Layers of the Earth's atmosphere
Atmosphere, derived from atmos meaning vapor and sphaira meaning sphere, is composed of gases that surround our planet. It is held in place by the Earth's gravity. It has about 78.08% nitrogen, 29.95% oxygen, 0.38% carbon dioxide and 0.93% argon. The atmosphere also contains very small amounts of other gases and about 1% water vapor.
Earth's atmosphere is thicker at the innermost radius around the Earth and starts becoming thinner towards outer space. It is composed of five layers, where each layer plays a significant role in supporting life on Earth. The outermost layer marks the Earth's territory from outer space. The proportions of gases and their temperatures vary across layers, thus making the composition of each layer different from the other. The composition of each atmospheric layer influences its properties and leads to various atmospheric phenomena.
Starting from the lowest, the 5 layers of the Earth's atmosphere are troposphere, stratosphere, mesosphere, thermosphere, and ionosphere. Troposphere is the first layer above the Earth's surface and contains around half of the Earth's atmosphere. Stratosphere, the stable layer is next, followed by mesosphere housing meteors and rock fragments. Thermosphere comes fourth. Above it lies the last atmospheric layer, ionosphere. It is a very thin layer that meets outer space through the exosphere.
The word is derived from Greek tropos meaning 'change' or 'turning'. It begins from the Earth's surface and extends to between 7 km at the poles and 17 km at the equator. It is wider at the equator and thins towards the poles.
Troposphere has a fairly uniform chemical composition, except for its water vapor content that decreases with height. Water vapor concentration is very less in the polar regions as compared to about 4% in the tropics. 99% of the water vapor in the Earth's atmosphere is contained in the troposphere.
The water vapor helps regulate air temperature as it absorbs solar energy and thermal radiation. The heating caused by solar energy reduces the density of air, causing it to rise. In the process of opposing the surrounding air, the air in this layer expends energy, resulting in a decrease in its temperature. The decreasing temperature causes the vapor content of the air to condense, resulting in an increase in the air mass. The troposphere contains about 80% of the total mass of the atmosphere.
Apart from water vapor, nitrogen (78%), oxygen (21%) and traces of argon and hydrogen ozone are present in this layer. The air we breathe is from the troposphere. Smog is also a constituent of the troposphere. Smoke and pollutants together with the fog present in this layer lead to production of smog. Lately, the percentage of carbon dioxide present in this layer has started rising, increasing the threat of global warming and greenhouse effect on Earth.
Weather changes occur in this layer of the atmosphere. The air at the equator is heated more than that at the poles, thus resulting in uneven heating by the Sun. This leads to convection currents in the air, causing movement of winds and moisture. Convection (heat transfer caused by motion of molecules within fluids or gases) facilitates vertical movement of heat in the troposphere. Horizontal heat transfer is achieved with the mechanism of advection (heat transfer caused by bulk motion of the fluid or gas).
Thunderclouds in troposphere
Troposphere hosts the water cycle. Friction between air and the Earth's surface propels dust particles in the air. Their coming in contact with moisture leads to cloud formation, further leading to precipitation in the form of rains or snow.
The temperature of the troposphere drops with increase in altitude. The temperature at the bottom of the troposphere is 59 degrees F. For every 1,000 feet increase in height, the temperature drop is of 3.6 degrees F. This rate of change of temperature is known as the lapse rate. The temperature at the top of the troposphere can reach up to -110 degrees F. The temperature may be higher in areas where temperature inversion occurs (is an inversion layer in the atmosphere where temperature does not decrease with increasing height).

Did You Know?
According to a recent study, living bacteria were found in the middle and upper troposphere. It is yet to be known how they got there, what they thrive on and what role they play in cloud formation and precipitation.
The boundary between the troposphere and the stratosphere is known as the tropopause. It can be defined as the point where the lapse rate changes from positive to negative. In other words, it's the point where air temperature stops decreasing with altitude.
The word is derived from the Latin word stratus meaning 'spreading out'. It begins above the troposphere and extends up to around 50 km. The bottom of the stratosphere starts at about 16 km near the equator and decreases up to 8 km near the poles.
It is the stratosphere which contains the ozone layer. The ozone layer is located between 15 and 35 km above the Earth's surface. The ozone layer contains ozone gas. This layer absorbs the sun's ultraviolet radiation, which can prove to be harmful to life on Earth.
The temperature-to-altitude relation in the stratosphere is opposite to that of the troposphere. Temperatures in this layer increase with a rise in altitude. That's the reason there is less convection and mixing in this layer, thereby making the layer relatively stable. Airflow from the stratosphere to the layers above or below it, occurs due to atmospheric waves and tides (oscillations in the atmosphere caused by factors like heat, gravity and the day-night cycle).
Most of the jet aircraft fly through the stratosphere avoiding the turbulence they face in the troposphere. Due to less convection in the layer, anything that gets there can stay there for long. This happens with CFCs from aerosols or other gases that are propelled into the stratosphere, by meteorites, volcanic eruptions or rockets.
Polar stratospheric clouds
The air in the stratosphere is dry and has very little water vapor. Due to lack of humidity, there is less cloud formation in this layer. In extremely low temperature conditions during the polar winters, clouds form in this layer. They are known as polar stratospheric clouds (PSCs) or nacreous clouds and they form at temperatures below -78 degrees C. They reflect sunlight to the ground and appear bright in color.
They differ in their chemical composition. Those containing water, nitric and sulfuric acids are responsible for ozone depletion in the polar regions. Those formed due to local cooling of the lower stratosphere contain water ice only and are observed in the northern polar regions.
Did You Know?
In 2009, Science Daily reported of the discovery of a new bacteria species in the upper stratosphere. It was discovered by Indian scientists at ISRO. With the joint efforts of CCMB, Hyderabad and NCCS, Pune in India, the samples were analyzed to conclude that there were 12 bacterial and 6 fungal colonies from which 9 were genetically similar to species on Earth.
Birds in the Stratosphere!
There have been reports of birds flying in the lower stratosphere. Ruppell's Vulture (considered as the highest-flying bird) had flown at a height of 11 km above sea level. Bar-headed geese are believed to have flown over Mt. Everest.
The boundary between the stratosphere and the mesosphere is known as the stratopause. In the stratosphere, temperature increases with height. Temperature attains a maximum value in the stratopause. It is located at 55 km above the Earth's surface.
The name is derived from Greek word mesos meaning 'middle'. It stretches from 50 km to around 80 to 85 km. The temperature of air in this layer decreases with increase in the height. Most of the meteors and rock fragments burn up in this layer before they can enter the Earth's stratosphere. It's due its composition, that meteors heat up and vaporize in this layer.
Polar mesospheric clouds
High up in the mesosphere, noctilucent clouds form over polar regions. They are also known as the polar mesospheric clouds. They form at heights unusual for any other type of clouds to form. They are thought to be made of ice crystals. They are resistant to UV radiation and glow brightly. The mesosphere also experiences lightnings called elves or sprites that are formed much above the thunderclouds in the troposphere.
Mesosphere has a sodium layer that is 5 km deep and made of un-ionized sodium atoms. This layer radiates and contributes to airglow (glowing of atmospheric air due to which the night sky never appears completely dark).
Air temperatures lower with increasing altitude. The top of the mesosphere called mesopause is the coldest part of the Earth's atmosphere. The temperature there is as low as -90 degrees C. Reduced solar heating and radiative emission of carbon dioxide leads to such low temperatures in this layer. This layer also shows tidal turbulence to gravity and planetary waves and also to zonal winds. Planetary waves and gravity waves generated in the lower layers propagate to the mesosphere and dissipate over there.
The air in the mesophere is very thin; so thin that gas atoms or molecules hardly ever interact with each other. It is too thin to give the thrust that aircraft require, making this layer unsuitable for planes to fly. It is this layer where 'space' begins.
Did You Know?
The height and composition of the mesosphere make it difficult to gather any data about it. Sounding rockets have been used to study this layer. Till today, scientists know very little about it.
The boundary between the mesosphere and the thermosphere is known as the mesopause. It can be defined as the area 80 km above the Earth's surface which forms the upper boundary of the mesosphere or the point in the upper mesosphere where atmospheric temperature reaches its lowest point.
The name is derived from Greek thermos meaning 'heat'. It extends from about 85 km to more than 640 km. The temperature of air in this layer increases with height in the lower regions after which it remains steady. A small change in energy can cause a large change in the air temperature of this layer. The temperature in this layer can rise up to 1,500 degrees Celsius or higher. It is the hottest layer in the atmosphere.
Oxygen present in this layer absorbs solar radiation, increasing the temperatures in this layer. The thermosphere is hotter during the day than at night. It absorbs most of Sun's heat. Sometimes the air expands, leading to an increase in height of the top of the thermosphere.
The air in this layer is very thin due to which gas particles hardly encounter each other. The high-energy UV and X-ray photons break down gas molecules. Thus, air in the upper thermosphere contains atomic oxygen, atomic nitrogen and helium. They even break apart electrons from gas particles, producing highly charged ions. These electrically charged ions and their collisions with electrically neutral gases give rise to electrical currents in some parts of the thermosphere.
Aurora is an interesting phenomenon that occurs in the thermosphere. At higher altitudes, charged particles collide with atoms and molecules exciting them into higher energy states. These excited atoms and molecules give out energy by emitting light which can be seen as a colorful aurora (also called northern and southern lights).
At a height of about 160 km, there exists an anacoustic zone. As the name suggests, it's the region where transmission of sound cannot take place. It's due to the thinness of air that does not allow molecules to interact, thereby making transmission of sound impossible.
Space shuttles orbit through the thermosphere. The International Space Station has an orbit between 320 and 380 kms in this layer. Due to changes in the density of air in this layer, satellites experience a drag force.
The outer boundary of the thermosphere is known as the thermopause. It is at a height of 500 to 1000 kms or higher.
That part of the atmosphere, which is ionized by solar radiation, is known as the ionosphere. As some scientists call it an extension of the thermosphere, it may not be regarded as a separate atmospheric layer. The temperature increases with increase in height. The ionosphere constitutes about 0.1% of the atmospheric mass. It forms the inner layer of the magnetosphere, or simply, the sphere of influence of Earth's magnetic/gravitational force. Even the ionosphere experiences auroras.
Based on which wavelength of sunlight is absorbed most frequently, the ionosphere is divided into three layers, namely D, E and F. The D region is roughly below 90 km and absorbs hard x-rays. The E region extends to about 150 km and absorbs soft x-rays. The F region extends up to about 600 km and absorbs extreme ultraviolet rays. It is further divided into layers F1 and F2. Layer F1 extends from 150 km to 240 km while F2 extends from 240 km to about 400 km. The density of electrons in this layer is higher than that in F1. The D and E layers weaken and even disappear during the night. They reflect radio waves of longer wavelengths (AM) while the F region reflects radio waves of shorter wavelengths. That is, layers D and E reflect low-frequency radio waves and the F layer reflects high-frequency radio waves.
The ionosphere allows the propagation of radio signals to distant places on Earth. It reflects radio waves back to Earth, thus facilitating radio communication. Free electrons in the ionosphere aid propagation of electromagnetic waves. Research is being done on the use of electromagnetic tethers (conducting wires or cables that use the ionosphere to extract energy from the Earth's magnetic field).
The word is derived from ancient Greek exo meaning 'outside' or 'external'. It is where the Earth's atmosphere meets the outer space. It houses free-moving particles that may migrate from the magnetosphere. It ranges from about 500-1000 km up to 10,000 km. Exosphere is the upper limit of the atmosphere. This layer is where atoms and molecules escape into space. The atmosphere becomes very thin in this layer.
This layer contains hydrogen and traces of helium, carbon dioxide and atomic oxygen. The air is so thin that it can be compared with the vacuum in space. Gas particles in the exosphere travel along curved paths. They either come down into the lower atmosphere due to Earth's gravitational pull or escape into outer space.
The lower boundary of the exosphere is known as the exobase or thermopause. It stretches from 500 to 1000 km. The height of the exobase varies with changes in the intensity of solar radiation. Below the thermopause, gas particles have atomic collisions. Above it, they have ballastic collisions.
The upper boundary is the distance at which the effect of pressure of the sun's rays on atomic hydrogen is more than that of the Earth's gravitational pull. Simply put, the sun's radiation exerts a force on the hydrogen atoms which is more than the force exerted by Earth's gravity. It is roughly at half the distance between the Earth and the Moon. 10,000 km is commonly considered as the boundary between the atmosphere and space.
At heights of 100,000 km, hydrogen atoms scatter UV radiation, creating a glow. It is known as geocorona.
Each layer of the Earth's atmosphere has a marked significance dictated by the layer's composition. The troposphere is home to all the weather changes while the ozone's protective envelop in the stratosphere helps life thrive on the planet. Owing to the mesospheric airglow, the night-sky is never pitch dark. The changing colors of the sky are a part of the optical phenomena, the atmospheric optics give rise to. Airplanes travel in the stratosphere, space shuttles travel through the thermosphere, while the ionosphere enables radio transmission. Atmosphere is the protective shield that makes the Earth a livable place.