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.
In this way, Earth's atmosphere can be divided called atmospheric stratification into five main layers. Excluding the exosphere, the atmosphere has four primary layers, which are the troposphere, stratosphere, mesosphere, and thermosphere. Exosphere The exosphere is the outermost layer of Earth's atmosphere i. 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.
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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. Thermosphere The thermosphere is the second-highest layer of Earth's atmosphere.
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 oxygenfor 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.
Mesosphere The mesosphere is the third highest layer of Earth's atmosphere, occupying the region above the stratosphere and below 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.
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Stratosphere The stratosphere is the second-lowest layer of Earth's atmosphere. It lies above the troposphere and is separated from it by the tropopause. It contains the ozone layer, which is the part of Earth's atmosphere that contains relatively high concentrations of that gas.
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 layerwhich 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. Troposphere The troposphere is the lowest layer of Earth's atmosphere. The troposphere is bounded above by the tropopausea boundary marked in most places by a temperature inversion i.
Thus, the lowest part of the troposphere i. Earth's surface is typically the warmest section of the troposphere. Fifty percent of the total mass of the atmosphere is located in the lower 5. Nearly all atmospheric water vapor or moisture is found in the troposphere, so it is the layer where most of Earth's weather takes place.
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 tropospherewhich 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.
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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.
In summary, the mass of Earth's atmosphere is distributed approximately as follows: 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 rightand 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. Osama bin Laden traveled to Afghanistan and helped organize Arab mujahideen to resist the Soviets.
Muslim legal scholars "have throughout Islamic history unanimously agreed that the jihad is an individual duty if the enemy destroys the Muslim countries", according to bin Laden. Osama bin LadenDeath of Osama bin Ladenand Videos of Osama bin Laden Osama bin Laden at about 40 years of age, Bin Laden orchestrated the attacks and initially denied involvement but later recanted his false statements.
In the video, bin Laden is seen talking to Khaled al-Harbi and admits foreknowledge of the attacks.
In the video, he said: It has become clear that the West in general and America in particular have an unspeakable hatred for Islam. It is the hatred of crusaders. Terrorism against America deserves to be praised because it was a response to injustice, aimed at forcing America to stop its support for Israel, which kills our people. We say that the end of the United States is imminent, whether Bin Laden or his followers are alive or dead, for the awakening of the Muslim umma nation has occurred but he stopped short of admitting responsibility for the attacks.
Shortly before the U. He admitted his direct link to the attacks and said they were carried out because: As you undermine our security, we undermine yours. He was then held at multiple CIA secret prisons and Guantanamo Bay where he was interrogated and tortured with methods including waterboarding.
Trials related to the September 11 attacks In "Substitution for Testimony of Khalid Sheikh Mohammed" from the trial of Zacarias Moussaouifive people are identified as having been completely aware of the operation's details. At the same time, another 17 al-Qaeda members were sentenced to penalties of between six and eleven years. He was sentenced to life without parole in the United States. Two particularly important publications were bin Laden's "Letter to America",  and a videotape by bin Laden.
Inal-Qaeda wrote, "for over seven years the United States has been occupying the lands of Islam in the holiest of places, the Arabian Peninsula, plundering its riches, dictating to its rulers, humiliating its people, terrorizing its neighbors, and turning its bases in the Peninsula into a spearhead through which to fight the neighboring Muslim peoples. Planning of the September 11 attacks Map showing the attacks on the World Trade Center the planes are not drawn to scale The idea for the attacks came from Khalid Sheikh Mohammedwho first presented it to Osama bin Laden in A series of meetings occurred in earlyinvolving Mohammed, bin Laden, and his deputy Mohammed Atef.
Bank Tower in Los Angeles because "there was not enough time to prepare for such an operation". Hazmi and Mihdhar arrived in the United States in mid-January