The Earth's atmosphere is a layer of gases surrounding the planet Earth that is retained by the Earth's gravity. It contains roughly (by molar content/volume) 78.08% nitrogen, 20.95% oxygen, 0.93% argon, 0.038% carbon dioxide, trace amounts of other gases, and a variable amount (average around 1%) of water vapor. This mixture of gases is commonly known as air. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation and reducing temperature extremes between day and night.
There is no definite boundary between the atmosphere and outer space. It slowly becomes thinner and fades into space. Three quarters of the atmosphere's mass is within 11 km of the planetary surface. An altitude of 120 km (~75 miles or 400,000 ft) marks the boundary where atmospheric effects become noticeable during re-entry. The Kármán line, at 100 km (62 miles or 328,000 ft), is also frequently regarded as the boundary between atmosphere and outer space.
Temperature and layers
The temperature of the Earth's atmosphere varies with altitude; the mathematical relationship between temperature and altitude varies among five different atmospheric layers (ordered highest to lowest, the ionosphere is part of the thermosphere):
Exosphere: from 500 – 1000 km (300 – 600 mi) up to 10,000 km (6,000 mi), free-moving particles that may migrate into and out of the magnetosphere or the solar wind.
exobase boundary
Ionosphere: the part of the atmosphere that is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere. It has practical importance because, among other functions, it influences radio propagation to distant places on the Earth. It is located in the thermosphere and is responsible for auroras.
thermopause boundary
Thermosphere: from 80 – 85 km (265,000 – 285,000 ft) to 640+ km (400+ mi), temperature increasing with height.
mesopause boundary
Mesosphere: From the Greek word "μέσος" meaning middle. The mesosphere extends from about 50 km (160,000 ft) to the range of 80 to 85 km (265,000 – 285,000 ft), temperature decreasing with height. This is also where most meteors burn up when entering the atmosphere.
stratopause boundary
Stratosphere: From the Latin word "stratus" meaning a spreading out. The stratosphere extends from the troposphere's 7 to 17 km (23,000 – 60,000 ft) range to about 50 km (160,000 ft). Temperature increases with height. The stratosphere contains the ozone layer, the part of the Earth's atmosphere which contains relatively high concentrations of ozone. "Relatively high" means a few parts per million—much higher than the concentrations in the lower atmosphere but still small compared to the main components of the atmosphere. It is mainly located in the lower portion of the stratosphere from approximately 15 to 35 km (50,000 – 115,000 ft) above Earth's surface, though the thickness varies seasonally and geographically.
tropopause boundary
Troposphere: From the Greek word "τρέπω" meaning to turn or change. The troposphere is the lowest layer of the atmosphere; it begins at the surface and extends to between 7 km (23,000 ft) at the poles and 17 km (60,000 ft) at the equator, with some variation due to weather factors. The troposphere has a great deal of vertical mixing because of solar heating at the surface. This heating warms air masses, which makes them less dense so they rise. When an air mass rises, it does work against gravity. This work changes some of the thermal (kinetic) energy into gravitational potential energy, so the temperature of the air mass decreases. As the temperature decreases, water vapor in the air mass may condense or solidify, releasing latent heat that further uplifts the air mass. This process determines the maximum rate of decline of temperature with height, called the adiabatic lapse rate. The troposphere contains roughly 80% of the total mass of the atmosphere. Fifty percent of the total mass of the atmosphere is located in the lower 5.6 km of the troposphere.
The average temperature of the atmosphere at the surface of Earth is 15 °C (59 °F).[1][2]
[edit]Pressure and thickness
Main article: Atmospheric pressure
Barometric Formula: (used for airplane flight) barometric formula
One mathematical model: NRLMSISE-00
The average atmospheric pressure, at sea level, is about 101.3 kilopascals (about 14.7 psi); total atmospheric mass is 5.1480×1018 kg [3].
Atmospheric pressure is a direct result of the total weight of the air above the point at which the pressure is measured. This means that air pressure varies with location and time, because the amount (and weight) of air above the earth varies with location and time. However the average mass of the air above a square meter of the earth's surface is known to the same high accuracy as the total air mass of 5148.0 teratonnes and area of the earth of 51007.2 megahectares, namely 5148.0/510.072 = 10.093 metric tonnes or 14.356 lbs (mass) per square inch. This is about 2.5% below the officially standardized unit atmosphere (1 atm) of 101.325 kPa or 14.696 psi, and corresponds to the mean pressure not at sea level but at the mean base of the atmosphere as contoured by the earth's terrain.
Were atmospheric density to remain constant with height the atmosphere would terminate abruptly at 7.81 km (25,600 ft). Instead it decreases with height, dropping by 50% at an altitude of about 5.6 km (18,000 ft). For comparison: the highest mountain, Mount Everest, is higher, at 8.8 km, which is why it is so difficult to climb without supplemental oxygen. This pressure drop is approximately exponential, so that pressure decreases by approximately half every 5.6 km (whence about 50% of the total atmospheric mass is within the lowest 5.6 km) and by 1/e = .368 every 7.64 km, the average scale height of Earth's atmosphere below 70 km. However, because of changes in temperature, average molecular weight, and gravity throughout the atmospheric column, the dependence of atmospheric pressure on altitude is modeled by separate equations for each of the layers listed above.
Even in the exosphere, the atmosphere is still present (as can be seen for example by the effects of atmospheric drag on satellites).
The equations of pressure by altitude in the above references can be used directly to estimate atmospheric thickness. However, the following published data are given for reference: [4]
50% of the atmosphere by mass is below an altitude of 5.6 km.
90% of the atmosphere by mass is below an altitude of 16 km. The common altitude of commercial airliners is about 10 km.
99.99997% of the atmosphere by mass is below 100 km. The highest X-15 plane flight in 1963 reached an altitude of 354,300 ft (108,000 m).
Therefore, most of the atmosphere (99.9997%) is below 100 km, although in the rarefied region above this there are auroras and other atmospheric effects.
