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Layers drawn to scale, objects within the layers are not to scale. Aurorae shown here at the bottom of the thermosphere can actually form at any altitude in this atmospheric layer. Principal layers In general, air pressure and density decrease with altitude in the atmosphere. However, temperature has a more complicated profile with altitude, and may remain relatively constant or even increase with altitude in some regions see the temperature section, below.

Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. This layer is mainly composed of extremely low densities of hydrogen, helium and several heavier molecules including nitrogen, oxygen and carbon dioxide closer to the exobase.

The atoms and molecules are so far apart that they can travel hundreds of kilometers without colliding with one another. Thus, the exosphere no longer behaves like a gas, and the particles constantly escape into space. These free-moving particles follow ballistic trajectories and may migrate in and out of the magnetosphere or the solar wind.

The exosphere is located too far above Earth for any meteorological phenomena to be possible. However, the aurora borealis and aurora australis sometimes occur in the lower part of the exosphere, where they overlap into the thermosphere. The exosphere contains most of the satellites orbiting Earth. The height of the thermopause varies considerably due to changes in solar activity. The temperature of the thermosphere gradually increases with height. Unlike the stratosphere beneath it, wherein a temperature inversion is due to the absorption of radiation by ozone, the inversion in the thermosphere occurs due to the extremely low density of its molecules.

The air is so rarefied that an individual molecule of oxygen , for example travels an average of 1 kilometre 0. This layer is completely cloudless and free of water vapor. However, non-hydrometeorological phenomena such as the aurora borealis and aurora australis are occasionally seen in the thermosphere.

Temperatures drop with increasing altitude to the mesopause that marks the top of this middle layer of the atmosphere. These are the highest clouds in the atmosphere and may be visible to the naked eye if sunlight reflects off them about an hour or two after sunset or a similar length of time before sunrise. They are most readily visible when the Sun is around 4 to 16 degrees below the horizon.

Lightning-induced discharges known as transient luminous events TLEs occasionally form in the mesosphere above tropospheric thunderclouds. The mesosphere is also the layer where most meteors burn up upon atmospheric entrance.

It is too high above Earth to be accessible to jet-powered aircraft and balloons, and too low to permit orbital spacecraft. The mesosphere is mainly accessed by sounding rockets and rocket-powered aircraft.

It lies above the troposphere and is separated from it by the tropopause. The stratosphere defines a layer in which temperatures rise with increasing altitude.

This rise in temperature is caused by the absorption of ultraviolet radiation UV radiation from the Sun by the ozone layer , which restricts turbulence and mixing.

Consequently, the stratosphere is almost completely free of clouds and other forms of weather. However, polar stratospheric or nacreous clouds are occasionally seen in the lower part of this layer of the atmosphere where the air is coldest.

The stratosphere is the highest layer that can be accessed by jet-powered aircraft. The troposphere is bounded above by the tropopause , a boundary marked in most places by a temperature inversion i. Thus, the lowest part of the troposphere i. Fifty percent of the total mass of the atmosphere is located in the lower 5. It has basically all the weather-associated cloud genus types generated by active wind circulation, although very tall cumulonimbus thunder clouds can penetrate the tropopause from below and rise into the lower part of the stratosphere.

Most conventional aviation activity takes place in the troposphere, and it is the only layer that can be accessed by propeller-driven aircraft.

Space Shuttle Endeavour orbiting in the thermosphere. The orange layer is the troposphere , which gives way to the whitish stratosphere and then the blue mesosphere. The ozone layer is contained within the stratosphere. In this layer ozone concentrations are about 2 to 8 parts per million, which is much higher than in the lower atmosphere but still very small compared to the main components of the atmosphere. The ionosphere is a region of the atmosphere that is ionized by solar radiation.

It is responsible for auroras. However, ionization in the mesosphere largely ceases during the night, so auroras are normally seen only in the thermosphere and lower exosphere. The ionosphere forms the inner edge of the magnetosphere. It has practical importance because it influences, for example, radio propagation on Earth. The homosphere and heterosphere are defined by whether the atmospheric gases are well mixed.

The surface-based homosphere includes the troposphere, stratosphere, mesosphere, and the lowest part of the thermosphere, where the chemical composition of the atmosphere does not depend on molecular weight because the gases are mixed by turbulence.

Above this altitude lies the heterosphere, which includes the exosphere and most of the thermosphere. Here, the chemical composition varies with altitude. This is because the distance that particles can move without colliding with one another is large compared with the size of motions that cause mixing. This allows the gases to stratify by molecular weight, with the heavier ones, such as oxygen and nitrogen, present only near the bottom of the heterosphere.

The upper part of the heterosphere is composed almost completely of hydrogen, the lightest element. During the day the planetary boundary layer usually is well-mixed, whereas at night it becomes stably stratified with weak or intermittent mixing.

Atmospheric pressure The average atmospheric pressure at sea level is defined by the International Standard Atmosphere as pascals This is sometimes referred to as a unit of standard atmospheres atm. Total atmospheric mass is 5. Atmospheric pressure is the total weight of the air above unit area at the point where the pressure is measured. Thus air pressure varies with location and weather. If the entire mass of the atmosphere had a uniform density from sea level, it would terminate abruptly at an altitude of 8.

It actually decreases exponentially with altitude, dropping by half every 5. However, the atmosphere is more accurately modeled with a customized equation for each layer that takes gradients of temperature, molecular composition, solar radiation and gravity into account.

By international convention, this marks the beginning of space where human travelers are considered astronauts. By comparison, the summit of Mt.

Meteors begin to glow in this region, though the larger ones may not burn up until they penetrate more deeply. Temperature and speed of sound Main articles: The instruments record microwaves emitted from oxygen molecules in the atmosphere. Because in an ideal gas of constant composition the speed of sound depends only on temperature and not on the gas pressure or density, the speed of sound in the atmosphere with altitude takes on the form of the complicated temperature profile see illustration to the right , and does not mirror altitudinal changes in density or pressure.

Density and mass Temperature and mass density against altitude from the NRLMSISE standard atmosphere model the eight dotted lines in each "decade" are at the eight cubes 8, 27, 64, Density of air The density of air at sea level is about 1.

Density is not measured directly but is calculated from measurements of temperature, pressure and humidity using the equation of state for air a form of the ideal gas law. Atmospheric density decreases as the altitude increases. This variation can be approximately modeled using the barometric formula.

More sophisticated models are used to predict orbital decay of satellites. The mean mass of water vapor is estimated as 1. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. October See also: Sunlight Solar radiation or sunlight is the energy Earth receives from the Sun. Earth also emits radiation back into space, but at longer wavelengths that we cannot see.

Part of the incoming and emitted radiation is absorbed or reflected by the atmosphere. In May , glints of light, seen as twinkling from an orbiting satellite a million miles away, were found to be reflected light from ice crystals in the atmosphere. If the light does not interact with the atmosphere, it is called direct radiation and is what you see if you were to look directly at the Sun.

Indirect radiation is light that has been scattered in the atmosphere. For example, on an overcast day when you cannot see your shadow there is no direct radiation reaching you, it has all been scattered. As another example, due to a phenomenon called Rayleigh scattering , shorter blue wavelengths scatter more easily than longer red wavelengths. This is why the sky looks blue; you are seeing scattered blue light. This is also why sunsets are red.

Much of the blue light has been scattered out, leaving the red light in a sunset. Different molecules absorb different wavelengths of radiation. For example, O2 and O3 absorb almost all wavelengths shorter than nanometers. When a molecule absorbs a photon, it increases the energy of the molecule. This heats the atmosphere, but the atmosphere also cools by emitting radiation, as discussed below.

The combined absorption spectra of the gases in the atmosphere leave "windows" of low opacity , allowing the transmission of only certain bands of light.

There are also infrared and radio windows that transmit some infrared and radio waves at longer wavelengths. For example, the radio window runs from about one centimeter to about eleven-meter waves. Emission electromagnetic radiation Emission is the opposite of absorption, it is when an object emits radiation. Objects tend to emit amounts and wavelengths of radiation depending on their " black body " emission curves, therefore hotter objects tend to emit more radiation, with shorter wavelengths.

Colder objects emit less radiation, with longer wavelengths. Because of its temperature, the atmosphere emits infrared radiation.

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