Atmosphere of Earth

more than other wavelengths by the gases in the atmosphere, surrounding Earth in a visibly blue layer at the stratosphere, above the clouds of the troposphere, when seen from space on board the ISS at an altitude of  (the Moon is visible as a crescent in the far background).]]
The Earth's **atmosphere** is the layer of gases around the Earth. It is held in place by Earth's gravity. It is today  made up mainly of nitrogen (78.1%). It also has plentiful oxygen (20.9%) and small amounts of argon (0.9%), carbon dioxide (~ 0.035%), water vapor, and other gases. The atmosphere protects life on Earth by absorbing (taking) ultraviolet rays from the Sun. It makes our days cooler and our nights warmer.

Solid particulates, including ash, dust, volcanic ash, etc. are small parts of atmosphere. They are important in making clouds and fog.

The atmosphere does not end at a specific place. The higher above the Earth, the thinner the atmosphere. There is no clear border between the atmosphere and outer space, though the Kármán line is sometimes treated as a border. Even higher, for some purposes the edge of the magnetosphere is treated as a border. 75% of the atmosphere is within  of the Earth's surface.

## History of Earth's atmosphere 
An important thing to remember about the Earth and other planets is that *they do not come from the Sun*.  They or their materials were picked up by the Sun's gravitation as it moved through space. The Sun is just composed of hydrogen, with a little bit of helium.  The material which makes up rocky planets and their satellites is almost entirely heavier elements whose origin was in earlier supernovae explosions.  The planets do give off small quantities of hydrogen and helium: this comes from the decay of larger radioactive molecules whose origin is also ancient supernovae.

Originally, the Earth's atmosphere had almost no free oxygen.
#The first atmosphere consisted of gases in the solar nebula, mainly hydrogen. There might also have been simple hydrides such as those now found in the gas giants (Jupiter and Saturn): water vapor, methane and ammonia. 
#The atmosphere gradually changed to mostly carbon dioxide and nitrogen. The lighter gases, like hydrogen and helium, cannot be held by the Earth's gravity, and would escape. For a long time (say two billion years or more), the atmosphere was dominated by carbon dioxide.
#In the Great Oxygenation Event the atmosphere changed to the kind we have now, with oxygen replacing the carbon dioxide. Our atmosphere is still mostly nitrogen, but most living organisms interact more with oxygen than with nitrogen. Oxygenation began with cyanobacteria making free oxygen by photosynthesis. Most organisms today need oxygen for their respiration: only a few anaerobic organisms can grow without oxygen.

## Composition 
Earth’s atmosphere is made up of a mixture of gases that surround the planet and are held in place by gravity. The three main gases in Earth's atmosphere are nitrogen, which makes up about 78% of the air, oxygen, which makes up about 21%, and argon, which makes up about 0.93%. Water vapor is also important and makes up about 0.25% of the atmosphere by mass. The amount of water vapor can change a lot. In very cold places, there is only a tiny amount, about 10 parts per million. In hot, humid areas, it can be as much as 5% of the air. Because water vapor changes so much, scientists often talk about the atmosphere in terms of "dry air" (air without water vapor). The rest of the gases in the atmosphere are called trace gases because they are only found in small amounts. These include important greenhouse gases like carbon dioxide, methane, nitrous oxide, and ozone. Greenhouse gases trap heat and help keep Earth warm. But, too much greenhouse gases can cause global warming. Besides argon, other noble gases such as neon, helium, krypton, and xenon are also present in very small amounts. The air can also contain tiny particles, called aerosols, which can come from nature or human activity. Natural aerosols include dust, pollen, sea spray, spores, and volcanic ash. Some pollutants from factories or vehicles can also be found in the air. These may include gases like chlorine, fluorine compounds, and mercury vapor, as well as sulfur compounds like hydrogen sulfide and sulfur dioxide. Some come from nature, while others are caused by human pollution.

### Major gases 
The Earth's atmosphere is made up of several major gases that help support life and shape our planet's climate. The most common gases are nitrogen, oxygen, and argon, which make up most of the air we breathe. There are also variable gases like water vapor, which change depending on the weather and location. Each of these gases plays an important role in the atmosphere and helps keep Earth livable.

* Nitrogen is the most common gas in Earth’s atmosphere. It makes up about 78% of the air we breathe. That means most of the air around us is nitrogen, not oxygen. Nitrogen is colorless, odorless, and does not easily react with other things. Nitrogen helps keep the air safe and stable. If there were too much oxygen in the air, things could catch fire too easily. Nitrogen acts like a buffer, or a cushion, that helps control this. It keeps the air from being too reactive. Nitrogen has been part of Earth’s atmosphere for a very long time. When Earth first formed over 4.5 billion years ago, the atmosphere was very different and had only some nitrogen. Over time, volcanoes released gases from inside the Earth, and more nitrogen entered the air. Because nitrogen is so stable, it stayed in the atmosphere, while lighter gases like hydrogen escaped into space. Over millions of years, nitrogen built up and became the main gas in the atmosphere, making up 78% of the air we breathe today.
* Oxygen is the second most common gas in Earth’s atmosphere. It makes up about 21% of the air we breathe. This means that out of every 100 parts of air, about 21 parts are oxygen. Even though there is more nitrogen in the atmosphere, oxygen is very important. This is because it is the gas that humans and animals need to breathe. Without oxygen, we could not survive. Plants also use oxygen in small amounts, especially at night, but they produce much more during the day. The form of oxygen we breathe is called O₂, which means two oxygen atoms are bonded together. Oxygen plays many important roles in the atmosphere. The most well-known role is supporting life. All animals, including humans, need oxygen for a process called cellular respiration. This is how our cells make energy from food. Oxygen is also involved in burning, or combustion. Fires cannot burn without oxygen. Oxygen has not always been common in Earth’s atmosphere. When the Earth first formed about 4.5 billion years ago, there was almost no oxygen in the air. Oxygen began to appear in large amounts about 2.4 billion years ago during a time called the Great Oxygenation Event. This happened because tiny microbes called cyanobacteria started to use sunlight to make energy in a process called photosynthesis. A by-product of this process is oxygen, which was released into the air. At first, oxygen built up slowly because it reacted with iron and other materials on Earth. But eventually, the oxygen levels rose high enough to change the atmosphere and allow new life forms to evolve. The rise of oxygen in the atmosphere was one of the most important events in Earth’s history. It allowed complex life forms like plants, animals, and eventually humans to appear. It also changed the chemistry of the oceans and rocks. Today, oxygen continues to be produced by plants, especially in forests and oceans, where tiny plants called phytoplankton live.
* Argon is a gas that makes up about 0.93% of the Earth's atmosphere. That means it is the third most common gas in the air, after nitrogen and oxygen. Even though it makes up less than 1% of the atmosphere, it is still more common than other gases like carbon dioxide and methane. Argon is a noble gas, which means it is very stable and does not easily react with other elements. Because it is so unreactive, argon mostly just "sits" in the atmosphere without changing or causing chemical reactions. Argon comes from the decay of a radioactive element in rocks called potassium-40. Over time, this decay releases argon gas, which escapes into the air. Since argon is very stable and does not break down or disappear, it has slowly built up in the atmosphere over millions of years. Unlike oxygen or water vapor, argon does not play a big role in supporting life, but its steady presence helps scientists understand the age and history of the atmosphere. Argon is also used in science labs, light bulbs, and welding because it does not react with the materials around it.
* Water vapor is the gas form of water. It is a variable gas in the Earth's atmosphere. This means its amount can change a lot from place to place and day to day. In hot and wet places, like rainforests or tropical oceans, water vapor can make up as much as 4% of the air. In cold and dry places, like deserts or the polar regions, it might be close to zero percent. This variability is because water vapor enters the atmosphere through evaporation, i.e when water turns into gas from oceans, lakes, rivers, and even from plants. When the air cools down, the water vapor can condense to form clouds, fog, or dew, and then return to the ground as rain or snow. Water vapor also acts as a powerful greenhouse gas. This means it helps trap heat in the Earth’s atmosphere, keeping the planet warm enough for life. In fact, water vapor is the most abundant greenhouse gas, and it works together with carbon dioxide and others to regulate Earth’s temperature. When the air gets warmer, it can hold more water vapor, and this can lead to even more warming, creating a feedback loop. Besides affecting temperature, water vapor is also important for weather. It helps form clouds, fog, and storms. Weather patterns like rain, snow, and humidity all depend on how much water vapor is in the air.

### Trace gases 
The Earth's atmosphere contains not only major gases like nitrogen and oxygen but also trace gases, which are present in very small amounts. Even though these gases make up less than 1% of the atmosphere, they are very important. Some trace gases, like carbon dioxide, methane, ozone, and nitrous oxide, help regulate Earth’s temperature and support life. Others come from natural sources or human activities and can affect the climate and air quality.

* Carbon dioxide, or CO₂, is a gas that makes up a small part of Earth’s atmosphere, but it plays a very important role. It makes up about 0.04% of the air. This may seem like a tiny amount, but this small percentage has a big effect on climate and life. Carbon dioxide is made of one carbon atom and two oxygen atoms. It is released into the atmosphere when people and animals breathe out, when plants and animals decay, and when things burn, like wood, coal, oil, or gas. It is also released by volcanoes and by certain natural processes in the ocean and soil. Even though CO₂ is a small part of the atmosphere, it is one of the main greenhouse gases. This means it traps heat in the Earth’s atmosphere, keeping the planet warm enough for life. Without carbon dioxide and other greenhouse gases, Earth would be far too cold to live on. Carbon dioxide is also part of the carbon cycle, which is the movement of carbon between the air, land, water, and living things. Plants take in carbon dioxide during photosynthesis, using it to make food and releasing oxygen back into the air. Animals eat the plants and get the carbon, and when they breathe or die, the carbon returns to the atmosphere. This cycle has kept CO₂ levels fairly balanced for millions of years. However, in recent times, human activities, especially burning fossil fuels and cutting down forests, have added large amounts of carbon dioxide to the air. This extra CO₂ is causing the planet to warm up, leading to climate change, which can result in rising sea levels, stronger storms, and changing weather patterns. Scientists around the world are studying carbon dioxide closely because managing its levels is important to protecting Earth’s environment.
* Besides argon, there are other noble gases in the Earth's atmosphere, but they are found in very tiny amounts. These trace noble gases include helium, neon, krypton, and xenon. They make up only a very small fraction of the air (less than 0.001% each). They are inert, which means they do not react easily with other elements. Because they are so unreactive, they mostly just "sit" in the atmosphere without changing or causing chemical reactions. Helium, for example, is the second lightest element and often escapes Earth's gravity into space. It mostly comes from radioactive decay deep in the Earth. It is released through volcanic activity. Neon is more common than krypton or xenon and is also very stable. It has useful properties, such as glowing when electricity passes through it, which is why it is used in neon signs. Krypton and xenon are even rarer than neon. They exist in tiny amounts but are important for scientific research and technology. These gases are used in certain types of lighting, lasers, and even spacecraft engines. Because they are so stable, they do not break down or combine with other gases, and they do not take part in processes like breathing or the greenhouse effect. These noble gases help scientists understand the history and structure of Earth’s atmosphere. Their stable nature allows researchers to use them to study air movements, age of ice cores, and the origin of gases in Earth’s atmosphere and beyond.
* Hydrogen is the lightest and most basic element in the universe. It can be found in very small amounts in the Earth’s atmosphere. It makes up only about 0.00005% of the air. Even though hydrogen is extremely common in the universe, especially in stars and interstellar clouds, it is rare in our atmosphere. This is because hydrogen is so light that it can easily escape Earth’s gravity and drift off into space. Most of the hydrogen in the atmosphere comes from natural processes, like the breakdown of methane or other gases, and from some human activities like burning fossil fuels or chemical industries. It can also be produced when water molecules are split by sunlight or electrical energy in a process called photolysis. Hydrogen is involved in certain chemical reactions in the upper atmosphere. For example, it can react with oxygen to form water vapor, and it can help control the amount of ozone in the air.
* Besides the more well-known gases, the Earth’s atmosphere also contains other trace gases that are present in tiny amounts but are still important. Some of these include methane (CH₄), ozone (O₃), and nitrous oxide (N₂O). Even though these gases make up less than 1% of the air, they have very powerful effects on the climate and air quality. Methane is a gas that comes from natural sources like wetlands, as well as human activities such as farming, landfills, and oil and gas production. It is a very strong greenhouse gas, even stronger than carbon dioxide, though it does not last as long in the atmosphere. Nitrous oxide, also known as “laughing gas,” is another greenhouse gas. It comes from soil bacteria, fertilizers, and burning fossil fuels. Though present in small amounts, it also traps heat and contributes to global warming. Another important trace gas is ozone, which is found in two main layers of the atmosphere. In the upper atmosphere, or stratosphere, ozone forms the ozone layer, which protects life on Earth by blocking harmful ultraviolet (UV) rays from the sun. This layer is essential for keeping plants, animals, and people safe from too much UV radiation. However, ozone near the ground level, in the lower atmosphere, can be harmful. It forms when sunlight reacts with pollution from cars and factories, creating smog that can cause breathing problems and other health issues. Scientists closely monitor them to understand climate change, air pollution, and how human actions are affecting the planet.

### Vertical composition 
As you move upward through the Earth's atmosphere, the types and amounts of gases begin to change. Near the ground, in the lowest layer called the troposphere, the air is thick and full of oxygen, nitrogen, carbon dioxide, water vapor, and other gases. This is where we live and breathe. It contains about 75% of all the air’s mass. Here, the gases are well-mixed because of winds and weather, so no matter where you go in this layer, the main gases stay in the same balance. Above the troposphere is the stratosphere, where the air is thinner, but the mix of gases like nitrogen and oxygen is still mostly the same. One important difference, though, is the ozone layer found in the stratosphere, which has a much higher concentration of ozone (O₃) than other layers. This ozone is important because it absorbs harmful ultraviolet (UV) rays from the sun, protecting life on Earth. This whole part of the atmosphere is called the homosphere. In the homosphere, the main gases, like nitrogen and oxygen, are mixed evenly. This means that, no matter where you go in this region, the air has about the same mix of gases. The homosphere stretches from the ground all the way up to about 80 kilometers (around 50 miles) above Earth. It includes the troposphere, stratosphere, and part of the mesosphere. The reason the gases stay so well mixed is because of winds, air currents, and turbulence that constantly stir the atmosphere. Even though water vapor and some tiny gases can change depending on the weather, the overall mix of air in the homosphere stays about the same.

As we keep going higher into the mesosphere and then the thermosphere, the air becomes very thin, and the gases begin to separate by their weight. This is the start of the heterosphere, where the gases are no longer mixed evenly. In this region, heavier gases like oxygen and nitrogen stay closer to the bottom, while lighter gases such as helium and hydrogen rise higher up. In the very top layer, called the exosphere, only the lightest gases remain, especially hydrogen and helium, and they are very spread out. Some of them even escape into outer space. Because the air is so thin up high, there is not enough mixing to keep the gases evenly spread out.

## Temperature and the atmospheric layers 
Some parts of the atmosphere are hot or cold, depending on height. Starting from the surface and climbing straight up, the air gets colder in the troposphere, but then it becomes hotter, higher in the stratosphere. These changes of temperature are divided into layers. These are like layers of an onion. The difference between the layers is the way the temperature changes.

These are the layers of the atmosphere, starting from the ground:
*Troposphere - Starts at the ground. Ends somewhere between . The higher, the colder. Weather in this layer affects our daily life.
*Stratosphere - Starts at . Ends at . The higher, the hotter. The heat comes from the Ozone layer at the top of the stratosphere. There is little water vapor and other substances in this layer. Airplanes fly in this layer because it is usually stable and air resistance is small.
*Mesosphere - Starts at . Ends at . The higher, the colder. Winds in this layer are strong, so the temperature is not stable.
*Thermosphere - Starts at . Ends at  or higher. The higher, the hotter. This layer is very important in radio communication because it helps to reflect some radio waves.
*Exosphere - Above the thermosphere. This is the top layer, and merges into interplanetary space.

Regions where one layer changes to the next have been named "-pauses." So the tropopause is where the troposphere ends ( high). The stratopause is at the end of the stratosphere. The mesopause is at the end of the mesosphere. 
These are boundaries.

The average temperature of the atmosphere at the surface of Earth is .

## Pressure 
The atmosphere has pressure. This is because even though air is a gas, it has weight. The average atmospheric pressure at sea level is about .

## Density and mass
The density of air at sea level is about 1.2 kilograms per cubic meter.  This density becomes less at higher altitudes at the same rate that pressure becomes less. The total mass of the atmosphere is about 5.1 &times; 1018 kg, which is only a very small part of the Earth's total mass.

## Related pages 
* Air
* Life timeline

## References 
## Other websites 
*The Atmosphere