What Is the ‘Atmosphere’?
The atmosphere of Planet Earth consists of a layer of gases (mostly nitrogen and oxygen) that we call “air”, held in place around the Earth by the force of gravity. Without the atmosphere, life would be impossible.
The atmosphere makes Earth habitable for humans in four ways. First, it contains the air we breathe. Second, it absorbs harmful ultraviolet rays from the sun. Third, it maintains a cosy surface temperature of 15°C (59°F) via the greenhouse effect – a mechanism which acts as a thermal blanket preventing some heat from escaping into space. Unfortunately, humans have unbalanced this mechanism by pumping too many greenhouse gases into the air, causing the planet to overheat. Fourth, through its clouds, it helps to minimize the drop in temperature from day to night (the so-called diurnal temperature variation).
The atmosphere also plays an integral role in several biogeochemical cycles, such as: the carbon cycle, the sulfur cycle, the nitrogen cycle and the water cycle. This helps it to regulate the climate system, although global warming is interfering with several of the normal geophysical processes.
- What Is Gravity?
- What Is The Atmosphere Made Of?
- How Big Is It?
- What Are The Main Layers?
- What Are The Main Pollutants In The Atmosphere?
- How Is The Atmosphere Connected To Climate Change?
- What Are The Main Greenhouse Gases In The Atmosphere?
- How Did The Atmosphere Evolve?
- How Does Earth’s Atmosphere Compare With Venus And Mars?
- Earth’s Other Spheres
What Is Gravity?
Gravity is a natural “pulling” force generated by anything with mass or energy (like a planet), that pulls toward it anything that has less mass or energy. Gravity is what gives “weight” to our bodies or any physical objects. At the beginning of the Universe, it was gravity that caused gaseous matter to come together forming stars and other structures in space. Without gravity, the atmosphere around Earth would gradually float off into space.
What Is The Atmosphere Made Of?
The atmosphere contains roughly 78 percent nitrogen, 21 percent oxygen, 0.93 percent argon, 0.04 percent carbon dioxide, plus trace amounts of hydrogen, helium, krypton, methane, neon, xenon, nitrous oxide, and ozone. 1
The atmosphere also contains a variable amount of water – either in gaseous form as water vapor, or in liquid form as clouds. On average, water content of the atmosphere is roughly 1 percent at sea level, or 0.25 percent overall.
The specific composition of air, as well as its temperature and pressure, varies according to altitude. Only the air closest to the Earth’s surface is suitable for humans who need oxygen to breathe, and for plants who need carbon dioxide to activate photosynthesis for energy and growth.
How Big Is It?
Earth’s atmosphere extends outwards for about 480 km (300 miles), although three quarters of it occupies the space closest to Earth, that is, within about 11 kilometers (36,000 feet) of the surface. This is the densest part. The higher you go, the thinner and thinner the atmosphere becomes. (There is also less oxygen to breathe.)
There is no natural dividing line between the atmosphere and outer space. Instead, the Karman line – an imaginary line 100 km above Earth’s surface beyond which the atmosphere is too thin to support aeronautical flight – is used as the border between the Earth’s atmosphere and outer space. 2
Like atmospheric density, air pressure also decreases the higher you go. At sea level, air pressure is roughly 1 kilogram per square cm (14.7 pounds per square inch). At 10,000 feet (3 km), the air pressure decreases to 0.7 kg per square cm (10 pounds per square inch).
What Makes Life Habitable On Earth?
Two things. First, Earth sits comfortably within the so-called “Goldilocks Zone”, the habitable zone around the Sun: if it was closer, it would burn up; if it was much further away, it would freeze. Second, it is the only planet in our solar system which has an atmosphere that can sustain life.
What Are The Main Layers?
Earth’s atmosphere is divided into five main layers: the troposphere, the stratosphere, the mesosphere, the thermosphere, and the exosphere. The air gets thinner with each ascending layer until all gas finally dissipates in space.
The troposphere rises from the Earth’s surface to an average height of 13km (8 miles). Because the troposphere is mostly heated via energy transfer from the surface, air is warmer near the ground and gets cooler the higher you go. The temperature ranges between 15°C (59°F) near the surface, to minus 51°C (minus 60°F) at the top. The troposphere varies in altitude. According to the National Weather Service, it’s about 11-12 miles (18-20 km) high at the equator; about 5½ miles high at mid-latitudes of 50°N and 50°S; and roughly 4 miles high at the poles. 3
The troposphere contains about four-fifths of the mass of Earth’s atmosphere. 4 About 50 percent of the total mass of the atmosphere is compressed into the first 5km (18,000 feet) of the troposphere. This includes the vast majority of all water vapor and moisture in the atmosphere, hence it is in the troposphere that most of Earth’s weather takes place. Air pressure drops, and temperatures get colder, as you climb higher in the troposphere.
The stratosphere sits above the troposphere and is separated from it by the tropopause. It extends roughly 12 to 50km (7 to 31 miles) above the Earth’s surface. The stratosphere’s temperature ranges from 51°C (minus 60°F) at its foot, to minus 15°C (5°F) at the top.
Like the troposphere, the stratosphere also varies in altitude. It is thicker over the poles and thinner over the equator.
The lower stratosphere contains the “ozone layer“. This absorbs harmful ultraviolet light from the Sun, notably UVB-type rays, converting them into heat. (Thus, unlike the troposphere, the stratosphere gets warmer the higher you go.)
Exposure to UVB radiation is associated with an increased risk of sunburn, skin cancer and serious eye problems. Because of its protective role, stratospheric ozone is sometimes called “good” ozone. At any rate it should not be confused with the “bad” ozone lower down in the troposphere, which is engendered by a chemical reaction between UV radiation and combustion gases from vehicle exhausts. 5 Tropospheric ozone is a key component of photochemical smog, linked to a range of serious respiratory complaints.
The stratospheric temperature gradient leads to very stable conditions, so the stratosphere has none of the air turbulence that is so prevalent in the troposphere. Because of this, the stratosphere is almost completely devoid of clouds (except for occasional polar clouds) and other forms of weather. As a result, most jets (and weather balloons) fly in the lower stratosphere (30,000–39,000 feet), just above the jet stream. This gives them a smoother, more fuel-efficient ride.
Amazingly, some bird species have been recorded flying in the upper levels of the troposphere. In November 1973, for instance, a vulture was sucked into a jet engine at a height of 37,000 feet above the Ivory Coast, in Africa. 6
On top of the stratosphere, the mesosphere extends roughly 50 to 80 km (31 to 50 miles) above the Earth’s surface. Here, temperature decreases with height – from minus 15°C (5°F) at its foot, to as low as minus 120°C (minus 184°F) at its top.
The top of the mesosphere, known as the mesopause, is the coldest place in Earth’s atmosphere, with temperatures averaging about minus 130°F (minus 90°C). The mesosphere is extremely difficult to study. It lies above the altitude for aircraft, while only its first few kilometers are accessible to weather balloons, for which the altitude record is 53.0 km.
The thermosphere sits on top of the mesosphere, roughly 80 to 700 km (50 to 440 miles) above the Earth’s surface. The thermosphere’s temperature increases with height and ranges from minus 120°C (minus 184°F) at its foot, to 2,000°C (3,600°F) at its top.
The reason for the high temperatures is that the thermosphere absorbs nearly all the incoming high-energy X-rays and extreme ultraviolet radiation (XUV) entering the atmosphere from the Sun.
The thermosphere is technically part of Earth’s atmosphere, but the air is so thin that most of it is normally thought of as outer space. Because of this it is completely free of clouds and water vapor. The thermosphere is where the space shuttles used to fly and where the International Space Station orbits at an altitude of between 350 and 420km (220 and 260 miles). It is also the layer where the auroras occur. Charged particles entering the atmosphere from outer space bump into atoms and molecules in the thermosphere, energizing them in the process, causing them to shed this excess energy by giving off light, which is visible from Earth as the Aurora Borealis and Aurora Australis.
The exosphere is the outermost layer of the atmosphere, reaching up to 6,200 miles (10,000 km) above the surface of the Earth. The exosphere’s temperature varies enormously from 0 to over 1700°C.
The exosphere is mainly composed of hydrogen and helium, along with some heavier molecules of nitrogen, oxygen and carbon dioxide. The exosphere is where the atmosphere merges into outer space, so not surprisingly the air is super-thin. In fact, the atoms and molecules are so far apart that they can travel hundreds of miles without bumping into one another. As a result, the exosphere no longer behaves like a gas, and its particles constantly leak into space.
Why Is The Sky Blue?
Sunlight is scattered in all directions by gases and particles in the atmosphere, a phenomenon known as Rayleigh scattering. Blue (and purple) light is dispersed more than the other colours because it travels as shorter, smaller waves. This alone would cause the sky to appear blue and purple, except our eyes see better at frequencies near the middle of the spectrum (yellow/green). Since blue is closer to yellow/green than purple is, the sky we see appears blue.
What Are The Main Pollutants In The Atmosphere?
Atmospheric pollutants include natural aerosols, such as volcanic ash and mineral dust – the Amazon rainforest receives 182 million tons of phosphorus-dust from the Sahara 7 – as well as wood-ash, pollen and spores.
Anthropogenic pollutants may also be present, such as hydrofluorocarbons, methane, mercury vapor, hydrogen sulfide and sulfur dioxide.
How Is The Atmosphere Connected To Climate Change?
Climate change is happening because Earth’s temperature is rising. Why is the temperature rising? Because humans are burning too many fossil fuels. This triggers the release of ever-increasing quantities of carbon dioxide (and other greenhouse gases) that then accumulate in the troposphere – a major carbon reservoir – where they trap heat given off by Earth, leading to global warming.
By digging up and burning fossil fuels like coal, peat, petroleum and natural gas – all of which are rich in ancient carbon – we are upsetting nature’s carbon cycle – a continuous process whereby Earth’s finite supply of carbon is shared by all living organisms for the benefit of the entire planet.
There are two basic ways of repairing the damage to the CO2-soaked atmosphere. First, fossil fuels must be replaced by renewable energy that doesn’t contain any carbon. Second, we must absorb extra carbon from the the atmosphere by conserving our stocks of trees and plants, that soak up CO2 during photosynthesis. In addition, we should plant more trees to enhance the absorption process.
What Are The Main Greenhouse Gases In The Atmosphere?
Based on U.S. figures, the most prevalent man-made greenhouse gases in Earth’s atmosphere are carbon dioxide (82 percent), methane (10 percent), nitrous oxide (6 percent) and various fluorinated gases (3 percent). 9 However, while carbon dioxide is far more prevalent than other greenhouse gases, it has a lower global warming potential (GWP) than the others. For example, over 20 years, methane is 84 times more powerful, and nitrous oxide is 264 times more powerful (IPCC Fifth Assessment Report 2013). 10
Clouds form from water vapor that condenses into liquid water when the surrounding air cools. The liquid water typically collects around particles of dust, ice or sea salt – all known as condensation nuclei. The type of cloud that forms is determined by temperature, wind and other conditions. For more, see: How Do Clouds Affect Climate?
How Did The Atmosphere Evolve?
Earth is thought to have been formed about 5 billion years ago. During the first phase – lasting some 500 million years – the vapor and gases that were expelled during the formation of Earth’s interior formed a dense atmosphere around the planet. It is believed that Earth’s original atmosphere consisted of methane, ammonia, water vapour, and the noble gas neon. 11 It was 100 times as dense as today’s atmosphere.
At the same time – around 4 billion years ago – the hydrosphere was formed as a result of condensation of water vapor, which created oceans of water.
The most notable aspect of this prehistoric environment was the complete absence of free oxygen, a state evidenced by anaerobic rock formations that are not replicated in any rocks less than 3 billion years old. This is because about 3.5 billion years ago, subterranean plate tectonics began rearranging the continental land masses, causing the appearance of new rock formations. These rocks absorbed enormous amounts of carbon dioxide – part of the slow carbon cycle – facilitating a shift to a more oxygen-abundant atmosphere. See: The Oxygen Cycle.
About one billion years ago, primitive aquatic microorganisms known as cyanobacteria (blue-green algae) began using energy from the Sun to split molecules of water and carbon dioxide in order to create energy and oxygen, in a process we now call photosynthesis. Some of the photosynthetically created oxygen combined with organic carbon to recreate CO2 molecules. The remaining oxygen accumulated in the atmosphere, although it did not exceed 10 percent of its present levels.
Up in the stratosphere, some oxygen (O2) molecules absorbed energy from the Sun’s ultraviolet (UV) rays and disassociated to form single oxygen atoms (O). These atoms combining with the surviving regular oxygen (O2) to form ozone (O3) molecules, which proved to be exceptionally good at absorbing solar UV rays.
By 600 million BC, enough ozone had been created in the stratosphere to protect the planet from biologically lethal UV radiation, to justify calling it an “ozone layer”. Up to this point, life had been confined to the ocean. Now, thanks to the protection offered by the ozone layer, organisms were able to develop on land. In this way, ozone played a pivotal role in the evolution of life on Earth.
How Does Earth’s Atmosphere Compare With Venus And Mars?
The atmosphere of Venus is predominantly carbon dioxide, with traces of nitrogen and sulfuric acid. The surface of the planet suffers from a runaway greenhouse effect, with enormous air pressure (more than 90 times heavier than Earth), and fiery surface temperatures approaching 465°C/ 870°F. The clouds are so dense that the surface is invisible in the light, and since little sun ever reaches the surface, Venus has no significant seasonal fluctuations in temperature.
The atmosphere on Mars is also mainly carbon dioxide, with traces of argon, carbon monoxide, nitrogen, oxygen and a number of other gases. The present atmosphere is about 100 times thinner than Earth’s – very different from the past, when water used to flow on the surface around 4.5 billion years ago. Scientists think that the Martian atmosphere gradually thinned because the sun stripped away the lighter molecules in the atmosphere, or perhaps was ripped away after the planet collided with an asteroid or comet. Mars experiences significant temperature swings partly due to the varying amounts of sunlight reaching the planet’s surface, which also impacts on its polar ice caps.
Compared to Venus and Mars, neither of whom have an oxygen-rich atmosphere or a manageable temperature, Earth looks decidedly fortunate. One very good reason why we should reduce our greenhouse gas emissions to prevent our atmosphere from being saturated with carbon dioxide. We don’t want to end up like our planetary sisters.
Earth’s Other Spheres
The Atmosphere connects with all the other ‘spheres’ that make up the different geophysical compartments of Earth. It connects with the lithosphere through volcanic eruptions of aerosols and other gaseous matter. It interfaces directly with the pedosphere (Earth’s soil layer), the hydrosphere (the oceans and other liquid water), and the cryosphere (Earth’s frozen water), with whom it shares biogeochemical pathways for carbon, nitrogen, water and other vital chemicals. And it also provides warmth and life-giving nutrients to the biosphere and its rich biodiversity of animals and plants.
For more about the timeline of our planet and the evolution of its atmosphere, please see: History of Earth in One Year (Cosmic Calendar).
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