. Air pressure drops, and temperatures get colder, as you climb higher in the troposphere.
The next layer up is called the stratosphere. The stratosphere extends from the top of the troposphere to about 50 km (31 miles) above the ground. The infamous ozone layer is found within the stratosphere. Ozone molecules in this layer absorb high-energy ultraviolet (UV) light from the Sun, converting the UV energy into heat. Unlike the troposphere, the stratosphere actually gets warmer the higher you go! That trend of rising temperatures with altitude means that air in the stratosphere lacks the turbulence and updrafts of the troposphere beneath. Commercial passenger jets fly in the lower stratosphere, partly because this less-turbulent layer provides a smoother ride. The jet stream flows near the border between the troposphere and the stratosphere.
Above the stratosphere is the mesosphere. It extends upward to a height of about 85 km (53 miles) above our planet. Most meteors burn up in the mesosphere. Unlike the stratosphere, temperatures once again grow colder as you rise up through the mesosphere. The coldest temperatures in Earth’s atmosphere, about -90° C (-130° F), are found near the top of this layer. The air in the mesosphere is far too thin to breathe; air pressure at the bottom of the layer is well below 1% of the pressure at sea level, and continues dropping as you go higher.
The layer of very rare air above the mesosphere is called the thermosphere. High-energy X-rays and UV radiation from the Sun are absorbed in the thermosphere, raising its temperature to hundreds or at times thousands of degrees. However, the air in this layer is so thin that it would feel freezing cold to us! In many ways, the thermosphere is more like outer space than a part of the atmosphere. Many satellites actually orbit Earth within the thermosphere! Variations in the amount of energy coming from the Sun exert a powerful influence on both the height of the top of this layer and the temperature within it. Because of this, the top of the thermosphere can be found anywhere between 500 and 1,000 km (311 to 621 miles) above the ground. Temperatures in the upper thermosphere can range from about 500° C (932° F) to 2,000° C (3,632° F) or higher. The aurora, the Northern Lights and Southern Lights, occur in the thermosphere.
Although some experts consider the thermosphere to be the uppermost layer of our atmosphere, others consider the exosphere to be the actual “final frontier” of Earth’s gaseous envelope. As you might imagine, the “air” in the exosphere is very, very, very thin, making this layer even more space-like than the thermosphere. In fact, the air in the exosphere is constantly – though very gradually – “leaking” out of Earth’s atmosphere into outer space. There is no clear-cut upper boundary where the exosphere finally fades away into space. Different definitions place the top of the exosphere somewhere between 100,000 km (62,000 miles) and 190,000 km (120,000 miles) above the surface of Earth. The latter value is about halfway to the Moon!
The ionosphere is not a distinct layer like the others mentioned above. Instead, the ionosphere is a series of regions in parts of the mesosphere and thermosphere where high-energy radiation from the Sun has knocked electrons loose from their parent atoms and molecules. The electrically charged atoms and molecules that are formed in this way are called ions, giving the ionosphere its name and endowing this region with some special properties.
Earth is not the only world with an atmosphere. All of the planets – and even a few moons – in our solar system have atmospheres. Some have clouds, wind, rain and powerful storms. Recently scientists have also begun to get glimpses of atmospheres of planets in other solar systems.
Each of the planets in our solar system has an uniquely structured atmosphere. The atmosphere of Mercury is extremely thin and is not very different from the vacuum of space. All four giant planets in our solar system – Jupiter, Saturn, Uranus and Neptune – have very thick, deep atmospheres. The smaller, rocky planets – Earth, Venus and Mars – have much thinner atmospheres hovering above their solid surfaces. The atmospheres on moons in our solar sytem are typically quite thin. Saturn’s moon Titan is an exception – air pressure at the surface of Titan is higher than on Earth! Of the five officially recognized dwarf planets, Pluto has a thin, seasonal atmosphere containing nitrogen, methane and carbon monoxide, and Ceres may have an extremely thin atmosphere of water vapor. But only Earth’s atmosphere has the layered structure that allows enough light energy to enter and be trapped for warmth, but also shields us from too much harmful radiation. This important balance is necessary to maintain life on Earth.